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1
SANGIOVANNI, GIORGIO. "The electron-phonon interaction in strongly correlated electron systems." Doctoral thesis, La Sapienza, 2004. http://hdl.handle.net/11573/917137.
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Yang, Xiaodong. "Effects of Electron-Phonon Interaction in Metals." Diss., Temple University Libraries, 2010. http://cdm16002.contentdm.oclc.org/cdm/ref/collection/p245801coll10/id/83903.
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PhysicsPh.D.
Phonons and electrons are two types of excitations which are responsible for many properties of condensed matter materials. The interaction between them plays an important role in condensed matter physics. In this thesis we present some theoretical investigations of the effects due to the interactions between phonons and electrons interactions. We show evidence that a structural martensitic transition is related to significant changes in the electronic structure, as revealed in thermodynamic measurements made in high magnetic fields. The effect of the magnetic field is considered unusual, as many influential investigations of martensitic transitions have emphasized that the structural transitions are primarily lattice dynamical and are driven by the entropy due to the phonons. We provide a theoretical frame-work which can be used to describe the effect of a magnetic field on the lattice dynamics in which the field dependence originates from the dielectric constant. The temperature-dependence of the phonon spectrum of alpha-uranium has recently been measured by Manley et al. using inelastic neutron scattering and x-ray scattering techniques. Although there is scant evidence of anharmonic interactions, the phonons were reported to show some softening of the optic modes at the zone boundary. The same group of authors later reported that an extra vibrational mode was observed to form at a temperature above 450 K. The existence of the proposed new mode is inconsistent with the usual theory of harmonic phonons, as applied to a structure composed of a monoclinic Bravais lattice with a two-atom basis. We investigate the effect that the f electron-phonon interaction has on the phonon spectrum and its role on the possible formation of a breathing mode of mixed electronic and phonon character. We examine the model by using Green’s function techniques to obtain the phonon spectral density. Some materials undergo phase transitions from a high temperature state with periodic translational invariance to a state in which the electronic charge density is modulated periodically. The wave vector of the modulation may be either commensurate or incommensurate with the reciprocal lattice vectors of the high temperature structure. In the case of an incommensurate charge density wave, the system supports phason excitation. For an incommensurate state, the new ground state has a lower symmetry than the high temperature state since the charge density does not have long-ranged periodic translational order. If the metal is ideal (with no impurities), a charge density wave should be able to slide throughout the crystal without resistance, resulting in current flow similar to that of a superconductor. The phason is an excitation of the charge density wave which is related to the collective motion of electrons. We estimate the phason density of states, and the phason contribution to the specific heat. Angle-resolved photoemission experiments have been performed on USb2, and very narrow quasiparticle peaks have been observed in a band which local spin-density approximation (LSDA) predicts to osculate the Fermi energy. The observed band is found to be depressed by 17 meV below the Fermi energy. The experimentally observed quasiparticle dispersion relation for this band exhibits a kink at an energy of about 23 meV below the Fermi energy. The kink is not found in LSDA calculations and, therefore, is attributable to a change in the quasiparticle mass renormalization by a factor of approximately 2. The existence of a kink in the quasiparticle dispersion relation of a band which does not cross the Fermi energy is unprecedented. The kink in the quasiparticle dispersion relation is attributed to the effect of the interband self-energy involving transitions from the osculating band into a band that does cross the Fermi energy.
Temple University--Theses
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Rösch, Oliver. "Electron phonon interaction in strongly correlated materials." [S.l. : s.n.], 2005. http://nbn-resolving.de/urn:nbn:de:bsz:93-opus-24707.
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Sica, Gerardo. "Electron-electron and electron-phonon interactions in strongly correlated systems." Doctoral thesis, Universita degli studi di Salerno, 2013. http://hdl.handle.net/10556/1418.
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2011 - 2012In this work we investigate some aspects of the physics of strongly correlated systems by taking into account both electron-electron and electron-phonon interactions as basic mechanisms for reproducing electronic correlations in real materials. The relevance of the electron-electron interactions is discussed in the first part of this thesis in the framework of a self-consistent theoretical approach, named Composite Operator Method (COM), which accounts for the relevant quasi-particle excitations in terms of a set of composite operators that appear as a result of the modification imposed by the interactions on the canonical electronic fields. We show that the COM allows the calculation of all the relevant Green’s and correlation functions in terms of a number of unknown internal parameters to be determined self-consistently. Therefore, depending on the balance between unknown parameters and self-consistent equations, exact and approximate solutions can be obtained. By way of example, we discuss the application of the COM to the extended t-U- J-h model in the atomic limit, and to the two-dimensional single-band Hubbard model. In the former case, we show that the COM provides the exact solution of the model in one dimension. We study the effects of electronic correlations as responsible for the formation of a plethora of different charge and/or spin orderings. We report the phase diagram of the model, as well as a detailed analysis of both zero and finite temperature single-particle and thermodynamic properties. As far as the single-band Hubbard model is concerned, we illustrate an approximated selfconsistent scheme based on the choice of a two-field basis. We report a detailed analysis of many unconventional features that arise in single-particle properties, thermodynamics and system’s response functions. We emphasize that the accuracy of the COM in describing the effects of electronic correlations strongly relies on the choice of the basis, paving the way for possible multi-pole extensions to the twofield theory. To this purpose, we also study a three-field approach to the single-band Hubbard model, showing a significant step forward in the agreements with numerical data with respect to the two-pole results. The role of the electron-phonon interaction in the physics of strongly correlated systems is discussed in the second part of this thesis. We show that in highly polarizable lattices the competition between unscreened Coulomb and Fröhlich interactions results in a short-range polaronic exchange term Jp that favours the formation of local and light pairs of bosonic nature, named bipolarons, which condense with a critical temperature well in excess of hundred kelvins. These findings, discussed in the framework of the so-called polaronic t-Jp model, are further investigated in the presence of a finite on-site potential ~U , coming from the competition between on-site Coulomb and Fröhlich interactions. We discuss the role of ~U as the driving parameter for a small-to-large bipolaron transition, providing a possible explanation of the BEC-BCS crossover in terms of the properties of the bipolaronic ground state. Finally, we show that a hard-core bipolarons gas, studied as a charged Bose-Fermi mixture, allows for the description of many non Fermi liquid behaviours, allowing also for a microscopic explanation of pseudogap features in terms of a thermal-induced recombination of polarons and bipolarons, without any assumption on preexisting order or broken symmetries. [edited by author]
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Hewett, Nicholas Peter. "The electron-phonon interaction in a two dimensional electron gas." Thesis, University of Nottingham, 1988. http://eprints.nottingham.ac.uk/14218/.
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At low temperatures the predominant energy loss mechanism for a Joule-heated two dimensional electron gas (2DEG) in a metal oxide semiconductor field effect transistor (MOSFET) is by acoustic phonon emission. By very accurately measuring the temperature gradient developed along the silicon substrate the phonon emission has been investigated as a function of electron concentration, device power, magnetic field and temperature. In zero magnetic field the results show the cut-off predicted theoretically in the maximum phonon momentum that can be emitted in the plane of the 2DEG for low electron concentrations. It is also found that the momentum of the emitted phonons perpendicular to the plane of the 2DEG is restricted by the width of the 2DEG for the high resistivity (1000 [omega]cm) substrates used. For carrier concentrations greater than 4.9 x 1016 m-2 phonon emission from an upper subband is seen. Electrical measurements indicate that the high mobility (1.2 m2 V-1 S-1) of the devices used leads to changes in the screening of scattering potentials by the electrons being important. This is also seen in the phonon emission experiments. Experiments performed in quantising magnetic fields up to 7 T show that for the powers used (0.2 uW mm-2 – 500 uW mm-2) the phonons emitted arise from Lars-Landau level scattering. Oscillations in the temperature of a thermometer situated directly opposite the middle of the 2DEG are attributed to the movement of the phonon emission to the corners of the 2DEG when the Fermi level is between Landau levels (the Quantum Hall regime). Other trends are attributed to the width of the Landau level limiting the maximum phonon energy that can be emitted. Attempts to use a stress tuned phonon filter to probe the frequency dependence of the phonon emission failed due to experimental difficulties.
6
Sica, G. "Electron-electron and electron-phonon interactions in strongly correlated systems." Thesis, Loughborough University, 2013. https://dspace.lboro.ac.uk/2134/12194.
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In this work we investigate some aspects of the physics of strongly correlated systems by taking into account both electron-electron and electron-phonon interactions as basic mechanisms for reproducing electronic correlations in real materials. The relevance of the electron-electron interactions is discussed in the first part of this thesis in the framework of a self-consistent theoretical approach, named Composite Operator Method (COM), which accounts for the relevant quasi-particle excitations in terms of a set of composite operators that appear as a result of the modification imposed by the interactions on the canonical electronic fields. We show that the COM allows the calculation of all the relevant Green s and correlation functions in terms of a number of unknown internal parameters to be determined self-consistently. Therefore, depending on the balance between unknown parameters and self-consistent equations, exact and approximate solutions can be obtained. By way of example, we discuss the application of the COM to the extended t-U-J-h model in the atomic limit, and to the two-dimensional single-band Hubbard model. In the former case, we show that the COM provides the exact solution of the model in one dimension. We study the effects of electronic correlations as responsible for the formation of a plethora of different charge and/or spin orderings. We report the phase diagram of the model, as well as a detailed analysis of both zero and finite temperature single-particle and thermodynamic properties. As far as the single-band Hubbard model is concerned, we illustrate an approximated self-consistent scheme based on the choice of a two-field basis. We report a detailed analysis of many unconventional features that arise in single-particle properties, thermodynamics and system's response functions. We emphasize that the accuracy of the COM in describing the effects of electronic correlations strongly relies on the choice of the basis, paving the way for possible multi-pole extensions to the two-field theory. To this purpose, we also study a three-field approach to the single-band Hubbard model, showing a significant step forward in the agreements with numerical data with respect to the two-pole results. The role of the electron-phonon interaction in the physics of strongly correlated systems is discussed in the second part of this thesis. We show that in highly polarizable lattices the competition between unscreened Coulomb and Fröhlich interactions results in a short-range polaronic exchange term Jp that favours the formation of local and light pairs of bosonic nature, named bipolarons, which condense with a critical temperature well in excess of hundred kelvins. These findings, discussed in the framework of the so-called polaronic t-Jp model, are further investigated in the presence of a finite on-site potential U, coming from the competition between on-site Coulomb and Fröhlich interactions. We discuss the role of U as the driving parameter for a small-to-large bipolaron transition, providing a possible explanation of the BEC-BCS crossover in terms of the properties of the bipolaronic ground state. Finally, we show that a hard-core bipolarons gas, studied as a charged Bose-Fermi mixture, allows for the description of many non Fermi liquid behaviours, allowing also for a microscopic explanation of pseudogap features in terms of a thermal-induced recombination of polarons and bipolarons, without any assumption on preexisting order or broken symmetries.
7
Galda, Alexey. "Electronic properties of Luttinger Liquid with electron-phonon interaction." Thesis, University of Birmingham, 2013. http://etheses.bham.ac.uk//id/eprint/4293/.
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This thesis addresses a theoretical study of the problem of a single impurity embedded in a one-dimensional system of interacting electrons in presence of electron-phonon coupling. First we consider a system with a featureless point-like potential impurity, followed by the case of a resonant level hybridised with a Luttinger Liquid. The stress is made on a more fundamental problem of a featureless scatterer, for which two opposite limits in the impurity strength are considered: a weak scatterer and a weak link. We have found that, regardless of the transmission properties of phonons through the impurity, the scaling dimensions of the conductance in these limits obey the duality condition, \( \triangle_{WS}\) \( \triangle_{WL}\) = 1, known for the Luttinger Liquid in the absence of phonons. However, in the case when the strength of phonon scattering is correlated with electron scattering by the impurity, we find a nontrivial phase diagram with up to three fixed points and a possibility of a metal-insulator transition. We also consider the case of a weakly interacting electron-phonon system in the presence of a single impurity of an arbitrary scattering potential. In the problem of a resonant level attached to the Luttinger Liquid we show that the electron-phonon coupling significantly modifies the effective energy-dependent width of the resonant level in two different geometries, corresponding to the resonant and anti-resonant transmission in the Fermi gas.
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Davenport, Anthony. "The electron-phonon interaction in graphitic materials and superconductors." Thesis, Open University, 2014. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.663225.
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In this thesis we study the effects of the interaction between electrons and phonon modes in condensed matter systems. We explore two theoretical outcomes of theelectron-phonon interaction: Charge wave order and superconductivity. The main aims of this thesis are to establish ways of making graphitic materials useful for digital computing, and to investigate unconventional forms of superconductivity. Low order perturbation theory is combined with a Green's function analysis to calculate electron band gaps in a bilayer graphitic material that. forms electron-phonon interactions via an adjacent polarisable substrate. Self-consistent equations are derived and computationally solved to examine band gap enhancement in bilayer graphene and bilayer boron nitride. We also compare results for several different systems to identify the most promising ones for future developments. Our results show a promising new method of gap creation, for gaps of up to leV, in a simple bilayer graphene system where the electron-phonon interaction causes enhanced charge density wave order. The possibility of three-dimensional high temperature bipolaronic superconductivity is examined numerically through continuous-time quantum Monte Carlo simulations backed up by an exact analytical approximation for large phonon-frequency. Bose-Einstein condensation of bipolarons in a cubic system is estimated to occur at temperatures as high as 90-120K at low carrier concentrations, where bipolarons are small and mobile. We also develop formalism for calculating the superconducting band gap of BCS like superconductivity in intercalated graphitic materials (IGMs). Green's function analysis combined with low order perturbation theory is used to derive a set of generalised self-consistent equations designed to accommodate the tight binding parameters of all IGMs.
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Okazaki, K., S. Sugai, Y. Muraoka, and Z. Hiroi. "Role of electron-electron and electron-phonon interaction effects in the optical conductivity of VO_2." The American Physical Society, 2006. http://hdl.handle.net/2237/7137.
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Rizzo, Francesco, Francesco Rizzo, Francesco Rizzo, FRANCESCO RIZZO, and Francesco Rizzo. "Transport Properties and Electron-Phonon Interaction in the Normal State of High Temperature Superconductors." Doctoral thesis, La Sapienza, 2005. http://hdl.handle.net/11573/917307.
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Cross, Andrew John. "The electron-phonon interaction in GaAs/(AlGa)As quantum wells." Thesis, University of Nottingham, 2001. http://eprints.nottingham.ac.uk/14316/.
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This thesis presents a study of the electron-phonon interaction in two dimensional electron gases (2DEGs), by measuring of the acoustic phonon emission from a sequence of n-type doped GaAs/(AlGa)As quantum wells. Previous studies of emISSIon from 2DEGs confined in GaAs heterojunctions (Chin et al., 1984) have shown a surprising absence of longitudinal acoustic (LA) mode phonon emission, in contrast with theoretical studies (Vass, 1987) which predict that deformation potential coupled LA mode emission should dominate the energy relaxation processes. This may be attributed to the finite width of the quasi-2D sheet, which imposes a restriction on the maximum emitted phonon wavevector component perpendicular to the 2DEG, leading to a suppression of the emission (the "1Iao cutoff') at smaller phonon wavevectors than predicted by the earlier theory. By using the quantum well width w as a means of modulating the thickness of the 2DEG, the dependence of the 1Iao cutoff on the phonon emission can be directly measured. In the present work, significant LA phonon emission from the quantum well samples is observed. To complement the experimental measurements, the theory of emission from a 2DEG has been modelled in detail using computer simulation techniques. Calculations of the electron-phonon interaction, including matrix element anisotropy and dynamic screening, as well as phonon focusing effects, can be combined to produce accurate predictions of the experimentally detected phonon emission energy spectra.
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Hardy, Thomas M. "Superconductivity with strongly correlated electrons and an electron-phonon interaction." Thesis, Loughborough University, 2009. https://dspace.lboro.ac.uk/2134/34947.
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The effect on the stability of the superconducting phase due the addition of an electron–phonon interaction to a repulsive Hubbard model is studied. Our Hubbard–Fröhlich Hamiltonian includes electron hoping, the on-site Coulomb repulsion, vibrating ions (phonons) and the electron–phonon interaction. A Lang–Firsov transformation is used to integrate out the phonon degrees of freedom. The transformation reduces the model to simple a Hubbard Hamiltonian with an additional long-range electron–electron attraction. A variational Monte Carlo technique, with a projected BCS trial function, is used to investigate the ground state energies of our transformed Hubbard–Fröhlich Hamiltonian. For various electron densities, with a d-wave superconducting order parameter, it is found that the inclusion of the electron-phonon interaction significantly enhances the condensation energy (the energy required to break paired electrons). We show that increasing the strength of the electron-phonon interaction increases the condensation energy. It is also found that even with an infinite on-site repulsion, where the resonating valence bond state cannot exist, the EPI does still lead to a d-wave superconducting state. In addition we examine, analytically, the coexistence of ferromagnetism and superconductivity. Allowing different masses for spin-up and spin-down electrons in a BCS-type Hamiltonian two new branches in the energy spectrum are found. Including a spatially varying order parameter a new expression for the pairing amplitude of finite momentum pairs is derived.
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Khalid, M. A. "A study of the electron-phonon interaction in the DHVA effect." Thesis, University of Sussex, 1987. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.380530.
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Wahab, Y. B. "Phonon emission and reflection by a 2DEG studied with superconducting tunnel junctions." Thesis, University of Nottingham, 1989. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.235361.
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Anderson, David Richard. "Phonon-limited electron transport in gallium nitride and gallium nitride-based heterostructures, 1760-1851." Thesis, University of York, 2002. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.270104.
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Russel, P. A. "Some aspects of the electron-phonon interaction in two and three dimensions." Thesis, University of Nottingham, 1988. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.381079.
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Roknabadi, M. R., M. Pahlevane, and M. Modorresi. "A study of inelastic electron-phonon interaction on tunneling magnetoresistance in polyacetylene." Thesis, Sumy State University, 2011. http://essuir.sumdu.edu.ua/handle/123456789/20562.
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Ferreyra, Romualdo A. "Electron – phonon interaction in multiple channel GaN based HFETs: Heat management optimization." VCU Scholars Compass, 2014. http://scholarscompass.vcu.edu/etd/3636.
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New power applications for managing increasingly higher power levels require that more heat be removed from the power transistor channel. Conventional treatments for heat dissipation do not take into account the conversion of excess electron energy into longitudinal optical (LO) phonons, whose associated heat is stored in the channel unless such LO phonons decay into longitudinal acoustic (LA) phonons via a Ridley path. A two dimensional electron gas (2DEG) density of ~5×1012cm-2 in the channel results in a strong plasmon–LO phonon coupling (resonance) and a minimum LO phonon lifetime is experimentally observed, implying fast heat removal from the channel. Therefore, it is desirable to shift the resonance condition to higher 2DEG densities, and thereby higher power levels. The more convenient way to attain the latter is by widening the 2DEG density profile via heterostructure engineering, i.e. by using multiple channel heterostructures. A single channel heterostructure (GaN/AlN/AlGaN), a basic heterostructure used to obtain a 2DEG, exhibits a resonance condition at low 2DEG densities (~0.65×1012 cm-2). Successful widening of the 2DEG density xv profile was predicted by simulation results for two types of multiple (Al)GaN channel heterostructures, i.e. coupled channel GaN/AlN/GaN/AlN/AlGaN and dual channel GaN/AlGaN/AlN/AlGaN. Because of a reduction of carrier confinement, it is experimentally observed that control of the channel is moderate in the case of dual channel heterostructures. On the other hand, carrier confinement provides a better control of the channel in coupled channel heterostructures. Furthermore, unlike in a dual channel heterostructure, alloy scattering does not affect carrier transport properties, which results in a higher cut-off frequency. It was found experimentally that the coupled channel heterostructure successfully reaches resonance condition at a 2DEG density that is 23% higher than in a single channel heterostructure. Multiple channel heterostructures therefore provide a convenient way to shift the plasmon-LO phonon resonance to higher 2DEG densities. However, in our grown heterostructures, high power levels under optimal channel working conditions and minimum heat accumulation, all desirable benefits for the development of high power transistors, were only observed in coupled channel heterostructures.
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Albrecht, Klaus Ferdinand [Verfasser], and Lothar [Akademischer Betreuer] Mühlbacher. "Nonequilibrium charge transport through quantum dots with electron-phonon interaction = Nichtgleichgewichts-Ladungstransport durch Quantenpunkte mit Elektron-Phonon Wechselwirkung." Freiburg : Universität, 2014. http://d-nb.info/1123478678/34.
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Geondzhian, Andrey. "Resonant inelastic X-ray scattering as a probe of exciton-phonon coupling." Thesis, Université Grenoble Alpes (ComUE), 2018. http://www.theses.fr/2018GREAY077/document.
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Les phonons contribuent à la diffusion inélastique résonante des rayons X (RIXS) du fait du couplage entre les degrés de liberté électronique et ceux du réseau. Contrairement à d'autres techniques sensibles aux interactions électron-phonon, la technique RIXS peut donner accès aux constantes de couplage dépendantes du moment. Des informations sur la dispersion de l'interaction électron-phonon sont très précieuses dans le contexte de la supraconductivité anisotrope conventionnelle et non conventionnelle.Nous avons considéré la contribution des phonons sur la diffusion RIXS d’un point de vue théorique. Contrairement aux études précédentes nous soulignons le rôle du couplage du réseau avec les trous de cœur. Notre modèle, avec les paramètres obtenus ab-initio, montre que même dans le cas d'un trou de coeur profond, la technique RIXS sonde le couplage exciton-phonon plutôt qu’un couplage direct électron-phonon.Cette différence conduit à des écarts quantitatifs et qualitatifs pour le couplage électron-phonon implicite par rapport à l'interprétation standard dans la littérature. Ainsi, notre objectif est de développer une approche rigoureuse pour quantifier le couplage électron-phonon dans le contexte des mesures de diffusion RIXS. La possibilité de reproduire avec précision les résultats expérimentaux à partir des calculs ab-initio, sans recourir à des paramètres ajustés, doit être considérée comme le test ultime d'une compréhension correcte de la contribution des phonons sur la diffusion RIXS.Nous commençons notre travail en considérant uniquement l’interaction trou de coeur-phonon dans le contexte de la spectroscopie par photoémission de rayons X. Nous combinons un calcul ab-initio de la fonction de réponse en espace réel avec des techniques de fonctions de Green à plusieurs corps pour reproduire les bandes latérales vibrationnelles dans les molécules SiX4 (X = H, F). L'approche que nous avons développée peut être appliquée aux matériaux cristallins.Nous examinons ensuite la contribution des phonons aux spectres d'absorption des rayons X. Contrairement aux excitations chargées générées par la photoémission par rayons X, l'absorption des rayons X crée une excitation neutre que nous approchons en tant que trou de cœur et électron excité. Nous résolvons d’abord la partie électronique du problème au niveau de l’équation de Bethe-Salpeter, puis nous habillons la quasi-particule excitonique à 2 particules résultante avec les interactions exciton-phonon en utilisant l’Ansatz des cumulants. La viabilité de cette méthode a été testée en calculant le seuil K XAS de la molécule N2 et le seuil K d’Oxygène de l’acétone. Les spectres vibrationnels obtenus concordent avec les résultats expérimentaux.Enfin, nous construisons une formulation hybride de la section transversale RIXS qui préserve la sommation explicite sur un petit nombre d'états finals, mais remplace la sommation sur les états intermédiaires, ce qui pourrait être extrêmement coûteux, par une fonction de Green. Nous avons obtenu un développement de la fonction de Green et dérivé des solutions analytiques exactes (dans la limite de non-recul) et approximatives. Le formalisme a de nouveau été testé sur le seuil K de l'acétone et est bien en accord avec l'expérience. En perspectives des travaux futurs, nous discutons de l’applicabilité de notre formalisme aux matériaux cristallinsPhonons contribute to resonant inelastic X-ray scattering (RIXS) as a consequence of the coupling between electronic and lattice degrees of freedom. Unlike other techniques that are sensitive to electron-phonon interactions, RIXS can give access to momentum dependent coupling constants. Information about the dispersion of the electron-phonon interaction is highly desirable in the context of understanding anisotropic conventional and unconventional superconductivity.We considered the phonon contribution to RIXS from the theoretical point of view. In contrast to previous studies, we emphasize the role of the core-hole lattice coupling. Our model, with parameters obtained from first principles, shows that even in the case of a deep core-hole, RIXS probes exciton-phonon coupling rather than a direct electron-phonon coupling.This difference leads to quantitative and qualitative deviations from the interpretation of the implied electron-phonon coupling from the standard view expressed in the literature. Thus, our objective is to develop a rigorous approach to quantify electron-phonon coupling within the context of RIXS measurements. The ability to accurately reproduce experimental results from first-principles calculations, without recourse to adjustable parameters, should be viewed as the ultimate test of a proper understanding of the phonon contribution to RIXS.We start by considering only the core-hole--phonon interaction within the context of X-ray photoemission spectroscopy. We combine an ab initio calculation of the real-space response function with many-body Green's functions techniques to reproduce the vibrational side-bands in SiX4 (X=H, F) molecules. The approach we developed is suitable for application to crystalline materials.We next consider the phonon contribution to X-ray absorption spectra. Unlike the charged excitations generated by X-ray photoemission, X-ray absorption creates a neutral excitation that we approximate as a core-hole and an excited electron. We first solved the electronic part of the problem on the level of the Bethe-Salpeter equation and then dressed the resulting 2-particle excitonic quasiparticle with the exciton-phonon interactions using the cumulant ansatz. The viability of this methodology was tested by calculating the N K-edge XAS of the N2 molecule and the O K-edge of acetone. The resulting vibronic spectra agreed favorably with experimental results.Finally, we construct a hybrid formulation of the RIXS cross section that preserves explicit summation over a small number of final states, but replaces the summation over intermediate states, which might be enormously expensive, with a Green's function. We develop an expansion of the Green's function and derive both analytically exact (in the no-recoil limit) and approximate solutions. The formalism was again tested on the O K-edge of acetone and agrees well with the experiment. To provide an outlook towards future work, we discuss application of the developed formalism to crystalline materials
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Lehmann, Dietmar. "Phonon Spectroscopy and Low-Dimensional Electron Systems." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2006. http://nbn-resolving.de/urn:nbn:de:swb:14-1138734990743-55381.
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The generation and propagation of pulses of nonequilibrium acoustic phonons and their interaction with semiconductor nanostructures are investigated. Such studies can give unique information about the properties of low-dimensional electron systems, but in order to interpret the experiments and to understand the underlying physics, a comparison with theoretical models is absolutely necessary. A central point of this work is therefore a universal theoretical approach allowing the simulation and the analysis of phonon spectroscopy measurements on low-dimensional semiconductor structures. The model takes into account the characteristic properties of the considered systems. These properties are the elastic anisotropy of the substrate material leading to focusing effects and highly anisotropic phonon propagation, the anisotropic nature of the different electron-phonon coupling mechanisms, which depend manifestly on phonon wavevector direction and polarization vector, and the sensitivity to the confinement parameters of the low-dimensional electron systems. We show that screening of the electron-phonon interaction can have a much stronger influence on the results of angle-resolved phonon spectroscopy than expected from transport measurements. Since we compare theoretical simulations with real experiments, the geometrical arrangement and the spatial extension of phonon source and detector are also included in the approach enabling a quantitative analysis of the data this way. To illustrate the influence of acoustic anisotropy and carrier confinement on the results of phonon spectroscopy in detail we analyse two different applications. In the first case the low-dimensional electron system acts as the phonon detector and the phonon induced drag current is measured. Our theoretical model enables us to calculate the electric current induced in low-dimensional electron systems by pulses of (ballistic) nonequilibrium phonons. The theoretical drag patterns reproduce the main features of the experimental images very well. The sensitivity of the results to variations of the confining potential of quasi-2D and quasi-1D electrons is demonstrated. This provides the opportunity to use phonon-drag imaging as unique experimental tool for determining the confinement lengths of low-dimensional electron systems. By comparing the experimental and theoretical images it is also possible to estimate the relative strength of the different electron-phonon coupling mechanisms.In the second application the low-dimensional electron system acts as the phonon pulse source and the angle and mode dependence of the acoustic phonon emission by hot 2D electrons is investigated. The results exhibit strong variations in the phonon signal as a function of the detector position and depend markedly on the coupling mechanism, the phonon polarization and the electron confinement width. We demonstrate that the ratio of the strengths of the emitted longitudinal (LA) and transverse (TA) acoustic phonon modes is predicted correctly only by a theoretical model that properly includes the effects of acoustic anisotropy on the electron-phonon matrix elements, the screening, and the form of the confining potential. A simple adoption of widely used theoretical assumptions, like the isotropic approximation for the phonons in the electron-phonon matrix elements or the use of simple variational envelope wavefunctions for the carrier confinement, can corrupt or even falsify theoretical predictions.We explain the `mystery of the missing longitudinal mode' in heat-pulse experiments with hot 2D electrons in GaAs/AlGaAs heterojunctions. We demonstrate that screening prevents a strong peak in the phonon emission of deformation potential coupled LA phonons in a direction nearly normal to the 2D electron system and that deformation potential coupled TA phonons give a significant contribution to the phonon signal in certain emission directionsDie vorliegende Arbeit beschäftigt sich mit der Ausbreitung von akustischen Nichtgleichgewichtsphononen und deren Wechselwirkung mit Halbleiter-Nanostrukturen. Güte und Effizienz moderner Halbleiter-Bauelemente hängen wesentlich vom Verständnis der Wechselwirkung akustischer Phononen mit niederdimensionalen Elektronensystemen ab. Traditionelle Untersuchungsmethoden, wie die Messung der elektrischen Leitfähigkeit oder der Thermospannung, erlauben nur eingeschränkte Aussagen. Sie mitteln über die beteiligten Phononenmoden und eine Trennung der einzelnen Wechselwirkungsmechanismen ist nur näherungsweise möglich ist. Demgegenüber erlaubt die in der Arbeit diskutierte Methode der winkel- und zeitaufgelösten Phononen-Spektroskopie ein direktes Studium des Beitrags einzelner Phononenmoden, d.h. in Abhängigkeit von Wellenzahlvektor und Polarisation der Phononen. Im Mittelpunkt der Arbeit steht die Fragestellung, wie akustische Anisotropie und Ladungsträger-Confinement die Ergebnisse der winkel- und zeitaufgelösten Phononen-Spektroskopie beeinflussen und prägen. Dazu wird ein umfassendes theoretisches Modell zur Simulation von Phononen-Spektroskopie-Experimenten an niederdimensionalen Halbleitersystemen vorgestellt. Dieses erlaubt sowohl ein qualitatives Verständnis der ablaufenden physikalischen Prozesse als auch eine quantitative Analyse der Messergebnisse. Die Vorteile gegenüber anderen Modellen und Rechnungen liegen dabei in dem konsequenten Einbeziehen der akustischen Anisotropie, nicht nur für die Ausbreitung der Phononen, sondern auch für die Matrixelemente der Wechselwirkung, sowie eine saubere Behandlung des Confinements der Elektronen in den niederdimensionalen Systemen. Dabei werden die Grenzen weit verbreiteter Näherungsansätze für die Elektron-Phonon-Matrixelemente und das Elektronen-Confinement deutlich aufgezeigt. Für den quantitativen Vergleich mit realen Experimenten werden aber auch solche Größen, wie die endliche räumliche Ausdehnung von Phononenquelle und Detektor, die Streuung der Phononen an Verunreinigungen oder die Abschirmung der Elektron-Phonon-Kopplung durch die Elektron-Elektron-Wechselwirkung berücksichtigt.Im zweiten Teil der Arbeit wird der theoretische Apparat auf typische experimentelle Fragestellungen angewandt. Im Falle der Phonon-Drag-Experimente an GaAs/AlGaAs Heterostrukturen wird der durch akustische Nichtgleichgewichtsphononen in zwei- und eindimensionalen Elektronensystemen induzierte elektrische Strom (Phonon-Drag-Strom) als Funktion des Ortes der Phononenquelle bestimmt. Das in der Arbeit hergeleitete theoretische Modell kann die experimentellen Resultate für die Winkelabhängigkeit des Drag-Stromes sowohl für Messungen mit und ohne Magnetfeld qualitativ gut beschreiben. Außerdem wird der Einfluss unterschiedlicher Confinementmodelle und unterschiedlicher Wechselwirkungsmechanismen studiert. Dadurch ist es möglich, aus Phonon-Drag-Messungen Rückschlüsse auf die elektronischen und strukturellen Eigenschaften der niederdimensionalen Elektronensysteme zu ziehen (Fermivektor, effektive Masse, Elektron-Phonon-Kopplungskonstanten, Form des Confinementpotentials). Als weiteres Anwendungsbeispiel wird das Problem der Energierelaxation (aufgeheizter)zweidimensionaler Elektronensysteme in GaAs Heterostrukturen und Quantentrögen untersucht. Für Elektronentemperaturen unterhalb 50 K werden die Gesamtemissionsrate als Funktion der Temperatur und die winkelaufgelöste Emissionsrate (als Funktion der Detektorposition) berechnet. Für beide Größen wird erstmals eine gute Übereinstimmung zwischen Theorie und Experiment gefunden. Es zeigt sich, dass akustische Anisotropie und Abschirmungseffekte zu überraschenden neuen Ergebnissen führen können. Ein Beispiel dafür ist der unerwartet große Beitrag der mittels Deformationspotential-Wechselwirkung emittierten transversalen akustischen Phononen, der bei einer Emission der Phononen näherungsweise senkrecht zum zweidimensionalen System beobachtet werden kann
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Rittweger, Florian [Verfasser]. "Influence of the electron-phonon interaction onto electronic transport in metals / Florian Rittweger." Halle, 2018. http://d-nb.info/1153007797/34.
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Al-Dossary, Omar M. M. "Macroscopic polar optical lattice vibrations and electron-phonon interaction in layered semiconductor structures." Thesis, University of Essex, 1993. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.359255.
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Sidorova, Mariia. "Timing Jitter and Electron-Phonon Interaction in Superconducting Nanowire Single-Photon Detectors (SNSPDs)." Doctoral thesis, Humboldt-Universität zu Berlin, 2021. http://dx.doi.org/10.18452/22296.
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Die vorliegende Doktorarbeit beschäftigt sich mit der experimentellen Studie zweier miteinander verbundener Phänomene: Dem intrinsischen Timing-Jitter in einem supraleitendenden Nanodraht-Einzelphotonen-Detektor (SNSPD) und der Relaxation der Elektronenenergie in supraleitenden Filmen. Supraleitende Nanodrähte auf einem dielektrischen Substrat als mikroskopische Grundbausteine jeglicher SNSPDs stellen sowohl für theoretische als auch für experimentelle Studien komplexe Objekte dar. Die Komplexität ergibt sich aus der Tatsache, dass SNSPDs in der Praxis stark ungeordnete und ultradünne supraleitende Filme verwenden, die eine akustische Fehlanpassung zu dem zugrundeliegenden Substrat aufweisen und einen Nichtgleichgewichts-Zustand implizieren. Die Arbeit untersucht die Komplexität des am weitesten in der SNSPD Technologie verbreiteten Materials, Niobnitrid (NbN), indem verschiedene experimentelle Methoden angewandt werden. Als eine mögliche Anwendung der SNSPD-Technologie wird ein Prototyp eines dispersiven Raman-Spektrometers mit Einzelphotonen-Sensitivität demonstriert.This Ph.D. thesis is based on the experimental study of two mutually interconnected phenomena: intrinsic timing jitter in superconducting nanowire single-photon detectors (SNSPDs) and relaxation of the electron energy in superconducting films. Microscopically, a building element of any SNSPD device, a superconducting nanowire on top of a dielectric substrate, represents a complex object for both experimental and theoretical studies. The complexity arises because, in practice, the SNSPD utilizes strongly disordered and ultrathin superconducting films, which acoustically mismatch with the underlying substrate, and implies a non-equilibrium state. This thesis addresses the complexity of the most conventional superconducting material used in SNSPD technology, niobium nitride (NbN), by applying several distinct experimental techniques. As an emerging application of the SNSPD technology, we demonstrate a prototype of the dispersive Raman spectrometer with single-photon sensitivity.
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Ye, Zhou. "Mechanism and the Effect of Microwave-Carbon Nanotube Interaction." Thesis, University of North Texas, 2005. https://digital.library.unt.edu/ark:/67531/metadc4919/.
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A series of experimental results about unusual heating of carbon nanotubes by microwaves is analyzed in this dissertation. Two of vibration types, cantilever type (one end is fixed and the other one end is free), the second type is both ends are fixed, have been studied by other people. A third type of forced vibration of carbon nanotubes under an alternating electromagnetic field is examined in this paper. Heating of carbon nanotubes (CNTs) by microwaves is described in terms of nonlinear dynamics of a vibrating nanotube. Results from the model provide a way to understand several observations that have been made. It is shown that transverse vibrations of CNTs during microwave irradiation can be attributed to transverse parametric resonance, as occurs in the analysis of Melde's experiment on forced longitudinal vibrations of a stretched elastic string. For many kinds of carbon nanotubes (SWNT, DWNT, MWNT, ropes and strands) the resonant parameters are found to be located in an unstable region of the parameter space of Mathieu's equation. Third order wave equations are used to qualitatively describe the effects of phonon-phonon interactions and energy transfer from microwaves to CNTs. This result provides another way to input energy from microwaves to carbon nanotubes besides the usual Joule heating via electron-phonon interaction. This model appears to be the first to point out the role of nonlinear dynamics in the heating of CNTs by microwaves.
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Hodgson, Michael John. "Bonding in semiconductors." Thesis, University of Cambridge, 1991. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.240971.
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Sidorova, Mariia [Verfasser]. "Timing Jitter and Electron-Phonon Interaction in Superconducting Nanowire Single-Photon Detectors (SNSPDs) / Mariia Sidorova." Berlin : Humboldt-Universität zu Berlin, 2021. http://d-nb.info/1226153380/34.
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Amavisca, Edward D. 1965. "Electron(hole)-phonon interaction in YBCO high temperature superconductor using quantum path integral molecular dynamics." Thesis, The University of Arizona, 1991. http://hdl.handle.net/10150/277899.
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In this research, we have implemented an original technique to study the electronic properties of a single electron placed in YBa2 at 300K. Using a discretized extension of Feynman's Quantum Path Integral, we have been able to characterize effective electron-phonon interactions, and electron location site probability. We find that the electron stabilizes at oxygen vacant sites in the copper-oxygen chains. In the copper-oxygen planes, the electron is unstable and moves into the chain. Upon complementing the quantum electron to a positive charge thereby simulating a hole, we then find that the hole moves into favorable sites in the copper-oxygen planes. These sites are surrounded by four oxygens and two copper ions. Next, by decoupling the electron and hole from the lattice, we obtain effective electron-phonon and hole-phonon coupling constants on the order of 30. These results indicate that the next area of research is to move toward a multi-electron system and allow for further study of the electrons near the Fermi level. Some of the difficulties associated with multi-electron systems such as "exchange", are briefly discussed.
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Maier, Stefan [Verfasser], and Andreas [Akademischer Betreuer] Komnik. "Effect of electron-phonon interaction in nanostructures and ultracold quantum gases / Stefan Maier ; Betreuer: Andreas Komnik." Heidelberg : Universitätsbibliothek Heidelberg, 2011. http://d-nb.info/1179782909/34.
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Lee, Chong Yong. "Electron-phonon interaction in barium-potassium-bismuth-oxygen superconductor by quantum path integral molecular dynamics (QPIMD)." Diss., The University of Arizona, 1990. http://hdl.handle.net/10150/185239.
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We have introduced two different techniques in this paper to investigate the high Tc superconductor BaK BiO₃. The first one is the rigid-ion molecular dynamics model to calculate classical properties of the crystal. This method provides the ionic anisotropic vibrational spectra. It is found that the oxygen modes dominate the vibrational spectrum from 150 cm⁻¹ up to 820 cm⁻¹. An increase in the number of substituted K around an oxygen favors high frequency vibrational modes. We have also determined by experiment IR spectrum. The absorption peaks are between 380 to 880 cm⁻¹. Our experimental and computational data are in good agreement in the high frequency region. The second technique used to investigate this high Tc superconductor is based on a quantum path integral molecular dynamics. It has been applied for the first time to the determination of electron-phonon interaction energy. We estimate the electron phonon coupling constant at about 1.34. This value represents a weak to moderate electron-phonon coupling in Ba₁₋ₓKₓBiO₃ in agreement with current views of this pairing mechanism in this material.
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Salami, Sana. "Conductance électronique et effet Seebeck dans des canaux conducteurs implantés sous la surface du diamant." Electronic Thesis or Diss., Lyon 1, 2024. http://www.theses.fr/2024LYO10287.
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L'interaction entre les électrons et les phonons dans la matière condensée est responsable du couplage de leurs équations de transport, ce qui donne lieu à des effets d’entrainements mutuels sous l'effet d'un gradient de température (effet Seebeck) ou d'un courant de charge (effet Peltier). Dans certains matériaux, ces effets, connus sous le terme anglophone de « phonon drag », conduisent à des valeurs de coefficient Seebeck très élevées. Ils se manifestent généralement à basse température lorsque le libre parcours moyen des phonons est important, près du maximum en température de la conductivité thermique de réseau. Historiquement, ces effets de « drag » entre les électrons et les phonons ont été mis de côté dans les stratégies de recherche en thermoélectricité car ils coïncident avec un maximum de la conductivité thermique du réseau, ce qui limite l'efficacité thermoélectrique. Toutefois, des études réalisées en 2008 sur des systèmes de silicium nanostructurés ont montré que des effets Seebeck significatifs dus à ces effets de « drag » peuvent coexister avec une faible conductivité thermique du réseau. Les travaux théoriques traitant les équations de transport couplées ont apporté de nouvelles perspectives, démontrant que les états de phonons dominant le transport de chaleur sont distincts de ceux qui sont principalement impliqués dans les effets de drag. L'objectif de ma thèse était de réaliser des expériences pour explorer un nouveau concept d’effets « drag » entre les électrons et les phonons, séparés par une interface. J'ai étudié les propriétés de transport des électrons confinés dans un canal conducteur sous la surface d'un diamant cristallin agissant comme un bain de phonon externe constitué d’états de phonon propagatifs. Mon travail inclut l’élaboration des systèmes, leur caractérisation structurale et le développement de protocoles de mesure pour la conductance et le coefficients Seebeck pour des systèmes conducteurs enterrés. J'ai étudié les propriétés de transport de canaux produits dans différentes conditions d'implantation et de recuit pour comprendre les liens entre leur microstructure et leurs propriétés de transport électronique. Dans la première étude, j'ai mesuré la conductance et le coefficient Seebeck dans des canaux épais (~145 nm) enfouis à environ 200 nm sous la surface du diamant. Ces canaux, créés par implantation ionique d'hélium à une fluence modérée suivie d'un recuit à haute température, présentaient principalement une microstructure graphitique. À température ambiante, la conductance est proche de celle du graphite massif. Le coefficient Seebeck présentait un comportement à basse température similaire à celui du graphite, avec un pic négatif attribué aux effets de traînée. Toutefois, ce pic est apparu à une température nettement plus élevée que dans le cas du graphite massif, ce qui pourrait être une première indication des contributions supplémentaires des phonons du diamant. Dans la seconde étude, j'ai examiné des canaux de dimensions similaires mais soumis à une fluence d'implantation plus faible ou recuits à des températures plus basses. Ces canaux présentent une microstructure désordonnée complexe composée d'agrégats d'atomes de carbone avec une hybridation sp, sp² et sp³. La conductance et le coefficient Seebeck en fonction de la température varient de manière significative par rapport à la première étude. Le coefficient Seebeck est positif et atteint plusieurs milliers de µV/K à basse température. Les propriétés de transport ont été interprétées à l'aide de la théorie du chaos quantique. Mon travail montre la faisabilité des mesures de conductance et du coefficient Seebeck à basse température à travers des canaux conducteurs enfouis sous la surface du diamant et met en évidence la richesse des propriétés de conduction de ces canaux qui peuvent être accordées en contrôlant leur microstructure par un choix judicieux des paramètres d'implantation et de recuitThe interaction between electrons and phonons in condensed matter is responsible for the coupling of their transport equations, which gives rise to mutual drag effects between them under the effect of a temperature gradient (Seebeck effect) or a charge current (Peltier effect). In some bulk materials, these drag effects, better known as “phonon drag”, lead to very high Seebeck coefficient values at low temperatures. It usually manifests at low temperatures when the phonon mean free path is large, near the maximum in temperature of the lattice thermal conductivity in dielectric crystals, which occurs due to reduced phonon-phonon scattering. Historically, phonon drag has been overlooked in thermoelectric research as it coincides with the maximum in the lattice thermal conductivity peak, potentially reducing overall thermoelectric efficiency. However, studies in 2008 on nanostructured silicon systems contradicted this, demonstrating that significant Seebeck effects from phonon drag can coexist with low lattice thermal conductivity. Theoretical works treating the coupled transport equations offered new insights, demonstrating that phonon states dominating heat transport are distinct in phase space, in wave-vector, and energy from those mainly involved in drag effects. The aim of my thesis was to carry out experiments to explore a novel concept of phonon drag between electron and phonon gases, separated by an interface. I studied transport properties of electrons confined to a conductive channel beneath the surface of a crystalline diamond acting as an external phonon bath with propagative phonon states. My thesis involved system construction, structural characterization, development of measurement protocols for conductance and Seebeck coefficients, and adaptation of these measurements for buried conductive systems. I analyzed the transport properties of channels produced under different implantation and annealing conditions to understand how the microstructure of the channel influences electronic transport properties. In the first study, I measured conductance and the Seebeck coefficient in thick channels (~145 nm) buried about 200 nm below the diamond surface. These channels, created via helium ion implantation at moderate fluence followed by high-temperature annealing, displayed mostly a graphitic microstructure. At room temperature, the conductance was close to that of bulk graphite. The Seebeck coefficient exhibited low-temperature behavior akin to graphite, with a negative peak attributed to drag effects. However, this peak occurred at a significantly higher temperature compared to solid graphite, which could be a first indication of additional contributions from diamond phonons. In the second study, I examined channels with similar dimensions but subjected to lower implantation fluence or annealed at lower temperatures. These channels exhibited a highly disordered microstructure composed of aggregates of carbon atoms with sp, sp², and sp³ hybridization. The temperature-dependent conductance and Seebeck coefficient varied significantly from the first study. The Seebeck coefficient is positive and reached several thousand µV/K at low temperatures. The transport properties were interpreted using quantum chaos theory. My work shows the feasibility of conductance and Seebeck coefficient measurements at low temperature through conductive channels buried beneath the diamond surface and highlights the rich conduction properties of such channels which can be tuned by controlling their microstructure through a careful choice of implantation and annealing parameters
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Richler, Kevin-Davis. "Modélisation du couplage électron-vibration dans les cellules solaires organiques : une approche par champ moyen dynamique inhomogène." Thesis, Université Grenoble Alpes (ComUE), 2019. http://www.theses.fr/2019GREAY052.
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Dans cette thèse, nous avons tout d’abord développé une théorie du champ moyen dynamique non homogène (I-DMFT) qui convient pour étudier les interactions électron- phonon dans des systèmes non invariants par translation. L’approche présentée, dont la seule hypothèse est celle d’une énergie propre locale dépendante du site, retrouve à la fois la solution exacte d’un électron pour un hamiltonien générique à liaisons fortes dans la limite de sans interaction et la solution DMFT pour le petit polaron dans les systèmes invariants par translation. Pour illustrer ses capacités, nous avons appliqué l’I-DMFT pour étudier la formation de polarons en présence d’impuretés isolées. Nous avons constaté que l’I- DMFT est plus précis que l’approximation de la moyenne inhomogène de la quantité de mouvement (IMA) et donne des résultats quantitatifs proches des calculs de Monte Carlo. Pour illustrer ses capacités numériques, nous utilisons ensuite l’I-DMFT pour étudier les effets des défauts intégrés sur une surface bidimensionnelle. Les cartes calculées de la densité locale d’états révèlent des oscillations de Friedel, dont la périodicité est déterminée par la masse du polaron. Ensuite, nous nous sommes concentrés sur le mécanisme de séparation interfacial électron-trou dans les systèmes prototypes de cellules solaires organiques. En utilisant l’I-DMFT, nous présentons une simulation entièrement quantique de la dynamique des porteurs de charge qui prend en compte les interactions électron-phonon, les désordres statiques et les champs électrostatiques. En particulier, ces simulations offrent la possibilité de calculer le taux d’injection de charge à l’interface donneur-accepteur, une quantité d’intérêt expérimental fondamental. Ceci constitue un premier pas vers un défi de longue date concernant le PVO, établissant ainsi un lien entre la chimie et la physique.Dans un premier temps, nous avons appliqué l’I-DMFT á un modèle générique unidimensionnel d’Hamiltonien, dont les paramètres modélisent la dynamique des porteurs de charge dans l’acide méthyl ester méthylique-C61 (PCBM) ou C60 systèmes accepteurs. Nos résultats montrent que les potentiels dynamiques (liés à la formation de polarons), comparés aux potentiels statiques aléatoires, peuvent présenter le principal mécanisme de perte préjudiciable dans les dispositifs PVO. Dans cette perspective, les molécules organiques avec des ́energies de r ́eorganisation mod ́er ́ees devraient ˆetre utilis ́ees pr ́ef ́erentiellement dans les matériaux de nouvelle génération, car l’augmentation de l’interaction électron-phonon entrave la gamme des énergies d’électrons entrantes appropriées en raison de la fragmentation de la densité d’états locale en sous-bandes polaroniques étroites. Enfin, nous avons appliqué l’I-DMFT à un hamiltonien modèle générique en trois dimensions, dont les paramètres modélisent les champs électrostatiques des charges dans les systèmes PCBM en trois dimensions et les systèmes accepteurs C60. Ces champs présentent un pic important sur une interphase étroite issue de la présence de petites molécules de fullerènes dans le matériau donneur. Nous montrons ensuite que de tels champs, lorsqu’ils sont suffisamment importants et correctement polarisés, peuvent exercer une influence positive sur l’efficacité de la séparation des charges à travers des interfaces organiques. Dans cette perspective, les matériaux présentant des champs électrostatiques et mésoéchelle forts et correctement polarisés doivent être sondés lors de la conception de dispositifs PVO de nouvelle génération, car ces champs étendent la gamme des énergies d’électrons entrantes appropriées et augmentent le rendement quantique. Globalement, la mise en œuvre numérique facile de l’I-DMFT et son application dans le traitement de la dynamique des porteurs de charge dans des systèmes donneur-accepteur tridimensionnels permettent d’étudier avec précision des problèmes auparavant difficiles d’accèsIn this thesis, we have developed an inhomogeneous dynamical mean-field theory (I-DMFT) that is suitable to investigate electron-phonon interactions in non-translationally invariant and/or inhomogeneous systems. The presented approach, whose only assumption is that of a local, site-dependent self-energy, recovers both the exact solution of an electron for a generic random tight-binding Hamiltonian in the non-interacting limit and the DMFT solution for the small polaron problem in translationally invariant systems. To illustrate its full apabilities, we have applied I-DMFT to study polaron formation in the presence of isolated impurities, where we have found that I-DMFT is more accurate than the inhomogeneous momentum average approximation (IMA) and gives quantitative accurate results when compared to the approximation free diagrammatic Monte Carlo calculations. We then use I-DMFT to study the effect of defects embedded on a two-dimensional surface. The computed maps of the local density of states reveal Friedel oscillations, whose periodicity is determined by the polaron mass. This can be of direct relevance for the interpretation of scanning-tunneling microscopy (STM) experiments on systems with sizable electron-lattice interactions. Next, we have focused on the interfacial electron-hole separation mechanism in prototypical organic photovoltaic donor-acceptor systems. Using I-DMFT, we present a fully quantum dynamical simulation of the charge carrier dynamics that considers the interplay of electron-vibration interactions, static-disorder, and electrostatic fields. In particular, these simulations provide the possibility to compute the charge injection rate at the donor-acceptor interface, a quantity of fundamental experimental interest, presenting one first step forward to a long-standing cumbersome challenge in OPV, thereby bridging between chemistry and physics. As a first step, we have applied the I-DMFT to a generic one-dimensional model Hamiltonian, whose parameters model the charge carrier dynamics in prototypical Phenyl- C61-butyric acid methyl ester (PCBM) or C60 acceptor systems. Our results show that dynamic potentials (related to polaron formation), when compared to random static po- tentials, can present the main detrimental lose mechanism in OPVs devices. From this perspective, organic molecules with moderate reorganization energies should be used pref- erentially in next-generation materials since increasing the electron-phonon interaction hinders the range of suitable incoming electron energies due to the fragmentation of the local density of states into narrow polaronic sub-bands. As a final step, we have applied the I-DMFT to a generic three-dimensional model Hamiltonian, whose parameters model charge splitting force fields at three-dimensional PCBM and C60 acceptor systems. These fields are sharply peaked over a narrow interphase stemming from the protrusion of small fullerene molecules into the donor domain. We then show that such fields, when dimensioned sufficiently and poled correctly, can provide an uplifting influence on the charge separation efficiency across organic interfaces. From this perspective, materials that present sufficiently and correctly poled strong electrostatic, mesoscale fields should be probed in the design of next-generation OPV devices since such fields extend the range of suitable incoming electron energies and increase the quantum yield. Overall, the easy numerical implementation of I-DMFT and its successful application in treating the charge carrier dynamics at prototypical three-dimensional donor-acceptor systems allows one to study accurately and without further difficulties problems that were previously difficult to access
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Ivanov, Victor V. "Separation de phases et interaction electron-phonon dans yba#2 cu#3 o#7## : une etude par spectroscopie raman." Nantes, 1998. http://www.theses.fr/1998NANT2023.
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La these de victor ivanov s'articule autour de deux axes de recherches pour lesquels la spectroscopie raman s'est revelee etre un outil bien adapte a l'etude des supraconducteurs a haute temperature critique reba#2cu#3o#7 (re = terre rare). La premiere partie de ce travail presente une etude theorique et experimentale du processus de mise en ordre et de diffusion de l'oxygene dans ces materiaux. M. Ivanov aborde en premier lieu le calcul de la distribution de temperature a travers l'echantillon sous l'effet de l'echauffement laser qui peut influer sur la distribution en oxygene. La microcristallinite des echantillons frittes induit un effet d'echauffement, negligeable dans le cas des films minces. Le comportement de la bande raman de l'oxygene apical dont la frequence varie avec le contenu en oxygene et l'ordre du sous reseau d'oxygene a mis en evidence des microphases dans les cristallites avec des domaines de taille minimum egale a 10 cellules elementaires. Une etude du compose isotopique du materiau sous-dope en oxygene yba#2cu#3#1#8o#6#. #2 a permis de mieux etudier les sites des lacunes en oxygene et l'influence du recuit laser sur l'echange isotopique entre les differents sites de l'oxygene. Dans la deuxieme partie de la these, le profil des bandes raman a haute resolution provenant des vibrations des atomes de baryum et de cuivre a ete etudie en fonction de la temperature entre 10 et 300 k sur des films minces de supraconducteurs yba#2cu#3o#7. Ces profils presentent une asymetrie caracteristique d'une forte interaction electron-phonon. La simple superposition d'une fonction lorentzienne asymetrique (fonction de fano) pour la raie du ba a 115 cm#-#1 et de lorentziennes pour les autres bandes ne peut rendre compte de facon satisfaisante de l'intensite spectrale entre 80 et 200 cm#-#1 et un calcul plus elabore s'impose. En considerant que les vibrations du baryum et du cuivre interagissent avec un continuum commun d'excitations electroniques, le spectre raman calcule est en tres bon accord avec le spectre raman experimental. Ce calcul permet de determiner l'intensite de l'interaction electron-phonon pour les deux phonons impliques (ba et cu) a differentes temperatures. Le continuum electronique est du a des transitions de basse energie puisque des phonons de basse energie sont deformes par l'interaction. Dans le schema de bandes de yba#2cu#3o#7 calcule par la methode k. P, deux bandes sont suffisamment proches entre elles et intersectent le niveau de fermi de facon a permettre des transitions electroniques interbandes en infrarouge lointain. La premiere hypothese pour l'origine du continuum implique des transitions electroniques interbandes de l'oxygene apical o(4)p#x o(4)p#y pres de la ligne sr (k = /a, /b, ) de la zone de brillouin. Dans la seconde hypothese, on remarque que la bande des chaines cuo croise une bande dite impaire du plan conducteur cuo#2 pres du niveau de fermi au point k = (0. 5/a, 0. 2/b, 0). Les transitions entre ces deux bandes ont des energies comprises entre 0 et 200 cm#-#1, donc proches des energies des phonons du sous-reseau metallique. Cette derniere hypothese parait la plus plausible car elle implique une anisotropie du gap supraconducteur, ainsi qu'il est observe experimentalement.
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Leão, Salviano de Araújo. "Espalhamento elétron-fônon ótico em fios quânticos de GaAs/Ga1-XAlXAs." Universidade de São Paulo, 1992. http://www.teses.usp.br/teses/disponiveis/54/54131/tde-08122010-100311/.
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Investigamos os efeitos de tamanho e do potencial de confinamento finito V0 nas taxas de espalhamento de absorção e de emissão de elétrons interagindo com os fônons longitudinais ópticos (fônons LO) de um fio quântico cilíndrico de GaAs à temperatura ambiente. Calculamos as taxas de espalhamento inter e intra-sub-banda e a taxa de espalhamento total para uma temperatura de 300 K, pois nesta temperatura o mecanismo de espalhamento dominante em semicondutores do tipo III-V é aquele devido aos fônons LO. Qualitativamente a taxa de emissão intra-sub-banda neste sistema tem o mesmo comportamento da sua correspondente em estruturas 2D. Para a absorção encontramos uma mudança suave de comportamento da taxa de absorção intra-sub-banda quando o raio do fio é da ordem do diâmetro do polaron (ou seja, da ordem de 80 ANGSTROM). Para raios pequenos ela tem um comportamento similar ao do bulk, mas para raios maiores ela cresce até atingir um máximo e depois cai monotonicamente à medida que aumentamos a energia do portador. Vimos que, o tamanho do fio e o potencial de confinamento têm grande influência na taxa de espalhamento total.We investigated the size effects and the effects of the finite confining potential V0 on the absorption and emission scattering rates of electron interacting with longitudinal optical (LO) phonons for a cylindrical GaAs quantum wire. We calculated the inter and intrasubband total scattering rate for a temperature of 300K, because in this temperature the dominant mechanism of scattering in semiconductors III-V is that due LO phonons. Qualitatively the intrasubband emission scattering rate in this system has the same behavior of the correspondent in 2D structures. For absorption we found a smooth change in the intrasubband absorption scattering rate behavior when the radius the wire is near the polaron diameter (ie, about 80 ANGSTROM). For small radius the scattering rate has a similar behavior as that of the bulk, but for large radius it increases until reach a maximum and after ir drops monotonicaly with increase of carrier energy. We found that the size effect and the confining potential have a large influence in the total scattering rate
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Magyar, Peter. "Quelques aspects du transport électronique bidimensionnel : études théoriques en champ magnétique faible et fort." Université Joseph Fourier (Grenoble ; 1971-2015), 1997. http://www.theses.fr/1997GRE10025.
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Ce travail presente deux etudes de transport des systemes d'electrons bidimensionnels dans les limites du champ magnetique faible et fort. Premierement, nous avons analyse l'utilite d'une nouvelle approche theorique, destinee a calculer la mobilite limitee par (a) des impuretes ionisees ou par (b) des dislocations. Ce traitement a pour but d'ameliorer la description de l'effet d'ecran. Notre etude a revele que la methode ne represente que des avantages tres limites dans le cas (a), tandis que dans le cas (b) les courbes calculees en fonction de la temperature et de la largeur du puits sont en accord avec les mesures. Deuxiemement, l'effet hall quantique entier a ete etudie dans un potentiel de desordre modele. Le systeme a ete couple a un bain thermique. A partir de l'etat stationnaire d'une equation de boltzmann quantique, les conductivites de hall et dissipative ont ete obtenues en fonction de la temperature pour tous les facteurs de remplissage. Nos resultats sont en accord qualitatif avec les experiences. En outre, ils montrent que, du aux phonons, les plateaux quantifies de la conductivite de hall sont sensiblement plus larges a tres basses temperatures que ceux de la conductivite dissipative.
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Lehmann, Dietmar. "Phonon Spectroscopy and Low-Dimensional Electron Systems: The Effect of Acoustic Anisotropy and Carrier Confinement." Doctoral thesis, Technische Universität Dresden, 2004. https://tud.qucosa.de/id/qucosa%3A24636.
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The generation and propagation of pulses of nonequilibrium acoustic phonons and their interaction with semiconductor nanostructures are investigated. Such studies can give unique information about the properties of low-dimensional electron systems, but in order to interpret the experiments and to understand the underlying physics, a comparison with theoretical models is absolutely necessary. A central point of this work is therefore a universal theoretical approach allowing the simulation and the analysis of phonon spectroscopy measurements on low-dimensional semiconductor structures. The model takes into account the characteristic properties of the considered systems. These properties are the elastic anisotropy of the substrate material leading to focusing effects and highly anisotropic phonon propagation, the anisotropic nature of the different electron-phonon coupling mechanisms, which depend manifestly on phonon wavevector direction and polarization vector, and the sensitivity to the confinement parameters of the low-dimensional electron systems. We show that screening of the electron-phonon interaction can have a much stronger influence on the results of angle-resolved phonon spectroscopy than expected from transport measurements. Since we compare theoretical simulations with real experiments, the geometrical arrangement and the spatial extension of phonon source and detector are also included in the approach enabling a quantitative analysis of the data this way. To illustrate the influence of acoustic anisotropy and carrier confinement on the results of phonon spectroscopy in detail we analyse two different applications. In the first case the low-dimensional electron system acts as the phonon detector and the phonon induced drag current is measured. Our theoretical model enables us to calculate the electric current induced in low-dimensional electron systems by pulses of (ballistic) nonequilibrium phonons. The theoretical drag patterns reproduce the main features of the experimental images very well. The sensitivity of the results to variations of the confining potential of quasi-2D and quasi-1D electrons is demonstrated. This provides the opportunity to use phonon-drag imaging as unique experimental tool for determining the confinement lengths of low-dimensional electron systems. By comparing the experimental and theoretical images it is also possible to estimate the relative strength of the different electron-phonon coupling mechanisms.In the second application the low-dimensional electron system acts as the phonon pulse source and the angle and mode dependence of the acoustic phonon emission by hot 2D electrons is investigated. The results exhibit strong variations in the phonon signal as a function of the detector position and depend markedly on the coupling mechanism, the phonon polarization and the electron confinement width. We demonstrate that the ratio of the strengths of the emitted longitudinal (LA) and transverse (TA) acoustic phonon modes is predicted correctly only by a theoretical model that properly includes the effects of acoustic anisotropy on the electron-phonon matrix elements, the screening, and the form of the confining potential. A simple adoption of widely used theoretical assumptions, like the isotropic approximation for the phonons in the electron-phonon matrix elements or the use of simple variational envelope wavefunctions for the carrier confinement, can corrupt or even falsify theoretical predictions.We explain the `mystery of the missing longitudinal mode' in heat-pulse experiments with hot 2D electrons in GaAs/AlGaAs heterojunctions. We demonstrate that screening prevents a strong peak in the phonon emission of deformation potential coupled LA phonons in a direction nearly normal to the 2D electron system and that deformation potential coupled TA phonons give a significant contribution to the phonon signal in certain emission directions.Die vorliegende Arbeit beschäftigt sich mit der Ausbreitung von akustischen Nichtgleichgewichtsphononen und deren Wechselwirkung mit Halbleiter-Nanostrukturen. Güte und Effizienz moderner Halbleiter-Bauelemente hängen wesentlich vom Verständnis der Wechselwirkung akustischer Phononen mit niederdimensionalen Elektronensystemen ab. Traditionelle Untersuchungsmethoden, wie die Messung der elektrischen Leitfähigkeit oder der Thermospannung, erlauben nur eingeschränkte Aussagen. Sie mitteln über die beteiligten Phononenmoden und eine Trennung der einzelnen Wechselwirkungsmechanismen ist nur näherungsweise möglich ist. Demgegenüber erlaubt die in der Arbeit diskutierte Methode der winkel- und zeitaufgelösten Phononen-Spektroskopie ein direktes Studium des Beitrags einzelner Phononenmoden, d.h. in Abhängigkeit von Wellenzahlvektor und Polarisation der Phononen. Im Mittelpunkt der Arbeit steht die Fragestellung, wie akustische Anisotropie und Ladungsträger-Confinement die Ergebnisse der winkel- und zeitaufgelösten Phononen-Spektroskopie beeinflussen und prägen. Dazu wird ein umfassendes theoretisches Modell zur Simulation von Phononen-Spektroskopie-Experimenten an niederdimensionalen Halbleitersystemen vorgestellt. Dieses erlaubt sowohl ein qualitatives Verständnis der ablaufenden physikalischen Prozesse als auch eine quantitative Analyse der Messergebnisse. Die Vorteile gegenüber anderen Modellen und Rechnungen liegen dabei in dem konsequenten Einbeziehen der akustischen Anisotropie, nicht nur für die Ausbreitung der Phononen, sondern auch für die Matrixelemente der Wechselwirkung, sowie eine saubere Behandlung des Confinements der Elektronen in den niederdimensionalen Systemen. Dabei werden die Grenzen weit verbreiteter Näherungsansätze für die Elektron-Phonon-Matrixelemente und das Elektronen-Confinement deutlich aufgezeigt. Für den quantitativen Vergleich mit realen Experimenten werden aber auch solche Größen, wie die endliche räumliche Ausdehnung von Phononenquelle und Detektor, die Streuung der Phononen an Verunreinigungen oder die Abschirmung der Elektron-Phonon-Kopplung durch die Elektron-Elektron-Wechselwirkung berücksichtigt.Im zweiten Teil der Arbeit wird der theoretische Apparat auf typische experimentelle Fragestellungen angewandt. Im Falle der Phonon-Drag-Experimente an GaAs/AlGaAs Heterostrukturen wird der durch akustische Nichtgleichgewichtsphononen in zwei- und eindimensionalen Elektronensystemen induzierte elektrische Strom (Phonon-Drag-Strom) als Funktion des Ortes der Phononenquelle bestimmt. Das in der Arbeit hergeleitete theoretische Modell kann die experimentellen Resultate für die Winkelabhängigkeit des Drag-Stromes sowohl für Messungen mit und ohne Magnetfeld qualitativ gut beschreiben. Außerdem wird der Einfluss unterschiedlicher Confinementmodelle und unterschiedlicher Wechselwirkungsmechanismen studiert. Dadurch ist es möglich, aus Phonon-Drag-Messungen Rückschlüsse auf die elektronischen und strukturellen Eigenschaften der niederdimensionalen Elektronensysteme zu ziehen (Fermivektor, effektive Masse, Elektron-Phonon-Kopplungskonstanten, Form des Confinementpotentials). Als weiteres Anwendungsbeispiel wird das Problem der Energierelaxation (aufgeheizter)zweidimensionaler Elektronensysteme in GaAs Heterostrukturen und Quantentrögen untersucht. Für Elektronentemperaturen unterhalb 50 K werden die Gesamtemissionsrate als Funktion der Temperatur und die winkelaufgelöste Emissionsrate (als Funktion der Detektorposition) berechnet. Für beide Größen wird erstmals eine gute Übereinstimmung zwischen Theorie und Experiment gefunden. Es zeigt sich, dass akustische Anisotropie und Abschirmungseffekte zu überraschenden neuen Ergebnissen führen können. Ein Beispiel dafür ist der unerwartet große Beitrag der mittels Deformationspotential-Wechselwirkung emittierten transversalen akustischen Phononen, der bei einer Emission der Phononen näherungsweise senkrecht zum zweidimensionalen System beobachtet werden kann.
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Dacosta, Fernandes Benoit. "Etude des propriétés électroniques et vibrationnelles de nano-objets métalliques et hybrides par spectroscopie femtoseconde." Thesis, Bordeaux, 2015. http://www.theses.fr/2015BORD0047/document.
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Ce travail a porté sur l’étude expérimentale de la dynamique électronique et vibrationnelle de nano-objets métalliques et hybrides par spectroscopie pompe-sonde femtoseconde. L’étude de la dynamique des échanges d’énergie électrons-réseau dans des systèmes métalliques bidimensionnels nous a permis de mettre en évidence une accélération du transfert d’énergie entre électrons et phonons due au confinement. Cette accélération est gouvernée par la plus petite dimension des nano-objets étudiés (nano-triangles d’argent 2D) lorsque celle-ci devient inférieure à environ 10 nm. Nous avons aussi étudié la dynamique vibrationnelle de nanoparticules métalliques, bimétalliques et métal diélectrique. Nous nous sommes intéressés aux modes de vibration longitudinaux et radiaux de bipyramides d’or en fonction de leurs dimensions, et à leurs évolutions consécutives à un dépôt d’argent. Une forte sensibilité des périodes et des amplitudes de vibration à de faibles dépôts a été mise en évidence. L’étude de l’évolution des modes de vibration de nano-objets de type coeur-coquille (argent-silice et or-silice) nous a permis d’obtenir des informations sur la qualité du contact mécanique à l’interface métal-diélectrique. Enfin, nous avons étudié les interactions électroniques dans des nano-systèmes hybrides métal/semi-conducteur (ZnO-Ag), et plus particulièrement les transferts de charge et les échanges d’énergie entre les deux composants.Nous avons montré la forte influence de l’environnement sur la dynamique électronique du ZnO et mis en évidence un transfert d’électron, photo-induit par une impulsion infrarouge, de la particule métallique vers la bande de conduction du semi-conducteurElectronic and vibrational dynamics have been studied in metallic and hybrid nano-object using femtosecond timeresolved spectroscopy. The study of electron-lattice energy exchanges in two-dimensional metallic systems showed an acceleration of the energy transfer between electrons and phonons due to confinement. This acceleration is governed by the smallest dimension of the nano-objects (2D-silver nano triangles ) when it becomes smaller than 10 nm. We also studied the vibrational dynamics of metallic nanoparticles, bimetallic and metal-dielectric. We investigated the longitudinal and radial modes of vibration of gold bipyramids which depend on their size, and studied their evolution under silver deposition. A high sensitivity of periods and amplitudes for small deposition were demonstrated. Our work on the evolution of acoustic vibrations of core-shell nano-objects (silver-silica and goldsilica) allowed us to obtain information on the quality of mechanical contact at the metal-dielectric interface. Finally, we studied the electronic interactions in hybrid metal / semiconductor (ZnO-Ag) nano-systems, and especially the charge transfer and energy exchanges between the two components. We showed a strong influence of the environment on the electron dynamics of ZnO and proved the existence of an electron transfer, photoinduced by an infrared pulse, from the metal particle to the semiconductor conduction band
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Lakehal, Massil. "Out of Equilibrium Lattice Dynamics in Pump Probe Setups." Thesis, Université de Paris (2019-....), 2020. http://www.theses.fr/2020UNIP7039.
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L'étude de la dynamique hors équilibre des systèmes fortement corrélés, à l'aide de laser femtoseconde, a révélé une variété de phénomènes sans analogue en physique d'équilibre. Dans cette thèse, nous étudions théoriquement la dynamique hors équilibre des degrés de liberté du réseau et leur signature en spectroscopie pompe-sonde. Nous développons une description microscopique des phonons cohérents displacive excité par le laser. La théorie capture la rétroaction de l'excitation des phonons sur le fluide électronique, qui manque dans la formulation phénoménologique actuelle. Nous montrons que cette rétroaction conduit à une oscillation avec une fréquence qui dépend du temps aux temps courts, même si le mouvement des phonons est harmonique. Pour les temps longs, cette rétroaction apparaît comme une phase résiduelle dans le signal oscillatoire. Nous appliquons la théorie au BaFe2As2, nous expliquons l'origine de la phase du signal oscillatoire rapporté dans des expériences récentes, et nous prédisons que le système oscille avec une fréquence décalé vers le rouge pour les grandes fluences. Notre théorie ouvre également la possibilité d'extraire des informations d'équilibre à partir la dynamique des phonons cohérents. Un autre phénomène intéressant qui a été observé en spectroscopie pompe-sonde est l'oscillation des fluctuations du réseau au double de la fréquence d’un phonon du système étudié. Ces oscillations sont interprétées comme une signature d'états de phonons squeezé macroscopique. Dans ce travail, nous identifions d'autres mécanismes d'oscillations à une fréquence double autre que le squeezing. Nous montrons qu'un quench de la température du bain thermique induite par la pompe, à laquelle le phonon est couplé, ou l'excitation d'un phonon cohérent pour lequel l'anharmonicité cubique est permise par symétrie peut également produire de telles oscillations en spectroscopie sans que le phonon soit dans un état squeezé. Nous concluons que, contrairement à ce qui est communément admis, les oscillations à double fréquence phononique en spectroscopie de bruit ne sont pas nécessairement une signature des phonons squeezés. Nous soulignons ce qui peut être un critère fiable pour identifier un phonon squeezé en utilisant la spectroscopie pompe-sondeThe study of the out of equilibrium dynamics of strongly correlated systems, using ultrafast pulses, uncovered a plethora of phenomena with no analog in equilibrium physics. In this thesis, we theoretically investigate the out of equilibrium dynamics of the lattice degrees of freedom and their signature in pump-probe spectroscopy. We develop a Hamiltonian-based microscopic description of laser pump induced displacive coherent phonons. The theory captures the feedback of the phonon excitation upon the electronic fluid, which is missing in the state-of-the-art phenomenological formulation. We show that this feedback leads to chirping at short timescales, even if the phonon motion is harmonic. At long times, this feedback appears as a finite phase in the oscillatory signal. We apply the theory to BaFe2As2, explain the origin of the phase in the oscillatory signal reported in recent experiments, and we predict that the system will exhibit redshifted chirping at larger fluence. Our theory also opens the possibility to extract equilibrium information from coherent phonon dynamics. Another interesting phenomenon that have been reported in pump-probe spectroscopy is the oscillation of the lattice fluctuations at double phonon frequency. These oscillations are invariably interpreted as a signature of macroscopic squeezed phonon states. In this work, we identify other mechanisms of double phonon frequency oscillations that do not involve squeezing. We show that a pump induced temperature quench of the bath, to which the phonon is coupled to, or exciting a coherent phonon for which cubic anharmonicity is allowed by symmetry can also produce such oscillations in noise spectroscopy without squeezing the phonon state. We conclude that, in contrast with what is commonly believed, double phonon frequency oscillations in noise spectroscopy are not necessarily a signature of macroscopic phonon squeezing. We point out what can be a reliable criterion to identify a squeezed phonon using pump-probe spectroscopy
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Dulong, Jean-Luc. "Etude de la fonction diélectrique infrarouge de métaux nobles à haute température, par réflectométrie différentielle." Paris 6, 1986. http://www.theses.fr/1986PA066397.
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Conception et mise au point d'un appareil permettant de mesurer la différence de réflectivité entre un échantillon chauffé jusqu'à 600°C et un échantillon à température ambiante dans un domaine spectral allant de 1,5 à 10 microns. Etude des interactions électron-phonon et électron-électron dans l'argent et le cuivre à partir de leurs propriétés optiques dans l'infrarouge. Influence des défauts de volume et de surface. Détermination de l'indice complexe de couches minces peu absorbantes, dans l'infrarouge.
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Langot, Pierre. "Etude femtoseconde de la thermalisation des porteurs libres dans l'arseniure de gallium." Palaiseau, Ecole polytechnique, 1996. http://www.theses.fr/1996EPXX0021.
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Ce travail porte sur l'etude des interactions porteur-porteur et porteur-phonon dans l'arseniure de gallium par des techniques pompe-sonde de saturation d'absorption femtoseconde a haute sensibilite. Pour cela, une source femtoseconde a deux couleurs a ete developpee par conversion de frequence non-lineaire des impulsions fournies par un oscillateur femtoseconde a saphir dope titane. Nous avons tout d'abord etudie les mecanismes d'interaction des trous libres dans l'asga. Les mesures realisees sur l'echauffement de trous froids en fonction de la densite de porteurs photo-generes, de leur exces d'energie initial et de la temperature du reseau nous ont permis de mesurer directement le temps d'interaction trou-phonon et d'en deduire le potentiel de deformation optique. Une etude sur le ralentissement de la thermalisation electron-reseau du a la creation d'une population de phonons hors equilibre en centre de zone de brillouin est ensuite presentee. Les differentes mesures que nous avons realisees nous ont permis de demontrer que la thermalisation electrons-reseau aux temps longs est gouvernee par la duree de vie des phonons optiques, en accord avec un modele de simulations numeriques
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Yudiarsah, Efta. "Charge Transport through Molecules: Structural and Dynamical Effects." Ohio University / OhioLINK, 2008. http://rave.ohiolink.edu/etdc/view?acc_num=ohiou1219343872.
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Huang, Wenyu. "Fundamental studies of the interaction between femtosecond laser and patterned monolayer plasmonic nanostructures." Diss., Atlanta, Ga. : Georgia Institute of Technology, 2007. http://hdl.handle.net/1853/24786.
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Thesis (Ph. D.)--Chemistry and Biochemistry, Georgia Institute of Technology, 2008.Committee Chair: El-Sayed, Mostafa A.; Committee Member: Perry, Joseph W.; Committee Member: Srinivasarao, Mohan; Committee Member: Whetten, Robert L.; Committee Member: Zhang, Z. John.
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Dib, Elias. "Interaction électron-phonon dans le cadre du formalisme des fonctions de Green hors-équilibre : application à la modélisation de transistors MOS de type p." Thesis, Aix-Marseille, 2013. http://www.theses.fr/2013AIXM4357.
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Depuis que les dimensions des nano-dispositifs ont atteint l’échelle nanométrique, la simulation quantique est devenue incontournable dans le domaine de la nanoélectronique. Parmi les différents phénomènes physiques, l’interaction électron-phonon représente un processus majeur limitant la mobilité des porteurs de charge à température ambiante. En combinant la théorie multibandes k.p avec le formalisme quantique des fonctions de Green hors-équilibre, nous avons étudié et comparé deux types de dispositifs double-grille dopés p: le transistor MOS «conventionnel» et celui dit «sans jonction». L’influence de l’orientation cristalline, du matériau semi-conducteur, de la longueur de grille et de l’épaisseur du substrat a été étudiée afin d’optimiser les performances de ces dispositifs aux dimensions ultimes. D’un point de vue plus fondamental, l’interaction avec les phonons est habituellement implémentée à partir de l’approche auto-cohérente de Born (SCBA). Nous avons exploré la validité des approches non auto-cohérentes numériquement moins coûteuse qui conservent le courant : Lowest Order Approximation (LOA). Une comparaison entre SCBA, LOA et son prolongement analytique (LOA+AC) en modèle multi-bande a été menéeDevice simulation has attracted large interest since the dimensions of electronic devices reached the nanoscale. Among the new physical phenomena observed we focus on interaction-induced effects. Particular emphasis is placed on electron-phonon interactions as it is one of the most important carrier mobility-limiting mechanisms in nanodevices. Using the k.p multiband theory combined with the Non-Equilibrium Green's Function formalism, we model 2 types of double-gate devices: p-type MOSFETs and junctionless p-type MOSFETs. The 2D architecture of the double-gate device enables us to investigate the influence of confinement in one direction, infinite propagation in the other direction and connection to semi-infinite reservoirs in the last one. Different crystallographic orientation, channel materials, gate lengths and channel widths are investigated. From a fundamental point of view, phonon scattering is usually implement via the so-called Self-Consistent Born Approximation (SCBA°. We explore the validity of a one shot current conserving method based on the Lowest Order Approximation (LOA). A comparison between SCBA, LOA and its analytic continuation (LOA+AC) in multiband models is discussed
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Kokott, Sebastian. "First-principles Investigation of Small Polarons in Metal Oxides." Doctoral thesis, Humboldt-Universität zu Berlin, 2018. http://dx.doi.org/10.18452/19535.
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Ein limitierender Faktor der Leitfähigkeit ist die Wechselwirkung der Ladungsträger mit polaren Phononenmoden; das resultierende Quasiteilchen wird als Polaron bezeichnet. Die Stärke der Elektron-Phonon (el-ph)-Wechselwirkung bestimmt die Stärke der Lokalisierung des Polarons, die z.B. die Charakteristik der Temperaturabhängigkeit der Mobliltät definiert. Wir fokussieren uns auf Metalloxide mit starker (el-ph)-Kopplung, bei der sich kleine Polaronen bilden.
Die Dichtefunktionaltheorie wird häufig für zur Simulation von Polaronen verwendet. Jedoch treten hierbei zwei Schwierigkeiten auf: Die Sensitivität der berechneten Eigenschaften in Abhängigkeit der Fehler im Austausch-Korrelations (XC)-Funktional und der Effekt der endlichen Superzellgröße. Beide Probleme werden in dieser Arbeit untersucht. Die Polaroneneigenschaften werden auf einer modifizierten Potentialoberfläche (PES) berechnet. Durch Variierung des Anteils der exakten Austauschenergie im hybriden HSE-Funktional zeigen wir, dass das modifizierte PES-Modell deutlich die Abhängigkeit der Polaroneneigenschaften vom XC-Funktional reduziert. Basierend auf dem Potential der el-ph-Kopplung von Pekar leiten wir das korrekte elastische langreichweitige Verhalten des Polarons und darauf aufbauend eine Korrektur für den Fehler durch die endliche Superzellgröße her. Diese Erkenntnisse werden durch ausgiebige Tests an MgO und Rutil TiO2 überprüft.
Die oben beschriebene Methode wird zur Untersuchung des Einflusses der Kristallstruktur auf die Bildung von Polaronen in Rutil und Anatas TiO2 und in der β- und κ-Phase von Ga2O3 angewendet. Während in Rutil nur kleine Elektronpolaronen stabil sind, finden wir in Anatas nur stabile Lochpolaronen. Hingegen existieren in beiden Phasen von Ga2O3 nur stabile Lochpolaronen, jedoch mit deutlich unterschiedlichen Bindungsenergien. Dadurch kann durch Verwendung unterschiedlicher Kristallstrukturen Eigenschaften wie Leitfähigkeit und Mobilität der Ladungsträger beeinflusst werden.An important factor limiting the conductivity is the interaction of the charge carrier with polar phonon modes. Such a phonon-dressed charge carrier is called polaron. The strength of the electron-phonon (el-ph) interaction determines the localization of the polaron, which in turn e.g. defines its characteristic temperature dependence for the charge-carrier mobility. We focus on metal oxides with strong el-ph coupling, where small polarons are formed. Density-functional theory is often used for calculating properties of polarons. However, there are two challenges: sensitivity of the calculated properties to the errors in exchange-correlation (XC) treatment and finite-size effects in supercell calculations. In this work, we develop an approach that addresses both challenges. The polaron properties are obtained using a modified neutral potential-energy surface (PES). By changing the fraction of exact exchange in the hybrid HSE functional we show that the modified PES model significantly reduces the dependence of the polaron properties on the XC functional. Based on Pekar's potential for the long-range el-ph coupling, we derive the proper elastic long-range behavior of the polaron and a finite-size correction for the polaron properties. These findings are proofed by an extensively test for rock salt MgO and rutile TiO2. Finally, the approach is used to investigate the influence of the crystal structure on the polaron properties for rutile and anatase TiO2, as well as for the monoclinic β- and orthorhombic κ-phase of Ga2O3. While in rutile TiO2 only small electron polarons are stable, only small hole polarons are found in anatase. Further, small hole polarons exist in both Ga2O3 polymorphs but have significantly different binding energies. Thus, we conclude that growing crystals of the same material but with different structure can be used to manipulate conductivity and charge-carrier mobility.
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Fratini, Simone. "Cristallisation des polarons à basse densité et transition isolant-métal : effets des interactions coulombiennes à longue portée." Université Joseph Fourier (Grenoble), 1999. http://www.theses.fr/1999GRE10057.
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Cette these est consacree a l'etude de la transition isolant-metal dans les systemes polaires dopes. Les premiers chapitres constituent une breve introduction a la physique des polarons. Apres avoir passe en revue les differents modeles et approximations disponibles pour decrire la formation de polarons, nous nous concentrons sur le modele de frohlich, qui permet de traiter sur le meme pied la formation des polarons et les interactions coulombiennes a longue portee. Nous observons que la repulsion entre les particules est dominante dans un systeme de n polarons larges a basse densite, ce qui provoque leur cristallisation. Le troisieme chapitre est consacre a l'etude du cristal de polarons dans l'approximation de champ moyen de wigner. Les proprietes de l'etat fondamental sont analysees en fonction du couplage electron-phonon, de la densite et de la temperature, a travers la methode des integrales de chemin de feynman. En nous basant sur le critere de lindemann, nous montrons qu'il existe deux mecanismes en competition pour la transition isolant-metal : la fusion du cristal conduite par les fluctuations des particules localisees, relevante en couplage faible et intermediaire, et la dissociation des polarons, valable en couplage fort lorsque la polarisation est trop lente pour suivre le mouvement rapide des electrons. Le dernier chapitre concerne l'inclusion des interactions dipolaires entre les particules localisees. Nous developpons un modele quadratique, base sur l'approche de feynman, qui permet de decrire en premiere approximation le spectre d'excitations de basse energie et les proprietes dielectriques du cristal. La stabilite de la phase cristallisee par rapport aux excitations collectives est egalement analysee et nous proposons une signature experimentale pour la transition isolant-metal.
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Mizrahi, Simon. "Dynamique ultrarapide dans les boites quantiques." Electronic Thesis or Diss., Institut polytechnique de Paris, 2024. http://www.theses.fr/2024IPPAE006.
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Cette thèse porte sur l’étude fondamentale et les applications optoélectroniques des boîtes quantiques colloïdales de calcogénide de mercure, avec une prépondérance pour le tellurure de mercure (HgTe). Celui-ci possède grâce au confinement quantique des porteurs, d’incroyables propriétés optiques lui permettant de modifier sa bande interdite et de couvrir l’intégralité du spectre infrarouge. Dans le cadre de cette thèse, nous nous sommes focalisés sur une gamme comprise entre 2 et 4 microns, qui présente un grand intérêt pour les instruments de guidage mais aussi la détection de certains polluants. Cette étude a principalement été réalisée par l’utilisation de lasers femtosecondes, dans le cadre de la réalisation d’expériences de spectroscopie pompe-sonde. Cette technique a permit la caractérisation des propriétés électroniques des porteurs qui sont indispensable au développement et à la réalisation de sources lumineuses ainsi que de photodétecteurs. En effet, les données expérimentales obtenues dans le cadre de cette thèse ont abouti à la réalisation de la première LED électroluminescente centrée à 2 microns ainsi que d’un photodétecteur centré à 4 microns et dont la température de fonctionnement a pu atteindre 200 K, à l’opposé des 80 K des photodétecteurs moyen infrarouge commerciaux. Pour finir, cette étude présente aussi le comportement du HgTe et du bismuth monocristallin en fonction de la température. Ce dernier nous ayant servi d’étalon pour cette partie de la thèseThis thesis focuses on the fundamental study and the optoelectronic applications of colloidal quantum dots of mercury calcogenide, with a preponderance for mercury telluride (HgTe). Thanks to the quantum confinement of the carriers, HgTe has incredible optical properties allowing it to modify its band gap and cover the entire infrared spectrum. In this thesis, we focused on a range between 2 and 4 microns, which is of great interest for guidance instruments but also for the detection of certain pollutants. This study was mainly carried out using femtosecond lasers, in the context of pump-probe spectroscopy experiments. This technique allowed the characterization of the electronic properties of the carriers that are essential to the development and realization of light sources and photodetectors. Indeed, the experimental data obtained in the framework of this thesis led to the realization of the first light-emitting LED centered at 2 microns as well as a photodetector centered at 4 microns and whose operating temperature could reach 200 K, unlike the 80 K of commercial mid-infrared photodetectors. Finally, this study also presents the behaviour of HgTe and monocrystalline bismuth as a function of temperature. The latter served as a standard sample for this part of the thesis
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Phinney, Isabelle Y. "Probing electron-electron and electron-phonon interactions in twisted bilayer graphene." Thesis, Massachusetts Institute of Technology, 2020. https://hdl.handle.net/1721.1/127092.
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Thesis: S.B., Massachusetts Institute of Technology, Department of Physics, 2020Cataloged from the official PDF of thesis.
Includes bibliographical references (pages 81-86).
Two-dimensional systems, and, most recently, moire systems, have risen to the forefront of condensed matter physics with the advent of experimental techniques that allow for controlled stacking of van der Waals heterostructures [17, 54]. For example, it was recently discovered that when two pieces of atomically thin carbon (graphene) are twisted at 1.1° with respect to one another, they display a variety of effects, including superconducting behavior [10]. Experimental investigation of the behavior of small-angle twisted bilayer graphene (SA-TBG) as a function of twist angle is imperative to understanding the mechanisms that play into the many interesting, strongly-interacting phenomena that the moire system displays. In this thesis, I present three experiments which explore electron-electron and electron-phonon interactions in SA-TBG. I first consider SA-TBG as a host for a viscous electron fluid and look for the onset of fluid behavior via electron transport. Then I investigate the temperature dependence of resistivity in SA-TBG devices at a number of angles. The final experiment examines magnetophonons in three devices above the magic angle and compares the findings to theoretical results.
by Isabelle Y. Phinney.
S.B.
S.B. Massachusetts Institute of Technology, Department of Physics
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Sergueev, Nikolai. "Electron-phonon interactions in molecular electronic devices." Thesis, McGill University, 2005. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=102171.
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Over the past several decades, semiconductor electronic devices have been miniaturized following the remarkable "Moores law". If this trend is to continue, devices will reach physical size limit in the not too distance future. There is therefore an urgent need to understand the physics of electronic devices at nano-meter scale, and to predict how such nanoelectronics will work. In nanoelectronics theory, one of the most important and difficult problems concerns electron-phonon interactions under nonequilibrium transport conditions. Calculating phonon spectrum, electron-phonon interaction, and their effects to charge transport for nanoelectronic devices including all atomic microscopic details, is a very difficult and unsolved problem. It is the purpose of this thesis to develop a theoretical formalism and associated numerical tools for solving this problem.In our formalism, we calculate electronic Hamiltonian via density functional theory (DFT) within the nonequilibrium Green's functions (NEGF) which takes care of nonequilibrium transport conditions and open device boundaries for the devices. From the total energy of the device scattering region, we derive the dynamic matrix in analytical form within DFT-NEGF and it gives the vibrational spectrum of the relevant atoms. The vibrational spectrum together with the vibrational eigenvector gives the electron-phonon coupling strength at nonequilibrium for various scattering states. A self-consistent Born approximation (SCBA) allows one to determine the phonon self-energy, the electron Green's function, the electronic density matrix and the electronic Hamiltonian, all self-consistently within equal footing. The main technical development of this work is the DFT-NEGF-SCBA formalism and its associated codes.
A number of important physics issues are studied in this work. We start with a detailed analysis of transport properties of C60 molecular tunnel junction. We find that charge transport is mediated by resonances due to an alignment of the Fermi level of the electrodes and the lowest unoccupied C60 molecular orbital. We then make a first step toward the problem of analyzing phonon modes of the C60 by examining the rotational and the center-of-mass motions by calculating the total energy. We obtain the characteristic frequencies of the libration and the center-of-mass modes, the latter is quantitatively consistent with recent experimental measurements. Next, we developed a DFT-NEGF theory for the general purpose of calculating any vibrational modes in molecular tunnel junctions. We derive an analytical expression for dynamic matrix within the framework of DFT-NEGF. Diagonalizing the dynamic matrix we obtain the vibrational (phonon) spectrum of the device. Using this technique we calculate the vibrational spectrum of benzenedithiolate molecule in a tunnel junction and we investigate electron-phonon coupling under an applied bias voltage during current flow. We find that the electron-phonon coupling strength for this molecular device changes drastically as the bias voltage increases, due to dominant contributions from the center-of-mass vibrational modes of the molecule. Finally, we have investigated the reverse problem, namely the effect of molecular vibrations on the tunneling current. For this purpose we developed the DFT-NEGF-SCBA formalism, and an example is given illustrating the power of this formalism.
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Leadley, David Romwald. "Electron-phonon interactions in low dimensional structures." Thesis, University of Oxford, 1989. http://ora.ox.ac.uk/objects/uuid:3e8fe3de-4c61-48ac-a475-050b76901a6f.
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Transport properties of the two-dimensional electron gas (2DEG) in high magnetic fields are used to investigate scattering processes affecting the resistivity of GaAs-GaAlAs and GaInAs-InP heterojunctions and quantum wells: especially coupling of electrons to acoustic and optic phonons; and transitions between electric subbands. The experiments fall into two groups: A systematic study of magnetophonon resonance (MPR) between 30K and 300K. Resonance positions indicate a coupling substantially below the LO phonon energy, expected from 3D measurements. GaAs-GaAlAs hetero junctions show amplitudes varying smoothly with electron density (ns) and closely related to the 4K mobility. On rotation in magnetic field they decrease rapidly as the resonance position returns to the LO value. In modulation doped structures the damping factor is determined by remote impurity scattering. As ns is increased in GaInAs-InP the coupling frequency decreases dramatically from the GaAs-like LO at 272cm-1 to the InAs-like TO at 226cm-1. At higher electric fields the 'normal' MPR maxima invert, starting at low magnetic fields, to form 'hot electron' MPR minima, with maximum amplitude at ~60K. This is the first direct observation of HEMPR in 2D and is explained in a diffusion picture. At lower electric fields, additional resonances are identified with resonant cooling by inter-subband scattering. Comparisons are made with calculations and explanations sought including consideration of interface phonons; coupled plasmon-phonon modes; and shifts of the resonance positions due to the shape of the density of states. Low temperature magnetoresistance measurements in GaAs-GaAlAs heterojunctions with more than one occupied electric subband. Shubnikov-de Haas oscillations in perpendicular magnetic fields contain non-additive terms at electron temperatures > 2K where acoustic phonon mediated inter-subband scattering is comparable to intra-subband scattering. Subband separations and greatly enhanced g-factors [largest for electrons in the upper subband ] are deduced from the oscillations. Damping of the oscillations in field, gives values for quantum lifetimes (τs), much smaller than τtʼ, deduced from mobility. With two subbands occupied τs is always largest for the upper subband, while relative sizes of τt depend on sample quality. Study of electron energy loss rates, from thermal damping of the oscillations, shows enhancement in the region kTe ~ ħωcʼ, which is evidence for cyclotron phonon emission. Depopulating subbands in parallel fields causes the resistance to drop, by up to 60%, due to suppression of inter-subband scattering. Systematic studies show this scattering rate is independent of ns.
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Montgomery, M. J. "Ineleastic electron-phonon interactions in atomic wires." Thesis, Queen's University Belfast, 2004. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.411758.
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