Dissertations / Theses: 'Dephasing'

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Bonifacio, Paolo. "Spacetime conformal fluctuations and quantum dephasing." Thesis, Available from the University of Aberdeen Library and Historic Collections Digital Resources, 2009. http://digitool.abdn.ac.uk:80/webclient/DeliveryManager?application=DIGITOOL-3&owner=resourcediscovery&custom_att_2=simple_viewer&pid=33587.

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Treiber, Maximilian. "Dephasing in disordered systems at low temperatures." Diss., Ludwig-Maximilians-Universität München, 2013. http://nbn-resolving.de/urn:nbn:de:bvb:19-157822.

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The transition from quantum to classical behavior of complex systems, known as dephasing, has fascinated physicists during the last decades. Disordered systems provide an insightful environment to study the dephasing time \tau_\varphi, since electron interference leads to quantum corrections to classical quantities, such as the weak- localization correction \Delta g to the conductance, whose magnitude is governed by \tau_\varphi. In this thesis, we study one of the fundamental questions in this field: How does Pauli blocking influence the interaction-induced dephasing time at low temperatures? In general, Pauli blocking limits the energy transfer \omega of electron interactions to \omega \ll T, which leads to an increase of \tau_\varphi. However, the so-called 0D regime of dephasing, reached at T \ll E_{Th}, is practically the only relevant regime, in which Pauli blocking significantly influences the temperature dependence of \tau_\varphi. Despite of its fundamental physical importance, 0D dephasing has not been observed experimentally in the past. We investigate several possible scenarios for verifying its existence: (1) We analyze the temperature dependence of \Delta g in open and confined systems and give detailed instructions on how the crossover to 0D dephasing can be reliably detected. Two concrete examples are studied: an almost isolated ring and a new quantum dot model. However, we conclude that in transport experiments, 0D dephasing unavoidably occurs in the universal regime, in which all quantum corrections to the conductance depend only weakly on \tau_\varphi, and hence carry only weak signatures of 0D dephasing. (2) We study the quantum corrections to the polarizability \Delta \alpha of isolated systems, and derive their dependence on \tau_\varphi and temperature. We show that \tZeroD dephasing occurs in a temperature range, in which \Delta \alpha depends strongly (as a power-law) on \tau_\varphi, making the quantum corrections to the polarizability an ideal candidate to study dephasing at low temperatures and the influence of Pauli blocking.

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Völker, Axel. "Dephasing and phase coherence in disordered mesoscopic conductors." [S.l. : s.n.], 2000. http://deposit.ddb.de/cgi-bin/dokserv?idn=963574353.

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Marquardt, Florian [Verfasser]. "Models of dephasing at low temperatures / Florian Marquardt." Aachen : Shaker, 2003. http://d-nb.info/1172614032/34.

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Grüner, Barbara, Martin Schlesinger, Philipp Heister, Walter T. Strunz, Frank Stienkemeier, and Marcel Mudrich. "Vibrational relaxation and dephasing of Rb2 attached to helium nanodroplets." Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2014. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-138750.

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The vibrational wave-packet dynamics of diatomic rubidium molecules (Rb2) in triplet states formed on the surface of superfluid helium nanodroplets is investigated both experimentally and theoretically. Detailed comparison of experimental femtosecond pump–probe spectra with dissipative quantum dynamics simulations reveals that vibrational relaxation is the main source of dephasing. The rate constant for vibrational relaxation in the first excited triplet state 13Σ+g is found to be constant γ ≈ 0.5 ns−1 for the lowest vibrational levels v [less, similar] 15 and to increase sharply when exciting to higher energies
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Grüner, Barbara, Martin Schlesinger, Philipp Heister, Walter T. Strunz, Frank Stienkemeier, and Marcel Mudrich. "Vibrational relaxation and dephasing of Rb2 attached to helium nanodroplets." Royal Society of Chemistry, 2011. https://tud.qucosa.de/id/qucosa%3A27778.

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The vibrational wave-packet dynamics of diatomic rubidium molecules (Rb2) in triplet states formed on the surface of superfluid helium nanodroplets is investigated both experimentally and theoretically. Detailed comparison of experimental femtosecond pump–probe spectra with dissipative quantum dynamics simulations reveals that vibrational relaxation is the main source of dephasing. The rate constant for vibrational relaxation in the first excited triplet state 13Σ+g is found to be constant γ ≈ 0.5 ns−1 for the lowest vibrational levels v [less, similar] 15 and to increase sharply when exciting to higher energies.
Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG-geförderten) Allianz- bzw. Nationallizenz frei zugänglich.

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Ben, Taher Azza. "Strong Optical Field Ionization of Solids." Thesis, Université d'Ottawa / University of Ottawa, 2018. http://hdl.handle.net/10393/37151.

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Population transfer from the valence to conduction band in the presence of an intense laser field is explored theoretically in semiconductors and dielectrics. Experiments on intense laser driven dielectrics have revealed population transfer to the conduction band that differs from that seen in semiconductors. Our research explores two aspects of ionization in solids. (i) Current ionization theories neglect coupling between valence and conduction band and therewith the dynamic Stark shift. Our single-particle analysis identifies this as a potential reason for the different ionization behaviour. The dynamic Stark shift increases the bandgap with increasing laser intensities thus suppressing ionization to an extent where virtual population oscillation become dominant. The dynamic Stark shift plays a role dominantly in dielectrics which due to the large bandgap can be exposed to significantly higher laser intensities. (ii) In the presence of laser dressed virtual population of the conduction band, elastic collisions potentially transmute virtual into real population resulting in ionization. This process is explored in context of relaxation time approximation.

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Cardamone, David Michael. "Dephasing and Decoherence in Open Quantum Systems: A Dyson's Equation Approach." Diss., Tucson, Arizona : University of Arizona, 2005. http://etd.library.arizona.edu/etd/GetFileServlet?file=file:///data1/pdf/etd/azu%5Fetd%5F1271%5F1%5Fm.pdf&type=application/pdf.

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Anderson, Richard Lloyd. "Decoherence, dephasing and quantum tunnelling in molecules with large amplitude vibrations." Thesis, Bangor University, 2006. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.440960.

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Schneck, Jude Robert. "Femtosecond electronic dephasing and population relaxation of some novel semiconducting materials." Thesis, Boston University, 2012. https://hdl.handle.net/2144/34692.

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Thesis (Ph.D.)--Boston University
PLEASE NOTE: Boston University Libraries did not receive an Authorization To Manage form for this thesis or dissertation. It is therefore not openly accessible, though it may be available by request. If you are the author or principal advisor of this work and would like to request open access for it, please contact us at open-help@bu.edu. Thank you.
The dissipation of energy by excited carriers in semiconductors is crucial to device development. In particular, the carrier relaxation mechanisms are strongly modified by the degree of disorder introduced into the lattice via the growth process. The pump probe spectroscopic technique is ideally suited to monitor the energy dissipation process and elucidate the relaxation mechanisms contributing to the carrier decay. Additionally, phase breaking interactions of optical transitions, as measured via the photon echo spectroscopic technique, provides insight into the different homogeneous relaxation mechanisms contributing to the optical resonance. When compared to high quality semiconducting materials, the fundamental homogeneous relaxation mechanisms depend strongly on the disorder inherent in the material. The photon echo technique is ideal for quantifying the strength of these interactions. Femtosecond pump-probe responses of a GaN thin film excited above and below the UV band gap were measured to determine the kinetic relaxation pathways of carriers. A number of fluence dependent decay processes were identified, including carrier-carrier scattering, exciton decay, trapping to defect states, and hole state recovery. The characteristic timescales of these mechanisms ranged from <50 fs to >600 ps. In other measurements on GaN, two-pulse photon echoes due to the strongly dipole coupled excitons were observed as a function of temperature (1 0 - 295K). A biexponential decay of the dephasing rate was found from these measurements and attributed to free and bound excitons. The dynamics of the E22 transition of (6,5) single walled carbon nanotubes was studied over a range of fluences via pump-probe spectroscopy. A fluence dependent dephasing rate was deduced from an analysis of the pump-probe signal intensity at a fixed short delay time allowing an effective cross section for exciton-exciton interactions to be determined. The relaxation kinetics of optically excited E22 excitons was revealed by pump fluence dependent fits to the observed pump-probe responses. The model includes both Auger recombination from the E11 and E22 states due to exciton-exciton annihilation and a stretched exponential decay from E11 to the valence band. E11 and E22 diffusion coefficients and the defect density were determined from this analysis.
2031-01-01

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TRIPATHY, SUVRANTA K. "DEPHASING OF EXCITONS AND PHASE COHERENT PHOTOREFRACTIVITY IN ZnSe QUANTUM WELLS." University of Cincinnati / OhioLINK, 2006. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1163704159.

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Domínguez, Javier. "A theoretical study of coupling and dephasing in semiconductor quantum dots." Thesis, Bangor University, 2004. https://research.bangor.ac.uk/portal/en/theses/a-theoretical-study-of-coupling-and-dephasing-in-semiconductor-quantum-dots(5cd7f798-c000-4ec8-8e94-7fd1b910cb83).html.

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In recent years, advances in crystal growth techniques have made it possible to fabricate semiconductor microstructures, which are so small that their electronic and optical properties deviate substantially from those of bulk materials. In these microstructures the electrons and holes energies are confined in one or more directions to a region, which is still considerably larger than the lattice constant but so small that the electron envelope wavefunctions become quantised. Quantum dots (QD's) are nanometer-scale structures that provide carrier confinement in all three spatial dimensions. In these structures the carrier momentum is also quantised in all three directions and hence a carrier confined within a QD can only occupy certain discrete energy levels, which depend on various factors like the potential barrier depth or the shape of the structure. It is the discrete nature of these energy states that make quantum dots highly attractive for quantum computation and optoelectronic devices. In this thesis the quantum energy levels of single and coupled quantum dots of different shapes are investigated. Coupled QD systems, which may be considered as 'artificial molecules' have been investigated for their possible application in quantum computing. An understanding of how dots couple in quantum dot molecules is needed. We use the linear combination of quantum dot states approximation in order to obtain the two lowest energy levels of the system and then been able to study the coupling between dots when the potential and the separation between dots are varied. This coupling can be used as a quantum gate, which is a key to building a quantum processor. This gate is achieve using the coupling between QD's and a mechanism which make it possible to switch this coupling on and off. The time required for the 'switching' needs to be long enough for the operation to be performed coherently. In the last chapter we study a possible source of decoherence due to the LO-phonon interaction in self-assembled quantum dots (SAD's). We calculate single electron capture-escape rates due to LO-phonon interaction for single SAD and coupled SAD's systems.

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Fluegel, Brian Darrius. "Sub-relaxation and sub-dephasing dynamics of light-induced polarization in semiconductors." Diss., The University of Arizona, 1992. http://hdl.handle.net/10150/185839.

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Using laser pulses of duration comparable to semiconductor relaxation and dephasing times, the coherent phenomena common to two-level systems have been demonstrated in II-VI and III-V semiconductors. These light-matter interactions are characterized by electrons and holes in their initially excited states and/or a macroscopic polarization that persists after the excitation. A nonthermal carrier distribution was observed in bulk CdSe, demonstrating the extremely fast energy relaxation possible when carrier-LO-phonon scattering is included. Quantum-confined CdSe microcrystallites were then employed to limit the available decay avenues. The one and two-pair transitions were characterized, and using spectral hole-burning, energy relaxation was shown to be substantially slower than in bulk. Dipole dephasing remained very rapid. Persistent macroscopic polarization was demonstrated in semiconductors through pump and probe experiments conducted on time scales faster than the inverse linewidth of the exciton. In the first set of experiments, it was found that the exciton optical Stark shift deviates from that predicted by a steady-state theory. The shift is smaller than expected and lasts longer than the pump pulse. Both effects are explained by free evolution of the exciton's polarization. Transient exciton bleaching is observed and shown to be a result of adiabatic following. In a second study of coherent polarization, spectral oscillations at negative delay times were studied. Oscillations were measured in a wide variety of semiconductors, temporally preceding several different pump-probe phenomena. By comparison with a semiclassical theory, it was shown that spectral oscillations in the spectral region of the exciton are evidence of persistent exciton polarization. Oscillations in the band are due to four wave mixing of the pump and probe beams.

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Kozikov, Aleksey. "Interference and interaction of charge carriers in graphene." Thesis, University of Exeter, 2011. http://hdl.handle.net/10036/3080.

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Electron transport at low temperatures in two-dimensional electron systems is governed by two quantum corrections to the conductivity: weak localisation and electron-electron interaction in the presence of disorder. We present the first experimental observation of these quantum corrections in graphene, a single layer of carbon atoms, over a temperature range 0.02 - 200 K. Due to the peculiar properties of graphene, weak localisation is sensitive not only to inelastic, phase-breaking scattering events, but also to elastic scattering mechanisms. The latter includes scattering within and between the two valleys (intra- and inter-valley scattering, respectively). These specifics make it possible, for example, to observe a transition from weak localisation to antilocalisation. Our work reveals a number of surprising features. First of all the transition occurs not only as the carrier density is varied, but also as the temperature is tuned. The latter has never been observed in any other system studied before. Second, due to weak electron-phonon interaction in graphene, quantum interference of electrons survives at very high temperatures, up to 200 K. For comparison, in other two-dimensional (2D) systems the weak localisation effect is only seen below 50 K. The electron-electron interaction correction is also affected by elastic scattering. In a two-valley system, there are two temperature regimes of the interaction correction that depend on the strength of inter-valley scattering. In both regimes the correction has its own expression. We show that because of the intra-valley scattering, a third regime is possible in graphene, where the expression for the correction takes a new form. The study of weak localisation demonstrates that the third regime is realised in our experiments. We use the new expression to determine the Fermiliquid parameter, which turns out to be smaller than in other 2D systems due to the chirality of charge carriers. At very low temperatures (below 100 mK) we observe a saturation of the electron dephasing length. We study different mechanisms that could be responsible for the saturation and discuss in detail one of them – spin-orbit interaction. We determine the spin coherence length from studies of weak localisation and the temperature dependence of the conductivity and found good agreement between the two types of experiments. We also show the way to tune the spin coherence length by an order of magnitude by controlling the level of disorder. However, experiment shows contradictions with theory both in values of the spin coherence length and the type of spin relaxation. We speculate about another spin-related mechanism, spin flip by vacancies, which to some extent could also explain our observations. We also present electron transport in graphene irradiated by gallium ions. Depending on the dosage of irradiation the behavior of electrons changes. Namely, electron localisation can be tuned from weak to strong. At low dosages we observe the weak localisation regime, where the mentioned quantum corrections to the conductivity dominate at low temperatures. We found the electron scattering between the valleys to be enhanced, attributing it to atomically sharp defects (kicked out carbon atoms) produced by ion irradiation. We also speculate that gallium ions can be embedded in the substrate or trapped between silica and graphene. We draw this conclusion after investigation of the spin-orbit interaction in irradiated samples. At high dosages electrons become strongly localised and their transport occurs via variable-range hopping.

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Treiber, Maximilian [Verfasser], and Jan von [Akademischer Betreuer] Delft. "Dephasing in disordered systems at low temperatures / Maximilian Treiber. Betreuer: Jan von Delft." München : Universitätsbibliothek der Ludwig-Maximilians-Universität, 2013. http://d-nb.info/103610107X/34.

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Sibalic, Nikola. "Rydberg atom ensembles under dephasing and dissipation : from single- to many-body dynamics." Thesis, Durham University, 2017. http://etheses.dur.ac.uk/12224/.

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This thesis explores the effects of decoherence and dephasing on single- and many- body dynamics of spin-systems. A particular realisation of the spin systems that the work focuses on are highly excited, Rydberg states of atoms. A software library ARC (Alkali Rydberg Calculator) for calculating properties of Rydberg states in alkali metals is presented, with particular attention to the multi- atom and multi-level effects that influence many-body dynamics in realistic systems, and properties related to terahertz imaging with alkali atom Rydberg states. Dressed-state electromagnetically induced transparency (EIT) is proposed as a way of preparing uniform-phase spin-waves in ladder excitation schemes, making the collective excitation storage insensitive to motional dephasing. Proof of concept dressed state EIT experiments are presented. Strong resonant dressing is also theoretically analysed as a way of preparing velocity superposition of spin-waves. The developed theoretical model is in a good agreement with existing experimental data on single-photon many-atom quantum beats in diamond excitation schemes. By modelling the strongly driven Rydberg ensembles, many-body dynamics of driven-dissipative spin systems is analysed. Working in the limit of strong dephasing, the effects of fluctuations, the shape of interaction potential, spatial correlations and motion on non-equilibrium phase diagrams and the occurrence of bistability are examined. An ensemble averaged mean field model is introduced as an exact solution for completely uncorrelated ensembles. It is shown that the van der Waals interaction does not allow the occurrence of bistability, for which a finite dipolar core is required. The short-range interaction potential shape is found to have a profound influence on non-equilibrium phase diagrams, controlling the size of fluctuations in the dynamics. For a frozen system, several methods for identifying and quantifying bistable phases are introduced, and phase diagrams are reconstructed. It is shown that the temperature of external degrees of freedom, i.e. spin motion, can drive a non-equilibrium transition into the bistable phase.

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Ulhaq, Ata [Verfasser]. "Photon Statistics and Dephasing of the Resonance Fluorescence from Single Quantum Dots / Ata Ulhaq." München : Verlag Dr. Hut, 2012. http://d-nb.info/1024242846/34.

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Kleff, Silvia [Verfasser]. "Transport theory of ferromagnetic nanograins and Dephasing in qubits with environmental resonances / Silvia Kleff." Aachen : Shaker, 2003. http://d-nb.info/1174513292/34.

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Ribeiro, Wellington Luiz. "Evolution of a 1D bipartite fermionic chain under in?uence of a phenomenological dephasing." reponame:Repositório Institucional da UFABC, 2018.

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Orientador: Prof. Dr. Gabriel Teixeira Landi
Dissertação (mestrado) - Universidade Federal do ABC, Programa de Pós-Graduação em Física, Santo André, 2018.
Em sistemas microscópicos, grandezas como calor e trabalho devem ser tratadas como variáveis aleatórias. Neste trabalho foram estudados os fluxos de calor e de partículas entre dois sistemas unidimensionais fermiônicos A eB, inicialmente preparados separadamente em equilíbrio térmico com reservatórios de calor e partículas preparados a diferentes temperaturas e diferentes potenciais químicos. Calculando a evolução da matriz densidade, foram analisadas as implicações da presença de um ruído de dephasing no sistema, tais como a termalização, a produção de entropia e a evolução da informação mútua como uma forma de analisar a correlação entre os sistemas. Além disso, foi estudado também uma forma do teorema de flutuação do calor no caso onde há fluxo de partículas.
In microscopic systems, heat and work must be treated as random variables. In this work I studied the fluxes of heat and particles between two unidimentional fermionic systems A and B, initially prepared in thermal equilibrium with a reservoir of particles and heat, kept at diferent temperatures and chemical potentials. Computing the evolution of the density matrix, the implications of the presence of a dephasing noise in the system were analyzed, such as thermalization, entropy production and the evolution of mutual information as a way to analyze the correlation between the systems. Moreover, a shape for fluctuation theorems of the heat in the case where there is also a ?ux of particles and its validity was also studied.

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Lillianfeld, Robert Brian. "Experimental Observation of Geometric Phases in Narrow-Gap Semiconductor Heterostructures." Diss., Virginia Tech, 2011. http://hdl.handle.net/10919/26889.

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We have studied the electron quantum phase by fabricating low dimensional (d â ¤ 2) mesoscopic interferometers in high-quality narrow-gap semiconductor (NGS) heterostructures. The low effective-mass electrons in NGS heterostructures enable observation of delicate quantum phases; and the strong spin-orbit interaction (SOI) in the systems gives us means by which we can manipulate the quantum-mechanical spin of these electrons through the orbital properties of the electrons. This enables the observation of spin-dependent phenomena otherwise inaccessible in non-magnetic systems. We have performed low temperature (0.4 K â ¤ T â ¤ 8 K), low noise (â V ~ 1Î¼V ) transport measurements, and observed evidence of Aharonov-Bohm (AB) and Alâ tshuler-Aronov-Spivak (AAS) quantum oscillations in meso- scopic devices that we fabricated on these NGSs. Our measurements are unique in that we observe both AB and AAS in comparable magnitude in ballistic networks with strong SOI. We show that, with appropriate considerations, diffusive formalisms can be used to describe ballistic transport through rings, even in the presence of SOI. This work also contains an introduction to the physics of geometric phases in mesoscopic systems, and the experimental and analytic processes through which these phases are probed. A discussion of the results of our measurements presents the case that quantum interferometric measurements of geometric phases can be understood quite thoroughly, and that these measurements may have deeper utility in discovery than has yet been recognized.
Ph. D.

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Helzel, Andreas [Verfasser], and Christoph [Akademischer Betreuer] Strunk. "Dephasing and quantum noise in an electronic Mach-Zehnder interferometer / Andreas Helzel. Betreuer: Christoph Strunk." Regensburg : Universitätsbibliothek Regensburg, 2013. http://d-nb.info/1043102035/34.

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Marchisio, Pier Paolo. "Analysis of the dephasing and entanglement properties of few-particle states in quantum dot structures." Thesis, University of York, 2011. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.547336.

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Helzel, Andreas Verfasser], and Christoph [Akademischer Betreuer] [Strunk. "Dephasing and quantum noise in an electronic Mach-Zehnder interferometer / Andreas Helzel. Betreuer: Christoph Strunk." Regensburg : Universitätsbibliothek Regensburg, 2013. http://nbn-resolving.de/urn:nbn:de:bvb:355-epub-277705.

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Weiler, Stefanie [Verfasser], and Peter [Akademischer Betreuer] Michler. "Mollow triplet emission properties and dephasing effects in semiconductor quantum dots / Stefanie Weiler. Betreuer: Peter Michler." Stuttgart : Universitätsbibliothek der Universität Stuttgart, 2014. http://d-nb.info/1046564447/34.

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Rembold, Alexander [Verfasser], and Alexander [Akademischer Betreuer] Stibor. "Second-order correlation analysis of multifrequency dephasing in single-particle interferometry / Alexander Rembold ; Betreuer: Alexander Stibor." Tübingen : Universitätsbibliothek Tübingen, 2018. http://d-nb.info/1168148634/34.

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Völker, Axel [Verfasser], Peter [Akademischer Betreuer] Kopietz, and Reiner [Akademischer Betreuer] Kree. "Dephasing and phase-coherence in disordered mesoscopic conductors / Axel Völker. Gutachter: Reiner Kree. Betreuer: Peter Kopietz." Göttingen : Niedersächsische Staats- und Universitätsbibliothek Göttingen, 2001. http://d-nb.info/1045069493/34.

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Bohn, Bernhard Johann [Verfasser], and Jochen [Akademischer Betreuer] Feldmann. "Exciton dynamics in lead halide perovskite nanocrystals: recombination, dephasing and diffusion / Bernhard Johann Bohn ; Betreuer: Jochen Feldmann." München : Universitätsbibliothek der Ludwig-Maximilians-Universität, 2020. http://d-nb.info/1206096543/34.

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Heidkamp, Marcus. "Spin coherence and -dephasing of donor and free conduction band electrons across the metal-insulator transition in Si:GaAs." kostenfrei, 2004. http://deposit.ddb.de/cgi-bin/dokserv?idn=977727149.

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Uria, Valencia Mariano. "Implementación y medición de un sistema cuántico abierto con la dinámica “Dephasing Channel” en la base de caminos." Master's thesis, Pontificia Universidad Católica del Perú, 2019. http://hdl.handle.net/20.500.12404/14119.

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Las base de las tecnologías cuánticas que se proponen hoy en día se basan en la característica de superposición de estados, la cual se presenta como un fenómeno de interferencia, y que da lugar a lo que se conoce como coherencia; no obstante al estar un sistema en contacto con un entorno se produce el fenómeno de decoherencia, lo que conlleva a una pérdida de la información que se quería almacenar, es decir las coherencias, no obstante existen procesos en los que se pueden recuperar la información, estos procesos son llamados no-markovianos. En el presente trabajo hacemos un estudio sobre un sistema cuántico con una dinámica específica, llamada "Dephasing Channel", se usa una medida de coherencia para asegurarnos que trabajamos con un proceso no-markoviano y armamos un arreglo óptico en donde la mencionada dinámica está plasmada en el espacio de Hilbert de caminos. Para la realización de las medidas del experimento, encontramos un método análogo a los parámetros de Stokes en polarización, pero en la base de caminos.
The basis of the quantum technologies that are proposed nowadays are based on the characteristic of superposition of the quantum states, which is presented as an interference phenomenon, and which gives rise to what is known as coherence; however, when a system is in contact with an environment, the phenomenon of decoherence occurs, which leads to a loss of the information that was wanted to be stored, that is the coherence, however, there are processes in which information can be retrieved, these processes are called non-Markovians. In the present work, we make a study about a quantum system with a specific dynamics, called "Dephasing Channel", we use a measure of coherence to make sure that we work with a non-Markovian process and we assembled an optical arrangement where the aforementioned dynamics are embodied in the Hilbert space of paths. For the realization of the measurements of the experiment, we find a method analogous to Stokes parameters in polarization, but at the base of roads.
Tesis

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Das, Lopamudra. "CONFINING AN ORGANIC MOLECULE IN A NANOCAVITY: EFFECT ON ROTATIONAL/VIBRATIONAL MOTION AND ITS CONSEQUENCE FOR SPIN DEPHASING." VCU Scholars Compass, 2010. http://scholarscompass.vcu.edu/etd/2259.

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Recently, it was found that the strongly temperature-dependent spin dephasing time of bound electrons in the organic molecule tris(8-hydroxyquinoline aluminum), or Alq3, increases when a few of these molecules are confined within 1-2 nm sized nanocavities in porous alumina. What causes this increase is not well understood, but one possibility is that the rotational/vibrational modes of the molecule are altered inside a nanocavity, leading to suppression of spin coupling with these modes and a concomitant increase in the spin dephasing time. To test this possibility, we have carried out mid-infrared spectroscopy of few molecule clusters confined within 1-2 nm sized nanocavities. Their spectrum is considerably different from that of bulk powders of molecules, indicating that at least some vibrational modes (in the mid infrared range) have been altered. This may be a possible cause for weakening of spin-environment coupling inside a nanocavity, resulting in an increase in the spin dephasing time.

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Soares, de Araújo Filho Vanduir. "Aplicação da espectroscopia de ressonância magnética nuclear (RMN) à toxicologia forense." Universidade Federal de Pernambuco, 2006. https://repositorio.ufpe.br/handle/123456789/2034.

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A Ressonância Magnética Nuclear (RMN) e a Cromatografia em fase gasosa acoplada à espectrometria de massas (GC/MS) fornecem informações sobre a estrutura química de substâncias, bem como, sobre a constituição qualitativa e quantitativa de amostras sem a necessidade de padrões de análise ou pré-tratamento de amostras. Uma jovem estudante de 19 anos morreu de forma súbita após inalar um aerossol cujo componente principal, o 1-1-dicloro-1-fluoretano (HCFC-141b), foi identificado por meio de RMN e GC/MS. Nas vísceras e no sangue da vítima não foram detectados venenos, álcool ou outras drogas. Os resultados dos exames necroscópico, toxicológico e microscópico não excluem a hipótese de morte por inalação do HCFC-141b e estabelecem dois mecanismos de morte como viáveis: asfixia e arritmia cardíaca. As análises realizadas e os dados do inquérito policial suportam probabilisticamente a hipótese de morte por arritmia cardíaca fatal que pode ter ocorrido de forma associada com asfixia. Algumas técnicas de RMN como ¹H, ¹³C, Distortionless Enhancement by Polarization Transfer (DEPT), Cross-Polarization and Magic Angle Spinning (CP/MAS) and Dipolar Dephasing (DD) foram usadas para caracterizar amostras de cocaína. Os deslocamentos químicos dos espectros obtidos foram atribuídos por meio de Heteronuclear Correlation (HETCOR) e Heteronuclear Multiple Bond Correlation (HMBC). As amostras foram primariamente caracterizadas por espectrometria de massas cujos espectros foram comparados com dados obtidos na literatura. Os resultados obtidos indicam que ¹H, ¹³C e DEPT são técnicas bastante eficientes para caracterização de amostras ilícitas de cocaína no estado líquido e que por meio de CP/MAS e CP/MAS/DD RMN é possível distinguir o crack do cloridrato de cocaína de uma maneira não destrutiva. Portanto, a RMN é uma excelente ferramenta para caracterização de amostras ilícitas de cocaína

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Mallus, Maria Ilaria [Verfasser], Ulrich [Akademischer Betreuer] [Gutachter] Kleinekathöfer, Jürgen [Gutachter] Fritz, and Carles [Gutachter] Curutchet. "On Dephasing and Exciton Transfer in Light-Harvesting Complexes / Maria Ilaria Mallus ; Gutachter: Ulrich Kleinekathöfer, Jürgen Fritz, Carles Curutchet ; Betreuer: Ulrich Kleinekathöfer." Bremen : IRC-Library, Information Resource Center der Jacobs University Bremen, 2017. http://d-nb.info/1141379686/34.

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33

Zhang, Bo. "Experimental Studies of Quantum Dynamics and Coherent Control in Homonuclear Alkali Diatomic Molecules." Doctoral thesis, KTH, Physics, 2002. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-3420.

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The main theme covered in this thesis is experimentalstudies of quantum dynamics and coherent control in homonuclearalkali diatomic molecules by ultrafast laser spectroscopy iththe implementation of pump-probe techniques.

A series of experiments have been performed on the Rb2molecules in a molecular beam as well as in a thermal oven. Thereal-time molecular quantum dynamics of the predissociatingelectronically excited D(3)¹Πu state of Rb₂, which couples to/intersects several otherneighbouring states, is investigated using wavepackets. Thepredissociation of the D state, explored by this wavepacketmethod, arises from two independent states, the (4)³Σ_u⁺and (1)³∆_u, for which the second corresponds to a much fasterdecay channel above a sharp energy threshold around 430 nm. Thelifetime of the D state above the energy threshold is obtained,τ ≈ 5 ps, by measuring the decay time of thewavepacket in a thermal oven. Further experimentalinvestigation performed in a molecular beam together withquantum calculations of wavepacket dynamics on the D state haveexplored new probe channels of wavepacket evolution: theD′(3)1Σu+ channel, which exhibits vibrational motionin a shelf state and the (4)³Σu+ channel, where direct build-up of thewavefunction is observed due to its spin-orbit oupling to the Dstate.

The real-time quantum dynamics of wavepackets confined totwo bound states, A¹Σ_u⁺(0_u⁺) and b³Π_u(0_u⁺), have been studied by experiment andcalculations. It is shown that these two states are fullycoupled by spin-orbit interaction, characterised by itsintermediate strength. The intermediate character of thedynamics is established by complicated wavepacket oscillationatterns and a value of 75 cm^-1is estimated for the coupling strength at thestate crossing.

The experiments on the Li₂molecule are performed by coherent control ofrovibrational molecular wavepackets. First, the Deutsch-Jozsaalgorithm is experimentally demonstrated for three-qubitfunctions using a pure coherent superposition of Li₂rovibrational eigenstates. The functionscharacter, either constant or balanced, is evaluated by firstimprinting the function, using a phase-tailored femtosecond(fs) pulse, on a coherent superposition of the molecularstates, and then projecting the superposition onto an ionicfinal state using a second fs pulse at a specific delay time.Furthermore, an amplitude-tailored fs pulse is used to exciteselected rovibrational eigenstates and collision induceddephasing of the wavepacket signal, due to Li₂-Ar collisions, is studied experimentally. Theintensities of quantum beats decaying with the delay time aremeasured under various pressures and the collisional crosssections are calculated for each well-defined rovibrationalquantum beat, which set the upper limitsfor ure dephasingcross sections.

Keywords:Ultrafast laser spectroscopy, pump-probetechnique, predissociation, wavepacket, pin-orbit interaction,coherent control, (pure) dephasing

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Rayanov, Kristian. "Coherence of Matter and Light Waves in Localizing Media." Master's thesis, Universitätsbibliothek Chemnitz, 2012. http://nbn-resolving.de/urn:nbn:de:bsz:ch1-qucosa-88015.

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The phenomena of coherence and localization have gained enormous research interest during the past decades. Theoretical predictions of localization have been confirmed recently in a variety of experiments in the fields of condensed matter physics and optics. We consider the widely employed model of the one-dimensional discrete nonlinear Schrödinger equation which allows for the investigation of localization of linear and nonlinear waves. We establish a generic connection between coherence and localization by showing that localized solutions are necessarily coherent. The effects of a loss of coherence are investigated numerically by applying random dephasing. The onset of a diffusive spreading regime is observed as a generic feature for persistent dephasing, which eventually destroys localization. After finite integration times maximal delocalization is achieved for a certain rate and strength of dephasing, resulting from the competition between destroying the initial wave packet on the one hand and not suppressing spreading too much on the other. When dephasing is switched off at a certain time, a loss or gain of coherence in linear wave packets directly corresponds to delocalization or stronger localization. This leads to stable partially coherent wave packets. In contrast, localization of nonlinear waves after dephasing is connected to the efforts of establishing complete coherence, at least for asymptotically long times. On intermediate time scales, however, various interesting partially coherent regimes can be observed
Die Phänomene von Kohärenz und Lokalisierung haben sich in den letzten Jahrzehnten zum Schwerpunkt zahlreicher Forschungsinteressen entwickelt. Erst kürzlich wurden theoretische Vorhersagen von Lokalisierung in verschiedensten Experimenten in den Bereichen der Festkörperphysik und Optik bestätigt. In dieser Arbeit wird das häufig angewendete Modell der eindimensionalen diskreten nichtlinearen Schrödingergleichung betrachtet, welches die Untersuchung der Lokalisierung von linearen und nichtlinearen Wellen ermöglicht. Eine generelle Verbindung zwischen Kohärenz und Lokalisierung wird gezeigt, wonach lokalisierte Lösungen notwendigerweise kohärent sind. Die Auswirkungen eines Verlustes von Kohärenz werden numerisch durch Anwendung unterschiedlicher Methoden eines zufälligen Dephasierens untersucht. Ein permanentes Dephasieren führt stets zum Auftreten eines diffusiven Regimes, welches letztlich die Zerstörung von Lokalisierung bedingt. Nach einer endlichen Integrationszeit wird jedoch eine maximale Delokalisierung nur durch eine bestimmte Rate und Stärke des Dephasierens bewirkt, resultierend aus einem Wettstreit zwischen möglichst schneller Zerstörung des ursprünglichen Wellenpaketes auf der einen Seite und nicht zu starker Behinderung des Zerfließens auf der anderen. Wird das Dephasieren nach einer bestimmten Zeit abgeschaltet, so entspricht ein folglicher Verlust oder Zugewinn an Kohäarenz in einem linearen Wellenpaket direkt einem Verlust oder Zugewinn von Lokalisierung. Dabei treten im allgemeinen stabile teil-kohärente Wellenpakete auf. Im Gegensatz dazu ist Lokalisierung nichtlinearer Wellenpakete stets mit den Bemühungen verbunden, vollständige Kohärenz wiederherzustellen, zumindest für asymptotisch lange Zeiten. Auf mittleren Zeitskalen können jedoch verschiedene interessante teil-kohärente Strukturen beobachtet werden

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Olivíková, Gabriela. "Kvantový popis superzářivosti emitorů s plazmonicky zprostředkovanou interakcí." Master's thesis, Vysoké učení technické v Brně. Fakulta strojního inženýrství, 2017. http://www.nusl.cz/ntk/nusl-319702.

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Superradiance is an enhanced decay of an excited system of emitters resulting from their mutual coupling. This thesis is focused on superradiance of the emitters coupled via their interaction with a plasmonic nanoparticle. So-called plasmon-mediated superradiance results in even stronger enhancement of the decay rate as the nanoparticle serves as an additional decay chanel. We have developed a quantum model of the system of emitters coupled to a plasmonic nanoparticle, which allows us to differentiate between a pure dephasing and decay processes. We show that the pure dephasing can destroy the cooperative effect leading to superradiance. Furthermore, we have studied how the direct mutual coupling between emitters affects time evolution of the system in dependence on its configuration, and we show conditions when a decay of the system is dramatically decreased by direct coupling.

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Helm, Julius. "Classical vs. Quantum Decoherence." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2012. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-84542.

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Based on the superposition principle, any two states of a quantum system may be coherently superposed to yield a novel state. Such a simple construction is at the heart of genuinely quantum phenomena such as interference of massive particles or quantum entanglement. Yet, these superpositions are susceptible to environmental influences, eventually leading to a complete disappearance of the system's quantum character. In principle, two distinct mechanisms responsible for this process of decoherence may be identified. In a classical decoherence setting, on the one hand, stochastic fluctuations of classical, ambient fields are the relevant source. This approach leads to a formulation in terms of stochastic Hamiltonians; the dynamics is unitary, yet stochastic. In a quantum decoherence scenario, on the other hand, the system is described in the language of open quantum systems. Here, the environmental degrees of freedom are to be treated quantum mechanically, too. The loss of coherence is then a direct consequence of growing correlations between system and environment. The purpose of the present thesis is to clarify the distinction between classical and quantum decoherence. It is known that there exist decoherence processes that are not reconcilable with the classical approach. We deem it desirable to have a simple, feasible model at hand of which it is known that it cannot be understood in terms of fluctuating fields. Indeed, we find such an example of true quantum decoherence. The calculation of the norm distance to the convex set of classical dynamics allows for a quantitative assessment of the results. In order to incorporate genuine irreversibility, we extend the original toy model by an additional bath. Here, the fragility of the true quantum nature of the dynamics under increasing coupling strength is evident. The geometric character of our findings offers remarkable insights into the geometry of the set of non-classical decoherence maps. We give a very intuitive geometrical measure---a volume---for the quantumness of dynamics. This enables us to identify the decoherence process of maximum quantumness, that is, having maximal distance to the convex set of dynamics consistent with the stochastic, classical approach. In addition, we observe a distinct correlation between the decoherence potential of a given dynamics and its achievable quantumness. In a last step, we study the notion of quantum decoherence in the context of a bipartite system which couples locally to the subsystems' respective environments. A simple argument shows that in the case of a separable environment the resulting dynamics is of classical nature. Based on a realistic experiment, we analyze the impact of entanglement between the local environments on the nature of the dynamics. Interestingly, despite the variety of entangled environmental states scrutinized, no single instance of true quantum decoherence is encountered. In part, the identification of the classical nature relies on numerical schemes. However, for a large class of dynamics, we are able to exclude analytically the true quantum nature.

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Rittershofer, Wolf. "Laser wakefield acceleration in tapered plasma channels : theory, simulation and experiment." Thesis, University of Oxford, 2014. http://ora.ox.ac.uk/objects/uuid:be45ca7d-790c-496c-9e52-160ce4fe277d.

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Laser-plasma accelerators are of great interest because of their ability to sustain extremely large acceleration gradients, enabling compact accelerating structures. Laser-plasma acceleration is realized by using a high-intensity short pulse laser to drive a large plasma wave or wakefield in an underdense plasma. This thesis considers the effect of axial plasma density upramps on laser wakefield acceleration. Theoretical groundwork shows that tapered plasma channels can be used to mitigate one of the main limitations of laser plasma acceleration, that is, dephasing of an electron beam with respect to the plasma wave. It is shown that it is possible to maintain an electron bunch at constant phase in the longitudinal electric fields of the laser wake field. This leads to an increased energy gain of an electron trapped in the wakefield. The required shape of the density slope is difficult to implement in experiments. Therefore, a linear density ramp is also considered which is predicted to also increase the energy gain beyond that possible in a uniform density plasma. Towards an experimental implementation it was studied how a suitable gas density profile can be established in a capillary. This was done employing simulations using the computational fluid dynamics tool kit OpenFoam and comparing these to measurements of the axial density profile based on Raman scattering. It was demonstrated that a linear density ramp could be established by applying different pressures on the capillary gas inlets. The dependence of the density profile on the capillary parameters, such as, capillary diameter and length and inlet diameter were also studied. The results of the simulations and the measurement showed excellent agreement and demonstrate that approximately linear density ramps can be generated by flowing gas along a capillary of constant cross-section Laser wakefield acceleration in plasmas with longitudinally varying density was investigated in an experiment at the Astra Laser at Rutherford Laboratories. The experiment utilised ionisation injection in order to operate in the mildly non-linear regime of laser-wakefield acceleration. The measured electron energies agree well with the theoretical predictions. It was demonstrated that an increase in the energy gain can be obtained by driving the accelerator in a ramped plasma, the electron spectrum is more narrow and the injected charge increases significantly. Measurements of the X-ray spectrum emitted by the betatron motion of the accelerated electron bunch allowed the transverse radius of the bunch to be deduced. These measurements showed that retrieved electron bunch radius is inversely proportional to the longitudinal density gradient, that is a plasma density upramp (downramp) has a decreased (increased) electron bunch radius.

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Mendoza, Arenas Juan José. "Spin and energy transport in boundary-driven low-dimensional open quantum systems." Thesis, University of Oxford, 2014. http://ora.ox.ac.uk/objects/uuid:44b89c4d-e9eb-4136-a540-c80bcabeb6f6.

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In spite of being the subject of intense research, several key but complex questions on the nonequilibrium physics of correlated quantum systems remain controversial. For example, the nature of particle and energy transport in different interacting regimes, the relevance of integrability and the impact of environmental coupling are still under active debate. These problems can now be approached numerically, due to the development of powerful algorithms which allow the efficient simulation of the dynamics of correlated systems. In the present thesis we study numerically and analytically the transport properties of low-dimensional quantum systems. In particular, we consider the steady-state spin and energy conduction through XXZ boundary-driven spin-1/2 chains. In the first part, we analyse the transport through chains with only coherent processes in the bulk. For spin transport induced by a magnetisation imbalance between the boundaries, previously identified ballistic, diffusive and negative differential conductivity regimes are reproduced. We provide a comprehensive explanation of the latter. The energy conduction induced by this driving scheme features the same properties as spin transport. For thermally-driven chains, we discuss the nature of energy transport and the emergence of local thermal states when the integrability of the Hamiltonian is broken. In the second part of the thesis we analyse the effect of bulk incoherent effects on the transport properties previously discussed. First we find that for weak particle-particle interactions, pure dephasing degrades spin and energy conduction. In contrast, for strong interactions dephasing induces a significant transport enhancement. We identify the underlying mechanism and discuss its generality. Finally, motivated by the lattice structure of several organic conductors, we study the interplay between coherent and incoherent processes in systems of weakly-coupled chains. We find an enhancement effect due to incoherent interchain hopping, stronger than that by dephasing, which increases with the chain length and relates to superdiffusive transport.

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Dey, Prasenjit. "Exciton Dynamics and Many Body Interactions in Layered Semiconducting Materials Revealed with Non-linear Coherent Spectroscopy." Thesis, University of South Florida, 2016. http://pqdtopen.proquest.com/#viewpdf?dispub=10076073.

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Atomically thin, semiconducting transition metal dichalogenides (TMDs), a special class of layered semiconductors, that can be shaped as a perfect two dimensional material, have garnered a lot of attention owing to their fascinating electronic properties which are achievable at the extreme nanoscale. In contrast to graphene, the most celebrated two-dimensional (2D) material thus far; TMDs exhibit a direct band gap in the monolayer regime. The presence of a non-zero bandgap along with the broken inversion symmetry in the monolayer limit brands semiconducting TMDs as the perfect candidate for future optoelectronic and valleytronics-based device application. These remarkable discoveries demand exploration of different materials that possess similar properties alike TMDs. Recently, III-VI layered semiconducting materials (example: InSe, GaSe etc.) have also emerged as potential materials for optical device based applications as, similar to TMDs, they can be shaped into a perfect two-dimensional form as well as possess a sizable band gap in their nano-regime. The perfect 2D character in layered materials cause enhancement of strong Coulomb interaction. As a result, excitons, a coulomb bound quasiparticle made of electron-hole pair, dominate the optical properties near the bandgap. The basis of development for future optoelectronic-based devices requires accurate characterization of the essential properties of excitons. Two fundamental parameters that characterize the quantum dynamics of excitons are: a) the dephasing rate, γ, which represents the coherence loss due to the interaction of the excitons with their environment (for example- phonons, impurities, other excitons, etc.) and b) excited state population decay rate arising from radiative and non-radiative relaxation processes. The dephasing rate is representative of the time scale over which excitons can be coherently manipulated, therefore accurately probing the source of exciton decoherence is crucial for understanding the basic unexplored science as well as creating technological developments. The dephasing dynamics in semiconductors typically occur in the picosecond to femtosecond timescale, thus the use of ultrafast laser spectroscopy is a potential route to probe such excitonic responses.

The focus of this dissertation is two-fold: firstly, to develop the necessary instrumentation to accurately probe the aforementioned parameters and secondly, to explore the quantum dynamics and the underlying many-body interactions in different layered semiconducting materials. A custom-built multidimensional optical non-linear spectrometer was developed in order to perform two-dimensional spectroscopic (2DFT) measurements. The advantages of this technique are multifaceted compared to regular one-dimensional and non-linear incoherent techniques. 2DFT technique is based on an enhanced version of Four wave mixing experiments. This powerful tool is capable of identifying the resonant coupling, probing the coherent pathways, unambiguously extracting the homogeneous linewidth in the presence of inhomogeneity and decomposing a complex spectra into real and imaginary parts. It is not possible to uncover such crucial features by employing one dimensional non-linear technique.

Monolayers as well as bulk TMDs and group III-VI bulk layered materials are explored in this dissertation. The exciton quantum dynamics is explored with three pulse four-wave mixing whereas the phase sensitive measurements are obtained by employing two-dimensional Fourier transform spectroscopy. Temperature and excitation density dependent 2DFT experiments unfold the information associated with the many-body interactions in the layered semiconducting samples.

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Hillmann, Frank. "Untersuchungen zur Relaxation von Anregungszuständen im Lichtsammelkomplex des Photosystems II höherer Pflanzen sowie im Halbleiter Cadmiumsulfid mittels Vierwellenmischung." Doctoral thesis, Humboldt-Universität zu Berlin, Mathematisch-Naturwissenschaftliche Fakultät I, 2001. http://dx.doi.org/10.18452/14673.

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Methoden der transienten Vierwellenmischung mit Femtosekunden-Zeitauflösung werden angewendet, um die Phasen- und Energierelaxation optisch selektiv erzeugter Anregungszustände im Lichtsammelkomplex II höherer Pflanzen (LHC II) sowie im Halbleiter Cadmiumsulfid (CdS) bei verschiedenen Temperaturen zu untersuchen. Für den LHC II werden die Ergebnisse der Messungen des zeitaufgelösten und integrierten Zweipuls-Photonenechos mit Resultaten aus Pump-Test-Experimenten verglichen, um unter Einbeziehung von Literaturdaten Rückschlüsse über den Charakter der phasenzerstörenden Prozesse zu ziehen und Zusammenhänge zu Strukturdaten des Komplexes aufzudecken. Die vorliegende Arbeit liefert erstmals einen systematischen Überblick über die totalen Phasenrelaxationszeiten T2 im Bereich der Qy-Bande des LHC II von 640 bis 685 nm bei 5 K. Das bei 5 K beobachtete Photonenechosignal am LHC II zeigt in Abhängigkeit von der Verzögerung der beiden Anregungsimpulse ein multiexponentielles Abklingen, das auf die Überlagerung der Einflüsse mehrerer Relaxationsprozesse zurückgeführt wird. Dabei lassen sich drei charakteristische Bereiche der Phasenrelaxationszeit unterscheiden, die verschiedenen phasenzerstörenden Prozessen zugeordnet werden. Ein Vergleich mit Resultaten aus Pump-Test-Experimenten führt zu der Schlußfolgerung, daß die Phasenrelaxation im LHC II bei 5 K für Wellenlängen £ 675 nm im wesentlichen durch den Energietransfer auf einer sub-ps Zeitskala bestimmt wird. Für Wellenlängen > 675 nm steigt die Phasenrelaxationszeit stark an und wird insbesondere im Bereich der tiefsten Anregungszustände um 680 nm durch reine Phasenzerstörung dominiert. Ab 20 K setzt bei dieser Wellenlänge ein zusätzlicher phasenzerstörender Prozeß ein, der mit steigender Temperatur zu einem mäßigen linearen Anstieg der Phasenrelaxationsrate (T2)-1 führt. Die Ursache ist vermutlich ein Aufwärts-Energietransfer. Im Bereich der Chlorophyll a-Absorption vernichten außerdem (physiologisch irrelevante) Multiexzitoneneffekte die Kohärenz der angeregten Zustände, verursacht durch die hohe Anregungsintensität. Zusammenfassend kann festgestellt werden, daß die Erhaltung der Kohärenz für die Funktionalität des LHC II eine untergeordnete Rolle spielt. Die wesentlichen Prozesse sind der schnelle räumliche Energietransfer und die Energierelaxation auf das Niveau des primären Elektrondonators P680 im Reaktionszentrum. Am Halbleiter CdS wird erstmals ein mittels Zwei-Photonen-Absorption angeregtes Photonenecho beschrieben, das in Abhängigkeit von der Wellenlänge charakteristische Quantenbeats mit einer Periode von 700 bis 800 fs zeigt. Das stark gedämpfte periodische Echosignal tritt sowohl für positive als auch für negative Verzögerungszeiten t der Anregungsimpulse auf, wobei die Abklingzeit für t>0 mit 170±10 fs doppelt so groß ist wie für t
Transient four-wave-mixing experiments with femtosecond resolution are performed in order to investigate phase and energy relaxation processes of optically excited states in the light harvesting complex II of higher plants (LHC II) and in the semiconductor cadmium sulfide (CdS) at different wavelengths and temperatures. Extensive studies of the time resolved and integrated two-pulse photon echo on LHC II are combined with pump-probe experiments. Results of both methods together with literature data are used to characterize the nature of dephasing processes and to reveal connections with structural data of the complex. This study gives the first systematic survey of total dephasing times T2 in the spectral region of the Qy-absorption band of LHC II from 640 to 685 nm at 5 K. In the case of LHC II, the photon echo signal at 5 K monitored as a function of delay between both excitation pulses shows a multi-exponential decay which is attributed to the superposition of several relaxation processes. Three characteristic dephasing time domains can be distinguished, ascribed to different dephasing processes. Comparing photon echo and pump-probe results it can be concluded that dephasing in LHC II at 5 K and for wavelengths £ 675 nm is dominated by the fast excitation energy transfer on a sub-ps time scale. At wavelengths > 675 nm the total dephasing time increases drastically. The loss of coherence of the lowest excited states around 680 nm at 5 K is mainly determined by pure dephasing. An additional dephasing process, probably uphill energy transfer, occurs at temperatures higher than 20 K leading to a moderate linear rise of the dephasing rate (T2)-1 with increasing temperature. Furthermore, the dephasing in the spectral region of chlorophyll a absorption is affected by (physiologically irrelevant) multi-excitonic effects caused by the high excitation energy. In summary, it can be concluded that the preservation of coherence plays a minor role in the functionality of LHC II. The main processes are the fast spatial excitation energy transfer and the energy relaxation down to the energetic level of the primary electron donor P680 of the reaction center. Investigations of four-wave mixing signals of the semiconductor CdS resulted in the first description of a two-photon excited photon echo in CdS showing characteristic quantum beats with a period of 700 to 800 fs in dependence on wavelength. The strongly damped periodical echo signal is found for both positive and negative delay times t between the excitation pulses. The decay time for t>0 amounts to 170±10 fs and is twice as large as for t

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41

Szwer, David James. "High fidelity readout and protection of a 43Ca+ trapped ion qubit." Thesis, University of Oxford, 2009. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.515002.

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This thesis describes theoretical and experimental work whose main aim is the development of techniques for using trapped ⁴³Ca⁺ ions for quantum information processing. I present a rate equations model of ⁴³Ca⁺, and compare it with experimental data. The model is then used to investigate and optimise an electron-shelving readout method from a ground-level hyperfine qubit. The process is robust against common experimental imperfections. A shelving fidelity of up to 99.97% is theoretically possible, taking 100 μs. The laser pulse sequence can be greatly simplified for only a small reduction in the fidelity. The simplified method is tested experimentally with fidelities up to 99.8%. The shelving procedure could be applied to other commonly-used species of ion qubit. An entangling two-qubit quantum controlled-phase gate was attempted between a ⁴⁰Ca⁺ and a ⁴³Ca⁺ ion. The experiment did not succeed due to frequent decrystallisation of the ion pair, and strong motional decoherence. The source of the problems was never identified despite significant experimental effort, and the decision was made to suspend the experiments and continue them in an improved ion trap which is under construction. A sequence of pi-pulses, inspired by the Hahn spin-echo, was derived that is capable of greatly reducing dephasing of any qubit. If the qubit precession frequency varies with time as an nth-order polynomial, an (n+1) pulse sequence is theoretically capable of perfectly cancelling the resulting phase error. The sequence is used on a 43Ca+ magnetic-field-sensitive hyperfine qubit, with 20 pulses increasing the coherence time by a factor of 75 compared to an experiment without any spin-echo. In our ambient noise environment the well-known Carr-Purcell-Meiboom-Gill dynamic-decoupling method was found to be comparably effective.

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Bylsma, Jason Michael. "Multidimensional Spectroscopy of Semiconductor Quantum Dots." Scholar Commons, 2012. http://scholarcommons.usf.edu/etd/4001.

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The coherent properties of semiconductor nanostructures are inherently difficult to measure and one-dimensional spectroscopies are often unable to separate inhomogeneous and homogeneous linewidths. We have refined and improved a method of performing multidimensional Fourier transform spectroscopy based on four-wave mixing (FWM) experiments in the box geometry. We have modified our system with broadband beamsplitters in all interferometer arms, high-resolution translation stages and the ability to work in reflection geometry. By improving the phase-stability of our setup and scanning pulse delays with sub-optical cycle precision, we are able to reproduce 2DFT spectra of GaAs multiple quantum wells. With the FWM signal reflected from the sample surface instead of transmitted through, we show that very low pulse powers can be used to generate coherent 2D signals from colloidal PbS quantum dots. Dephasing times are particularly difficult to measure in small colloidal quantum dots due to environmental broadening effects from the colloidal growth. We show that low-temperature pure excitonic dephasing can be measured via time-integrated measurements as well as from the cross-diagonal linewidths of 2DFT spectra. Ultrafast sub-picosecond dephasing times are measured at 5 K in 3 nm PbS quantum dots, while excitation-density-dependence is investigated in these dots. By retrieving the global phase with an all-optical method, we are able to retrieve the real-part 2D spectra of PbS quantum dots.

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43

Herterich, Emmi. "Sudden Death of Entanglement for non-locality and concurrence : A review of entanglement sudden death behaviour of non-locality and concurrence in commonly used entangled state classes under inuence of decay and dephasing noise dynamics." Thesis, Uppsala universitet, Materialteori, 2019. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-397446.

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Cavalcante, Ary de Oliveira. "Forma de linha do espectro Raman do ânion croconato em diversos ambientes." Universidade de São Paulo, 2003. http://www.teses.usp.br/teses/disponiveis/46/46132/tde-09092014-161633/.

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A espectroscopia Raman é utilizada para obtenção de informações dinâmicas de líquidos. A habilidade do ânion croconato como sonda das interações é testada no estudo do processo de transição vítrea no sal croconato de tetra-n-butilamônio (CTBA), [(C4H9)4N]2C5O5.4H2O, um líquido muito viscoso com temperatura de transição vítrea, Tg, em cerca de 298K. As funções de correlação temporais foram obtidas pela transformada de Fourier do contorno dos modos do croconato, C5O5-2. Posteriormente, foram ajustados modelos teóricos para o defasamento vibracional a essas funções de correlação, como o modelo de Kubo. Nesse estudo foi possível encontrar assinaturas espectroscópicas da transição vítrea que são interpretadas do ponto de vista da dinâmica do ânion croconato e das interações deste ânion com o ambiente em que ele se insere. A miscibilidade do CTBA em acetonitrila permitiu que fossem feitos estudos do defasamento do ânion croconato neste solvente. Nestes estudos, nota-se um contraste com os resultados obtidos para a solução aquosa saturada do sal Li2C5O5. O valor encontrado para tempo de correlação da flutuação da freqüência vibracional, τc, na solução diluída em acetonitrila (τc ≈ 0,5 ps) é superior ao respectivo valor encontrado no CTBA. A relaxação lenta e o ambiente homogêneo experimentado pelo oxocarbono na solução diluída de CTBA em acetonitrila corroboram que as interações de curto alcance entre as moléculas de acetonitrila e o croconato não são decisivas para o defasamento neste sistema, já que na solução aquosa o valor de τc é determinado pela estrutura de gaiola das moléculas de água em torno do C5O5-2 unidas por ligações de hidrogênio fortes.
Raman spectroscopy is used for attainment of dynamic information of liquids. The ability of the croconate anion as a probe of the interactions is tested in the study of the glass transition in the salt tetra-n-butilammonium croconate (CTBA), [n-(C4H9)4N]2C5O5.4H2O, a very viscous liquid with glass transition temperature, Tg, at ca. 298K. The time correlation functions were obtained by Fourier transforming the contour of the Raman bands of the croconate\'s modes, C5O5-2. Theoretical models for vibrational dephasing were adjusted to these vibrational time correlation functions by the Kubo\'s equation. In this study, it was possible to find spectroscopic signatures of the glass transition that are interpreted in the light of the dynamics of the croconate anion, and the interactions of this anion with the surrounding environment. The miscibility of the CTBA with acetonitrile allowed for studies of the anion dephasing in this solvent. In these studies, a contrast with the results obtained for the saturated aqueous solution of the simple salt Li2C5O5 was noticed. The value of the correlation time of the fluctuation of the vibrational frequency, τc, in the diluted acetonitrile solution (τc ≈ 0,5 ps) is higher than the respective value in pure CTBA despite the high viscosity of the latter. The slow relaxation and the homogeneous environment probed by oxocarbon in the diluted solution of CTBA in acetonitrile corroborate the physical picture in which the short-range interactions between the molecules of acetonitrile and the croconate are not essential for the vibrational dephasing in this system, since the τc value in the aqueous solution is mainly determined by the tight structure of water molecules around the C5O5-2 due to the hydrogen bonds.

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45

Pernice, Ansgar. "The role of system-environment correlations in the dynamics of open quantum systems." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2013. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-110764.

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In the present thesis the dynamics of the correlations between an open quantum system and its environment is investigated. This becomes feasible by means of a very useful representation of the total system-environment state. General conditions for separability and entanglement of the latter are derived, and investigated in the framework of an open quantum two-level system, which is coupled to a dissipative and a dephasing environment.

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46

Raskin, Maxim. "Ultraschnelle Ladungsträger- und Spindynamik in II-VI und III-V Halbleitern mit weiter Bandlücke." Doctoral thesis, Universitätsbibliothek Chemnitz, 2013. http://nbn-resolving.de/urn:nbn:de:bsz:ch1-qucosa-125227.

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Die vorliegende Arbeit beschäftigt sich mit der Herstellung und Charakterisierung von verdünnten magnetischen II-VI und III-V Halbleiter-Dünnschichten. Diese Systeme bieten vereinfachte optische kohärente Kontrolle von Spin-basierten Prozessen und eignen sich hervorragend für den Einsatz in zukünftigen opto-magnetischen Anwendungen. ZnO-, ZnXO-, GaN- und GaXN-Proben (X = Mangan, Cobalt) sind mit Hilfe der naßchemischen Sol-Gel Synthese hergestellt worden. Sie werden mit Hilfe der Photolumineszenzspektroskopie untersucht. Die spektrale Position der elektronischen Niveaus in der Nähe der Bandkante dieser Materialien wird bestimmt, um in weiteren Experimenten die freien und gebundenen Exzitonen einzeln abzufragen. Mit der Methode der zeitaufgelösten differentiellen Transmissionsspektroskopie (TRDT) werden die Lebensdauern dieser Ladungsträger bestimmt und mit ultraschnellen Prozessen der optischen Anregung und Relaxation in Verbindung gebracht. Die Methode der zeitaufgelösten Faraday-Rotation-Spektroskopie (TRFR) wird angewandt, um die kohärente Spindynamik des optisch angeregten Teilchenensembles zu beschreiben. Die Kohärenz unterliegt den Störeinflüssen verschiedener Streumechanismen, die in der vorliegenden Arbeit identifiziert und quantitativ beschrieben werden. Bei einigen untersuchten Materialsystemen (ZnCoO, ZnMnO und GaMnN) wird die jeweilige spezifische Elektron-Ion Austauschenergie N0α bestimmt, welche die Kopplungsstärke der elektronischen Spins zu denen der Dotierionen beschreibt.

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47

Benedetti, C. "DECOHERENCE, NON-MARKOVIANITY AND QUANTUM ESTIMATION IN QUBIT SYSTEMS SUBJECT TO CLASSICAL NOISE." Doctoral thesis, Università degli Studi di Milano, 2015. http://hdl.handle.net/2434/254031.

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The unavoidable interaction of a quantum system with its environment usually degrades its coherence and quantumness. The environment and the decoherence process may be described as the interaction with a classical or quantized bath. As a matter of fact, the classical description becomes progressively more reliable as far as the environment has many degrees of freedom, i.e. it becomes complex, or when the interaction between a quantum system and a classical fluctuating field is taken into account. In this thesis, I consider a qubit system coupled to a stochastic classical field and address the decoherence and non-Markovianity induced by the external noise as well as the spectral characterization of the classical field by quantum-limited measurement on the qubit. I thus analyze the dynamics of quantum correlations between two non-interacting, initially entangled qubits subject to a classical noise generated by a stochastic process. Two relevant classes of noise are taken into account: Gaussian noise, such as the Ornstein-Uhlenbeck process, and non-Gaussian noise, such as the random telgraph noise and the colored noise with 1/f spectrum. I also discuss the evaluation of non-Markovianity of the induced dynamical map and link the presence of revivals of quantum correlations with the information backflow to the system. The precise characterization of the stochastic process generating the classical noise, possibly using minimal resources, is a crucial ingredient for the design of high-precision measurements and reliable communication protocols. To this purpose, I also address the characterization of the spectral parameters of classical noise by quantum probes, e.g. a qubit coupled to the stochastic process generating the noise. By using the tools of quantum estimation theory, I explore the performances of quantum measurements on the qubit and show that it is possible to effectively extract information about the noise.

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48

Bajracharya, Pradeep. "Relaxation Dynamics and Decoherence of Excitons in II-VI Semiconductor Nanostructures." University of Cincinnati / OhioLINK, 2007. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1186757546.

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49

Thon, Raphaël. "Dynamique vibrationnelle de métaux-carbonyles pièges en matrice cryogénique." Phd thesis, Université Paris Sud - Paris XI, 2013. http://tel.archives-ouvertes.fr/tel-00842721.

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Nous avons mis en place un dispositif permettant l'acquisition d'échos de photons stimulés infrarouges à l'échelle femtoseconde. Le but est d'examiner la dynamique vibrationnelle aux temps courts de métaux carbonyles (W(CO)₆ and Fe(CO)₅) piégés en matrice cryogénique (4-50 K). Cet environnement solide, issu de la condensation d'un mélange gazeux contenant une impureté et un gaz inerte (N₂, CH₄, Ar, etc.), est propice à l'étude de systèmes dans leur état fondamental. L'excitation d'une vibration moléculaire s'atténue toujours temporellement, ce qui correspond dans le domaine spectral à un élargissement des raies d'absorption. L'étude de la dynamique vibrationnelle vise à examiner les causes physiques à l'origine de cet élargissement spectral. Typiquement, elles sont de trois sortes : phénomènes intramoléculaires, interactions entre molécules piégées et interactions entre la molécule piégée et l'environnement. Les échos de photons permettent de distinguer les contributions homogènes et inhomogènes de l'élargissement spectral et de caractériser les processus de déphasage, de relaxation des populations et de diffusion spectrale. Parmi les résultats obtenus, nous avons mis en évidence l'influence des phonons spécifiques aux matrices moléculaires (ex : libration de N₂ et rotation de CH₄ ) sur le déphasage vibrationnel ainsi que l'influence de la transition de phase du méthane solide à 20 K sur la dynamique vibrationnelle. Nous avons également montré que la dynamique vibrationnelle était dépendante du site cristallographique dans lequel est piégée la molécule. Enfin, en excitant plusieurs modes de vibration simultanément, nous avons pu examiner les couplages intramoléculaires.

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50

Ruess, Frank Joachim Physics Faculty of Science UNSW. "Atomically controlled device fabrication using STM." Awarded by:University of New South Wales. Physics, 2006. http://handle.unsw.edu.au/1959.4/24855.

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We present the development of a novel, UHV-compatible device fabrication strategy for the realisation of nano- and atomic-scale devices in silicon by harnessing the atomic-resolution capability of a scanning tunnelling microscope (STM). We develop etched registration markers in the silicon substrate in combination with a custom-designed STM/ molecular beam epitaxy system (MBE) to solve one of the key problems in STM device fabrication ??? connecting devices, fabricated in UHV, to the outside world. Using hydrogen-based STM lithography in combination with phosphine, as a dopant source, and silicon MBE, we then go on to fabricate several planar Si:P devices on one chip, including control devices that demonstrate the efficiency of each stage of the fabrication process. We demonstrate that we can perform four terminal magnetoconductance measurements at cryogenic temperatures after ex-situ alignment of metal contacts to the buried device. Using this process, we demonstrate the lateral confinement of P dopants in a delta-doped plane to a line of width 90nm; and observe the cross-over from 2D to 1D magnetotransport. These measurements enable us to extract the wire width which is in excellent agreement with STM images of the patterned wire. We then create STM-patterned Si:P wires with widths from 90nm to 8nm that show ohmic conduction and low resistivities of 1 to 20 micro Ohm-cm respectively ??? some of the highest conductivity wires reported in silicon. We study the dominant scattering mechanisms in the wires and find that temperature-dependent magnetoconductance can be described by a combination of both 1D weak localisation and 1D electron-electron interaction theories with a potential crossover to strong localisation at lower temperatures. We present results from STM-patterned tunnel junctions with gap sizes of 50nm and 17nm exhibiting clean, non-linear characteristics. We also present preliminary conductance results from a 70nm long and 90nm wide dot between source-drain leads which show evidence of Coulomb blockade behaviour. The thesis demonstrates the viability of using STM lithography to make devices in silicon down to atomic-scale dimensions. In particular, we show the enormous potential of this technology to directly correlate images of the doped regions with ex-situ electrical device characteristics.

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Dissertations / Theses on the topic 'Dephasing'

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