Дисертації: "Distinct charge density wave"

1

Gaspar, Luis Alejandro Ladino. "CHARGE DENSITY WAVE POLARIZATION DYNAMICS." UKnowledge, 2008. http://uknowledge.uky.edu/gradschool_diss/643.

Повний текст джерела

Анотація:

We have studied the charge density wave (CDW) repolarization dynamics in blue bronze (K0.3MoO3) by applying symmetric bipolar square-wave voltages of different frequencies to the sample and measuring the changes in infrared transmittance, proportional to CDW strain. The frequency dependence of the electro-transmittance was fit to a modified harmonic oscillator response and the evolution of the parameters as functions of voltage, position, and temperature are discussed. We found that resonance frequencies decrease with distance from the current contacts, indicating that the resulting delays are intrinsic to the CDW with the strain effectively flowing from the contact. For a fixed position, the average relaxation time for most samples has a voltage dependence given by τ0 ∼ V −p, with 1 < p < 2. The temperature dependence of the fitting parameters shows that the dynamics are governed by both the force on the CDW and the CDW current: for a given force and position, both the relaxation and delay times are inversely proportional to the CDW current as temperature is varied. The long delay times (∼ 100 μs) for large CDW currents suggest that the strain response involves the motion of macroscopic objects, presumably CDW phase dislocation lines. We have done frequency domain simulations to study charge-density-wave (CDW) polarization dynamics when symmetric bipolar square current pulses of different frequencies and amplitudes are applied to the sample, using parameters appropriate for NbSe3 at T = 90 K. The frequency dependence of the strain at one fixed position was fit to the same modified harmonic oscillator response and the behavior of the parameters as functions of current and position are discussed. Delay times increase nonlinearly with distance from the current contacts again, indicating that these are intrinsic to the CDWwith the strain effectively flowing from the contact. For a fixed position and high currents the relaxation time increases with decreasing current, but for low currents its behavior is strongly dependent on the distance between the current contact and the sample ends. This fact clearly shows the effect of the phase-slip process needed in the current conversion process at the contacts. The relaxation and delay times computed (∼ 1 μs) are much shorter than observed in blue bronze (> 100 μs), as expected because NbSe3 is metallic whereas K0.3MoO3 is semiconducting. While our simulated results bear a qualitative resemblance with those obtained in blue bronze, we can not make a quantitative comparison with the K0.3MoO3 results since the CDW in our simulations is current driven, whereas the electro-optic experiment was voltage driven. Different theoretical models predict that for voltages near the threshold Von, quantities such as the dynamic phase velocity correlation length and CDW velocity vary as ξ ∼ |V/Von − 1| −ν and v ∼ |V/Von − 1|ξ with ν ∼ 1/2 and ζ = 5/6. Additionally, a weakly divergent behavior for the diffusion constant D ∼ |V/Von − 1|−2ν+ζ is expected. Motivated by these premises and the fact that no convincing experimental evidence is known, we carried out measurements of the parameters that govern the CDW repolarization dynamic for voltages near threshold. We found that for most temperatures considered the relaxation time still increases for voltages as small as 1.06Von indicating that the CDW is still in the plastic and presumably in the noncritical limit. However, at one temperature we found that the relaxation time saturates with no indication of critical behavior, giving a new upper limit to the critical regime, of |V/Von − 1| < 0.06.

Стилі APA, Harvard, Vancouver, ISO та ін.

2

Rai, Ram C. "ELECTRO-OPTICAL STUDIES OF CHARGE-DENSITY-WAVE MATERIALS." UKnowledge, 2004. http://uknowledge.uky.edu/gradschool_diss/427.

Повний текст джерела

Анотація:

A searched for narrow-band-noise (NBN) modulations of the infrared transmission in blue bronze has been performed. No modulations were observed, giving an upper limits for NBN changes in the absorption coefficient of )2000/(/3.0.andlt;.cmNBN. The implication of these results on proposed CDW properties and NBN mechanisms are discussed. An infrared microscope with a capability of doing both reflectance and transmission measurements has been integrated into the previous electro-transmission system with tunable diode lasers. Electro-optic experiments were done using the microscope for the studies of the CDW states of K0.3MoO3 (blue bronze) and orthorhombic TaS3. The electro-reflectance signal for blue bronze has been evidenced for the first time. The infrared reflectance of K0.3MoO3 varied with position when a voltage greater than the CDW depinning threshold is applied. The spatial dependence of .R/R was slightly different than for ./, in that the magnitude of .R/R decreased and, for low voltages and frequencies, the signal became inverted near the contacts. Perhaps the differences might be associated with changes in the CDW properties on the surface. For blue bronze, the electro-reflectance signal was measured to be smaller than electro-transmittance signal by one order of magnitude for light polarized transverse to the chain direction, while the electro-reflectance signal for parallel polarized light was found to be a few times smaller than for transverse polarized light. The fits of the electro-reflectance spectrum showed that the changes in background dielectric constant were ~ 0.05 % and/or oscillator strength and/or frequency shifts of the phonons were ~ 0.05 % and ~ 0.005 cm-1 in the applied electric field. We also found that parallel polarized phonons are affected by CDW strain, and these changes dominate the electro-reflectance spectrum. We have examined the electro-reflectance spectra associated with CDW current investigation for light polarized parallel to the conducting chains for signs of expected current-induced intragap states, and conclude that the density of any such states is at most a few times less than expected. We have observed a large (~1%) change in infrared reflectance of orthorhombic TaS3, when its CDW is depinned. The change is concentrated near one current contact. Assuming that the change in reflectance is proportional to the degree of CDW polarization, we have studied the dynamics of CDW repolarization through position dependent measurements of the variation of the electro-reflectance with the frequency of square wave voltages applied to the sample, and have found that the response could be characterized as a damped harmonic oscillator with a distribution of relaxation (i.e. damping) times. The average relaxation time, which increases away from the contacts, varies with applied voltage as with p ~ 3/2, but the distribution of times broadens as the voltage approaches the depinning threshold. Very low resonant frequencies (~ 1 kHz) indicate a surprisingly large amount of inertia, which is observable in the time dependence of the change in reflectance as a polarity dependent delay of ~ 100 s.

Стилі APA, Harvard, Vancouver, ISO та ін.

3

Ru, Nancy. "Charge density wave formation in rare-earth tritellurides /." May be available electronically:, 2008. http://proquest.umi.com/login?COPT=REJTPTU1MTUmSU5UPTAmVkVSPTI=&clientId=12498.

Повний текст джерела

Стилі APA, Harvard, Vancouver, ISO та ін.

4

Hite, Omar. "Controlling the Charge Density Wave in VSE2 Containing Heterostructures." Thesis, University of Oregon, 2018. http://hdl.handle.net/1794/23179.

Повний текст джерела

Анотація:

Exploring the properties of layered materials as a function of thickness has largely been limited to semiconducting materials as thin layers of metallic materials tend to oxidize readily in atmosphere. This makes it challenging to further understand properties such as superconductivity and charge density waves as a function of layer thickness that are unique to metallic compounds. This dissertation discusses a set of materials that use the modulated elemental reactants technique to isolate 1 to 3 layers of VSe2 in a superlattice in order to understand the role of adjacent layers and VSe2 thickness on the charge density wave in VSe2. The modulated elemental reactants technique was performed on a custom built physical vapor deposition to prepare designed precursors that upon annealing will self assemble into the desired heterostructure. First, a series of (PbSe)1+δ(VSe2)n for n = 1 – 3 were synthesized to explore if the charge density wave enhancement in the isovalent (SnSe)1.15VSe2 was unique to this particular heterostructure. Electrical resistivity measurements show a large change in resistivity compared to room temperature resistivity for the n = 1 heterostructure. The overall change in resistivity was larger than what was observed in the analogous SnSe heterostructure. v A second study was conducted on (BiSe)1+δVSe2 to further understand the effect of charge transfer on the charge density wave of VSe2. It was reported that BiSe forms a distorted rocksalt layer with antiphase boundaries. The resulting electrical resistivity showed a severely dampened charge density wave when compared to both analogous SnSe and PbSe containing heterostructures but was similar to bulk. Finally, (SnSe2)1+δVSe2 was prepared to further isolate the VSe2 layers and explore interfacial effects on the charge density wave by switching from a distorted rocksalt structure to 1T-SnSe2. SnSe2 is semiconductor that is used to prevent adjacent VSe2 layers from coupling and thereby enhancing the quasi two-dimensionality of the VSe2 layer. Electrical characterization shows behavior similar to that of SnSe and PbSe containing heterostructures. However, structural characterization shows the presence of a SnSe impurity that is likely influencing the overall temperature dependent resistivity. This dissertation includes previously published and unpublished co-authored materials.

Стилі APA, Harvard, Vancouver, ISO та ін.

5

Boshoff, Ilana. "Ultrafast electron diffraction on the charge density wave compound 4Hb-TaSe2." Thesis, Stellenbosch : Stellenbosch University, 2012. http://hdl.handle.net/10019.1/20062.

Повний текст джерела

Анотація:

Thesis (MSc)--Stellenbosch University, 2012.
ENGLISH ABSTRACT: Ultrafast electron diffraction is a powerful method to study atomic movement in crystals on sub-picosecond timescales. This thesis consists of three parts. In part one the ultrafast electron diffraction machine is described, followed by improvements that were made and techniques that were developed in order to bring the system to state of the art level and enable the acquisition of suffcient data to obtain information on the structural dynamics in crystals. The second part contains a description of the sample which was studied in our fi rst time-resolved measurements, the transition-metal dichalcogenide 4Hb-TaSe2. This particular crystal is an example of a strongly coupled electronic system which develops a charge density wave (CDW) accompanied by a periodic lattice distortion (PLD). An overview of the formation of electron diffraction patterns and what can be learned from them are also given, followed by the results of the ultrafast electron diffraction experiments done with 4Hb-TaSe2. Part three describes an alternative source to study dynamics in crystalline samples, namely laser plasma-based ultrafast X-ray diffraction. The ultrafast electron diffraction group functions as a unit, but my tasks ranged from sample preparation and characterisation of the electron beam to the setting up and execution of experiments. I was involved in analysing the data and contributed small parts to the data analysis software.
AFRIKAANSE OPSOMMING: Ultravinnige elektron diffraksie is a metode om die beweging van atome in kristalle op sub-pikosekonde tydskale te bestudeer. Hierdie tesis bestaan uit drie dele. In deel een van die tesis word die ultravinnige elektron diffraksie masjien beskryf, gevolg deur verbeteringe wat aangebring is en tegnieke wat ontwikkel is om die sisteem tot op 'n wêreldklas vlak te bring waar die insameling van genoegsame data om inligting oor die strukturele dinamika in kristalle te bekom, moontlik is. Die tweede deel bevat 'n beskrywing van die monster wat in ons eerste tydopgeloste eksperimente gebruik is, naamlik die oorgangsmetaaldichalkogenied 4Hb-TaSe2. Hierdie kristal is 'n voorbeeld van 'n sterk gekoppelde elektroniese sisteem wat 'n ladingsdigtheid-golf en 'n gepaardgaande periodiese versteuring van die kristalrooster ontwikkel. 'n Oorsig van die formasie van elektron diffraksiepatrone en wat ons daaruit kan leer word ook gegee. Daarna word die resultate van die ultravinnige elektron diffraksie eksperimente wat op 4Hb-TaSe2uitgevoer is beskryf en bespreek. In deel drie word 'n alternatiewe metode om die dinamika in kristalmonsters te bestudeer, naamlik laser plasma-gebaseerde ultravinnige X-straal diffraksie, beskryf. Die ultravinnige elektron diffraksie groep funksioneer as 'n eenheid, maar my verantwoordelikhede het gestrek van die voorbereiding van monsters en die karakterisering van die elektron bundel tot die opstel en uitvoer van eksperimente. Ek was ook betrokke by die analisering van data en het dele van die data analise sagteware geskryf.

Стилі APA, Harvard, Vancouver, ISO та ін.

6

Yetman, Paul John. "Experimental studies on the size dependence of sliding charge-density wave phenomena." Thesis, University of Bristol, 1990. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.279769.

Повний текст джерела

Стилі APA, Harvard, Vancouver, ISO та ін.

7

Ren, Yuhang. "Time-resolved optical studies of colossal magnetoresistance and charge -density wave materials." W&M ScholarWorks, 2003. https://scholarworks.wm.edu/etd/1539623421.

Повний текст джерела

Анотація:

This thesis presents measurements of collective modes and ultrafast carrier relaxation dynamics in charge-density-wave (CDW) conductors and colossal magnetoresistance (CMR) manganites. A femtosecond laser pump pulse excites a broad frequency spectrum of low-energy collective modes and electron-hole pairs thereby changing its optical properties. The low-energy collective excitations and quasiparticle relaxation and recombination processes are monitored by measuring the resulting photoinduced absorption as a function of probe pulse wavelength and time delay.;A general model was developed for the photogeneration and detection mechanism of collective modes based on light absorption in two-color pump-probe experiments. A broad spectrum of collective modes (phasons and amplitudons) with frequencies down to a few GHz is excited and propagates normal to the surface into the material. The dispersion of the long-wavelength phason and amplitudon can be measured by changing the probe wavelength.;The first pump-probe spectroscopy was performed from the ultraviolet to mid-infrared wavelength range to study low-frequency collective excitations, including temperature evolution, dispersion, damping, and anisotropy of amplitude mode and transverse phason in quasi-one dimensional CDW conductors, K 0.3MoO3 and K0.33MoO3 on ultrafast time scale. The transverse phason exhibits an acoustic-like dispersion relation in the frequency range from 5--40 GHz. The phason velocity is strongly anisotropic with a very weak temperature dependence. In contrast, the amplitude mode exhibits a weak (optic-like) dispersion relation with a frequency of 1.66 THz at 30 K.;The studies were extended to doped perovskite manganite thin films and single crystals. A low-energy collective mode is observed and discussed in terms of the opening of a pseudogap resulting from charge/orbital ordering phases. The softening of the collective mode is necessary to explain by combining a cooperative Jahn-Teller type distortion of the MnO6 octahedra with the collective mode. The quasiparticle dynamics in the vicinity of the metal-insulator transition is strongly affected by the presence of a pseudogap, phase separation and percolation, which are strongly dependent on temperature. A very long-lived relaxation process is observed due to a slow spin relaxation process. The dynamics of the spin system is further investigated in strained and unstrained thin films, which show a strong strain effect.

Стилі APA, Harvard, Vancouver, ISO та ін.

8

Bellec, Ewen. "Study of charge density wave materials under current by X-ray diffraction." Thesis, Université Paris-Saclay (ComUE), 2019. http://www.theses.fr/2019SACLS437/document.

Повний текст джерела

Анотація:

Ce manuscrit a pour sujet principal la diffraction par rayons X des matériaux ondes de densité de charges (ODC). Nous avons étudié le cristal quasi-1D NbSe3 ainsi que le quasi-2D TbTe3. Plusieurs grands instruments ont été utilisés pour cette étude, le synchrotron ESRF de Grenoble sur la ligne ID01 ainsi que le laser à électron libre LCLS à Stanford. Premièrement, grâce à la cohérence du faisceau X à LCLS, nous avons pu observer une perte de cohérence transverse dans NbSe3 lors de l’application d’un courant électrique au-dessus d’un certain seuil ainsi qu’une compression longitudinale de l’ODC. Ensuite, à l’ESRF, nous avons utilisé un faisceau X focalisé au micromètre par une Fresnel zone plate pour scanner l’ODC localement par diffraction sur NbSe3 puis ensuite sur TbTe3. Lorsqu’un courant est appliqué sur l’échantillon, nous avons observé une déformation transverse indiquant que l’ODC est bloquée au niveau de la surface de l’échantillon dans NbSe3. Dans le cas de TbTe3, l’ODC tourne sous courant présentant un cycle d’hystérésis lorsque le courant passe continument de positif à négatif. Nous avons aussi pu constater dans plusieurs régions, toujours pour TbTe3, la création de défauts d’irradiation localisés induisant une compression-dilatation de l’ODC. Dans une dernière partie théorique, nous montrons comment la théorie du transport électrique de l’ODC par un train de solitons portants chacun une charge ainsi que la prise en compte du blocage de l’ODC sur la surface de l’échantillon que nous avons vu expérimentalement permet de comprendre plusieurs mesures de résistivité en fonction des dimensions de l’échantillon trouvées dans la littérature. Nous présentons ensuite plusieurs idées pour expliquer du blocage de l’ODC sur les surfaces au niveau microscopique et proposons l’hypothèse d’une ODC commensurable en surface (et incommensurable dans le volume)
The main subject of this manuscript is the X-ray diffraction of charge density wave (CDW) materials. We studied the quasi-1D NbSe3 crystal and the quasi-2D TbTe3. Several large instruments facilities were used for this study, the ESRF synchrotron in Grenoble on the ID01 line and the LCLS free electron laser in Stanford. First, thanks to the coherence of the X-beam at LCLS, we were able to observe a loss of transverse coherence in NbSe3 when applying an electrical current above a certain threshold as well as a longitudinal compression of the CDW. Then, at the ESRF, we used an X-ray beam focused on the micrometer scale by a Fresnel zone plate to scan the CDW locally by diffraction on NbSe3 and on TbTe3. When a current is applied to the sample, we observed a transverse deformation indicating that the CDW is pinned on the sample surface in NbSe3. In the case of TbTe3, the CDW rotates under current showing a hysteresis cycle when one is continuously changing from positive to negative current. We have also observed in several regions, in TbTe3, the creation of localized irradiation defects inducing a compression-dilation of the CDW. In a last theoretical part, we show how the theory of electric transport in the CDW state by a train of charged solitons, as well as taking into account the CDW pinning on the surface of the sample that we have seen experimentally, allows us to understand several resistivity measurements, found in the literature, made on samples with different dimensions. Finally, we present several ideas for an explanation of the CDW pinning at the surfaces on a microscopic level and propose the hypothesis of a commensurate CDW on the surface (and incommensurate in volume)

Стилі APA, Harvard, Vancouver, ISO та ін.

9

Edkins, Stephen David. "Visualising the charge and Cooper pair density waves in cuprates." Thesis, University of St Andrews, 2016. http://hdl.handle.net/10023/9888.

Повний текст джерела

Анотація:

The study of cuprate high-temperature superconductors has undergone a recent resurgence due to the discovery of charge order in several families of cuprate materials. While its existence is now well established, little is known about its microscopic origins or its relationship to high-temperature superconductivity and the pseudogap. The aim of the research presented in this thesis is to address these questions. In this thesis I will report on the use of spectroscopic-imaging scanning tunnelling microscopy (SI-STM) to visualise the short-ranged charge density wave (CDW) in Bi₂Sr₂CaCu₂O₈₊ₓ and NaxCa₂₋ₓCuO₂Cl₂. Building on previous measurements of the intra unit-cell electronic structure of cuprates, I introduce sub-lattice segregated SISTM to individually address the atomic sub-lattices in the CuO₂ plane with spatial phase sensitivity. Using this technique I establish that the CDW in Bi₂Sr₂CaCu₂O₈+x and NaxCa₂₋ₓCuO₂Cl₂ has a previously unobserved d-symmetry form factor, where a breaking of rotational symmetry within the unit cell is modulated periodically in space. Towards identifying a mechanism of CDW formation, I establish that the amplitude of CDW modulations in the electronic structure are maximal at the pseudogap energy-scale and that these modulations exhibit a spatial phase difference of π between filled and empty states. Together with the doping evolution of the CDW wave-vector this highlights the role of the low-energy electronic structure of the pseudogap regime in CDW formation. To elucidate the relationship between the CDW and the superconducting condensate I will introduce nanometer resolution scanned Josephson tunnelling microscopy (SJTM). In this approach the Cooper pair (Josephson) tunnelling current between a Bi₂Sr₂CaCu₂O₈₊ₓ sample and a scan-able Bi₂Sr₂CaCu₂O₈₊ₓ nano-flake STM tip is used to directly visualise the superconducting condensate. I will report the observation of a periodic modulation in the Cooper pair condensate at the same wave-vector as the CDW, the first direct detection of a periodically modulating condensate in any superconductor.

Стилі APA, Harvard, Vancouver, ISO та ін.

10

Yi, Tianyou. "Modeling of dynamical vortex states in charge density waves." Phd thesis, Université Paris Sud - Paris XI, 2012. http://tel.archives-ouvertes.fr/tel-00768237.

Повний текст джерела

Анотація:

Formation of charge density waves (CDW) is a symmetry breaking phenomenon found in electronic systems, which is particularly common in quasi-one-dimensional conductors. It is widely observed from highly anisotropic materials to isotropic ones like the superconducting pnictides. The CDW is seen as a sinusoidal deformation of coupled electronic density and lattice modulation; it can be also viewed as a crystal of singlet electronic pairs. In the CDW state, the elementary units can be readjusted by absorbing or rejecting pairs of electrons. Such a process should go via topologically nontrivial configurations: solitons and dislocations - the CDW vortices. An experimental access to these inhomogeneous CDW states came from studies of nano-fabricated mesa-junctions, from the STM and from the X-ray micro-diffraction. Following these requests, we have performed a program of modeling stationary states and of their transient dynamic for the CDW in restricted geometries under applied voltage or at passing normal currents. The model takes into account multiple fields in mutual nonlinear interactions: the two components of the CDW complex order parameter, and distributions of the electric field, the density and the current of normal carriers. We were using the Ginzburg-Landau type approach and also we have derived its extension based on the property of the chiral invariance. We observed the incremental creation of static dislocations within the junctions. The transient dynamics is very rich showing creation, annihilation and sweeping of multiple vortices. The dislocations cores concentrate the voltage drops thus providing self-tuned microscopic junctions where the tunneling creation of electron-hole pairs can take place. The results obtained from this model agree with experiment observations. The methods can be extended to other types of charge organization known under the general name of the Electronic Crystal. It takes forms of Wigner crystals at hetero-junctions and in nano-wires, CDWs in chain compounds, spin density waves in organic conductors, and stripes in doped oxides. The studied reconstruction in junctions of the CDW may be relevant also to modern efforts of the field-effect transformations in strongly correlated materials with a spontaneous symmetry breaking.

Стилі APA, Harvard, Vancouver, ISO та ін.

11

Latt, Kyaw Zin. "Manipulation of Molecular Charge Density Waves and Molecular Transport Systems." Ohio University / OhioLINK, 2019. http://rave.ohiolink.edu/etdc/view?acc_num=ohiou1557418915977344.

Повний текст джерела

Стилі APA, Harvard, Vancouver, ISO та ін.

12

Ramakrishnan, Sitaram [Verfasser], and Smaalen Sander [Akademischer Betreuer] van. "Charge-density-wave ordering in three-dimensional metallic compounds / Sitaram Ramakrishnan ; Betreuer: Sander van Smaalen." Bayreuth : Universität Bayreuth, 2020. http://d-nb.info/1213348951/34.

Повний текст джерела

Стилі APA, Harvard, Vancouver, ISO та ін.

13

ROMANIN, DAVIDE. "Electron-phonon interactions in low-dimensional carbon systems: superconductivity and charge-density waves." Doctoral thesis, Politecnico di Torino, 2021. http://hdl.handle.net/11583/2872341.

Повний текст джерела

Стилі APA, Harvard, Vancouver, ISO та ін.

14

Rocha, Matthew Paul. "A high-resolution study of the electronic structure of NbSe3 in normal and charge density wave states with angle resolved photoemission spectroscopy /." view abstract or download file of text, 2003. http://wwwlib.umi.com/cr/uoregon/fullcit?p3113025//.

Повний текст джерела

Анотація:

Thesis (Ph. D.)--University of Oregon, 2003.
Typescript. Includes vita and abstract. Includes bibliographical references (leaves 166-167). Also available for download via the World Wide Web; free to University of Oregon users.

Стилі APA, Harvard, Vancouver, ISO та ін.

15

Sellam, Mohamed Amine. "Competition between charge density wave and metallic/superconducting states in metastable 1T-TaS2 and 1T-VS2 synthesised under high pressure." Paris 6, 2011. http://www.theses.fr/2011PA066585.

Повний текст джерела

Анотація:

L’objectif de cette Thèse est une étude de la stabilité des phases d’onde de densité de charge (CDW) et supraconductrice en compétition dans les dichalcogénures lamellaires de métaux de transition. Dans ce but, nous avons employé une technique de synthèse sous haute pression pour induire des modifications de la structure cristalline dans deux composés 1T-TaS2 et 1T-VS2 ayant la même structure 1T (ou de type CdI2) et la même configuration électronique d1. Dans le cas de 1T-TaS2, notre étude de diffraction de rayons X synchrotron et de neutrons sur poudre montre que des conditions de synthèse de 2-6 GPa et 400 °C induisent un désordre dans les plans S et une contraction (expansion) notable de l’axe a dans les plans (c hors plan). Ceci suggère un modèle de désordre d’orientation de ces plans correspondant à l’empilement aléatoire de couches TaS2 de type 1T ou 2H qui correspondent à une coordinance octaédrique ou trigonal prismatique des ions Ta. Ce modèle est en accord avec nos mesures magnétiques et de transport. Cette phase désordonnée est caractérisée par deux phases CDW, une non commensurée, l’autre quasi-commensurée (NCCDW, ICCDW), observées à température ambiante et à basse température respectivement. Ces phases diffèrent de celles observées dans les composés 1T ou 2H synthétisés sous conditions standard. Cette différence met en évidence la métastabilité du composé synthétisé sous haute pression et explique son comportement paramagnétique et métallique/supraconducteur avec Tc ~ 2. 5 K. Afin d’étudier les propriétés supraconductrices, nous avons intercalé le carbone entre les plans à des concentrations x = 0. 05-0. 3 sous les mêmes conditions de pression et de température. Une augmentation notable de Tc a été observée. Notamment, la dépendance de Tc avec x montre un maximum de 4 K pour x=0. 2. Considérant que l’électronégativité du carbone est identique à celle du souffre, ce résultat démontre que la stabilisation de la phase supraconductrice est uniquement due à la séparation des plans et l’effet du dopage sujet à controverse dans la littérature peut être exclu. Dans le cas de 1T-VS2, nous avons réussi la synthèse de la phase stoichiométrique de haute pureté sous forme de poudre à des conditions de haute pression (P = 4 GPa) et à T = 700 °C. Des travaux précédents concernent uniquement la synthèse par dé-intercalation de Li à partir de LiVS2, donc ce résultat montre que 1T-VS2 est une phase métastable à haute pression. Dans ce cas la synthèse à haute pression produit un composé bien cristallisé et sans désordre, ce qui a permis un affinement structural de haute qualité entre 5 et 300 K par diffraction de rayons X synchrotron. Les données montrent que la structure reste de type CdI2 dans toute cette gamme de température. Cependant, une expansion thermique négative de l’axe c est observée dans la gamme 50-150 K, ce qui reflète une augmentation anormale de la distance V-S. Cette anomalie est accompagnée d’un crossover marqué du comportement Curie-Weiss; à haute température la constante de Weiss  = – 43 K révèle un échange antiférromagnétique qui devient négligeable à basse température, alors que le moment localisé et le terme paramagnétique de Pauli restent inchangés. Cela reflète un changement dans les propriétés de cohésion de l’orbitale hybride 3d(V)-3p(S). Le crossover de régime magnétique est corrélé à un crossover de régime de transport; la résistivité électrique dc montre un comportement métallique à haute température et faiblement isolante au dessous de 200 K. Ce résultat contredit la prédiction d’une phase métallique très stable selon des calculs ab initio dans le cadre de la théorie de la fonctionelle densité. Ce désaccord conduit à formuler l’hypothèse que l’interaction d’échange joue un rôle déterminant sur l’état fondamental et que un état métallique peut être stabilisé sous haute pression

Стилі APA, Harvard, Vancouver, ISO та ін.

16

Souza, Valdeci Pereira Mariano de [UNESP]. "Estudo das propriedades elétricas não lineares de polímeros conjugados." Universidade Estadual Paulista (UNESP), 2003. http://hdl.handle.net/11449/91942.

Повний текст джерела

Анотація:

Made available in DSpace on 2014-06-11T19:25:32Z (GMT). No. of bitstreams: 0 Previous issue date: 2003-02-18Bitstream added on 2014-06-13T20:27:50Z : No. of bitstreams: 1 souza_vpm_me_rcla.pdf: 535132 bytes, checksum: d3bce6e40a2c0ef0a0bc8cc227fe99dd (MD5)
Neste trabalho, apresentamos um estudo experimental das propriedades elétricas não lineares de polímeros conjugados, como por exemplo, curvas I(V) não lineares para baixos valores de campo elétrico aplicado e existência de um campo elétrico de threshold. Investigamos o comportamento elétrico não linear em pastilhas prensadas de poli(3-metiltiofeno) (P3MT) oxidadas, obtidas através do processo de síntese eletroquímica. Das medidas elétricas realizadas, na faixa de temperatura (~9 K - ~297 K), os resultados experimentais obtidos: curvas I(V), condutividade versus freqüência e constante dielétrica versus freqüência, foram comparados com os diversos modelos teóricos existentes na literatura. Os dados obtidos em toda a faixa de temperatura mencionada mostraram boa concordância com a teoria de tunelamento para CDW (charge density wave deppining) proposta por J. Bardeen.
In this work, we present an experimental study of the non-linear electrical properties in conjugated polymers, as for example, non-linear I(V) curves at low electric fields and existence of a threshold electric field. We investigated the non linear electrical behavior of pressed pellets of poly(3-methilthiopene) (P3MT), which were obtained through electrochemical synthesis oxidized. From the electrical measurements, in the temperature range (~9K - ~297K), the experimental results: I(V) curves, conductivity versus frequency and dielectric constant versus frequency, were compared with the several theoretical models discussed in the literature. The data obtained in the whole temperature range have shown good agreement on the tunneling theory for CDW systems (charge density wave depinning) proposed by J. Bardeen.

Стилі APA, Harvard, Vancouver, ISO та ін.

17

Ren, Zhensong. "Combined neutron, transport and material based investigation in Ca₃Ir₄Sn₁₃." Thesis, Boston College, 2015. http://hdl.handle.net/2345/bc-ir:104538.

Повний текст джерела

Анотація:

Thesis advisor: Stephen D. Wilson
This dissertation investigates the cubic type II superconductor, Ca₃Ir₄Sn₁₃, discovered by Remeika and the coauthors more than 30 years ago. It was originally discovered be to a superconductor and later suggested to host ferromagnetic spin fluctuations, which lead to a peak-like anomaly in thermodynamic and transport measurements. Later detailed x-ray single crystal structural refinement associated the peak-like anomaly in transport and magnetization measurements with a charge density wave phase transition at the same temperature. The potential charge density wave phase transition T* can be suppressed either by pressure or chemical potential through substitution on the Ca and Ir site such that a temperature-pressure/composition phase diagram can be constructed. Upon investigating magnetism in this compound, polarized neutron scattering and μSR data from our group and other researchers did not reveal any magnetic order or magnetic spin fluctuations at the time scale of μSR . However, through the partial substitution of Ir by Rh, we realized a structural quantum critical point at ambient pressure with 30% of Ir substituted by Rh--providing the research community a valuable material's platform for studying the interplay between 3D charge density wave order and superconductivity. On the other hand, our surprising discovery of the weak HHL (L=odd) type of super-lattice peaks from neutron scattering led us to a tentative model of a distorted Ca sublattice in this material. The similarity of the lattice instabilites of the Remeika compound and A15 superconductors are discussed, which may give us more insight into its role in the formation of the superconducting phase
Thesis (PhD) — Boston College, 2015
Submitted to: Boston College. Graduate School of Arts and Sciences
Discipline: Physics

Стилі APA, Harvard, Vancouver, ISO та ін.

18

Saint-Lager, Marie-Claire. "Etude de la cohérence de phase des ondes de densité de charge." Grenoble 1, 1988. http://www.theses.fr/1988GRE10090.

Повний текст джерела

Анотація:

Lorsque l'onde de densite de charge (odc) est en mouvement, la coherence de phase tend a etre de l'ordre de la longueur de l'echantillon. A 3 d, le raccordement entre deux regions de vitesse differente est assure par des dislocations de phase se deplacant dans le plan perpendiculaire au mouvement de l'odc. Un modele est propose, ou les dislocations, comme l'odc elle-meme, sont sensibles aux impuretes. La condition de raccordement est gouvernee par leur piegeage qui est represente par un terme de frottement statique

Стилі APA, Harvard, Vancouver, ISO та ін.

19

Jurczek, Edmund. "A local charge density wave instability in alloys with strong electron phonon coupling with application to Ba Bi-(1-x) Pb-x 0-3 /." Zürich, 1985. http://e-collection.ethbib.ethz.ch/show?type=diss&nr=7880.

Повний текст джерела

Стилі APA, Harvard, Vancouver, ISO та ін.

20

Al-Hadeethi, Ali Khairi Abdul-Qader [Verfasser], and Christine A. [Akademischer Betreuer] Kuntscher. "Dc transport and optical measurements on the charge-density-wave compounds (K,Rb)0.3(Mo,W)O3 / Ali Khairi Abdul-Qader Al-Hadeethi. Betreuer: Christine A. Kuntscher." Augsburg : Universität Augsburg, 2013. http://d-nb.info/107770285X/34.

Повний текст джерела

Стилі APA, Harvard, Vancouver, ISO та ін.

21

Chen, Mengsu. "Exact diagonalization study of strongly correlated topological quantum states." Diss., Virginia Tech, 2019. http://hdl.handle.net/10919/87436.

Повний текст джерела

Анотація:

A rich variety of phases can exist in quantum systems. For example, the fractional quantum Hall states have persistent topological characteristics that derive from strong interaction. This thesis uses the exact diagonalization method to investigate quantum lattice models with strong interaction. Our research topics revolve around quantum phase transitions between novel phases. The goal is to find the best schemes for realizing these novel phases in experiments. We studied the fractional Chern insulator and its transition to uni-directional stripes of particles. In addition, we studied topological Mott insulators with spontaneous time-reversal symmetry breaking induced by interaction. We also studied emergent kinetics in one-dimensional lattices with spin-orbital coupling. The exact diagonalization method and its implementation for studying these systems can easily be applied to study other strongly correlated systems.
PHD
Topological quantum states are a new type of quantum state that have properties that cannot be described by local order parameters. These types of states were first discovered in the 1980s with the integer quantum Hall effect and the fractional quantum Hall effect. In the 2000s, the predicted and experimentally discovered topological insulators triggered studies of new topological quantum states. Studies of strongly correlated systems have been a parallel research topic in condensed matter physics. When combining topological systems with strong correlation, the resulting systems can have novel properties that emerge, such as fractional charge. This thesis summarizes our work that uses the exact diagonalization method to study topological states with strong interaction.

Стилі APA, Harvard, Vancouver, ISO та ін.

22

Erasmus, Nicolas. "Ultrafast structural dynamics in 4Hb-TaSe2 observed by femtosecond electron diffraction." Thesis, Stellenbosch : Stellenbosch University, 2013. http://hdl.handle.net/10019.1/79934.

Повний текст джерела

Анотація:

Thesis (PhD)--Stellenbosch University, 2013.
ENGLISH ABSTRACT: In this thesis the structural dynamics, upon photo-excitation, of the charge-densitywave (CDW) material 4Hb-TaSe2 is investigated on the time-scale of atomic motion and simultaneously on the spatial-scale of atomic dimensions. CDW materials have been of interest since their discovery in the 1970’s because of their remarkable non-linear and anisotropic electrical properties, gigantic dielectric constants, unusual elastic properties and rich dynamical behaviour. Some of these exotic properties were extensively investigated in thermal equilibrium soon after their discovery but only recently have ultrafast techniques like femtosecond spectroscopy become available to study their out-of-equilibrium behaviour on the time-scale of atomic motion. By studying their behaviour on this time-scale a more in-depth understanding of their macroscopic properties can be gained. However, to do investigations on the atomic time-scale and simultaneously directly observe the evolution of the atomic arrangements is another challenge. One approach is through the previously mentioned technique of femtosecond pump-probe spectroscopy but converting the usual ultrashort optical probing source to an ultrashort electron or x-ray source that can diffract off the sample and reveal structural detail on the atomic level. Here, the femto-to-picosecond out-of-equilibrium behaviour upon photo-excitation in 4Hb-TaSe2 is investigated using an ultrashort electron probe source. Two variations of using an electron probe source are used: conventional scanning Femtosecond Electron Diffraction (FED) and a new approach namely Femtosecond Streaked Electron Diffraction (FSED). The more established FED technique, based on femtosecond pumpprobe spectroscopy, is used as the major investigating tool while the FSED technique, based on ultrafast streak camera technology, is an attempt at broadening the scope of available techniques to study structural dynamics in crystalline material on the subpicosecond time-scale. With these two techniques, the structural dynamics during the phase transition from the commensurate- to incommensurate-CDW phase in 4Hb-TaSe2 is observed through diffraction patterns with a temporal resolution of under 500 fs. The study reveals strong coupling between the electronic and lattice systems of the material and several time-constants of under and above a picosecond are extracted from the data. Using these time-constants, the structural evolution during the phase transition is better understood and with the newly gained knowledge, a model of all the processes involved after photo-excitation is proposed.
AFRIKAANSE OPSOMMING: In hierdie tesis word die strukturele dinamika van die lading-digtheid-golf (LDG) materiaal 4Hb-TaSe2 ondersoek op die tydskaal van atomiese bewegings en gelyktydig op die ruimtelikeskaal van atomiese dimensies. LDG materie is al van belang sedert hul ontdekking in die 1970’s as gevolg van hul merkwaardige nie-lineêre en anisotrope elektriese eienskappe, reuse diëlektriese konstantes, ongewone elastiese eienskappe en ryk dinamiese gedrag. Sommige van hierdie eksotiese eienskappe is omvattend ondersoek in termiese ewewig kort na hul ontdekking, maar eers onlangs is dit moontlik deur middle van ultravinnige tegnieke soos femtosekonde spektroskopie om hulle uit-ewewigs gedrag te bestudeer op die tydskaal van atomiese beweging. Deur die gedrag op hierdie tydskaal te bestudeer kan ’n meer insiggewende begrip van hul makroskopiese eienskappe verkry word. Om ondersoeke in te stel op die atomiese tydskaal en gelyktydig direk die evolusie van die atoom posisie te waarneem is egter ’n moeilike taak. Een benadering is deur middle van femtosekonde “pump-probe” spektroskopie maar dan die gewone optiese “probe” puls om te skakel na ’n electron of x-straal puls wat van die materiaal kan diffrak en dus strukturele inligting op die atomiese vlak kan onthul. Hier word die femto-tot-pico sekonde uit-ewewig gedrag in 4Hb-TaSe2 ondersoek met behulp van elektron pulse. Twee variasies van die gebruik van ’n elektron bron word gebruik: konvensionele “Femtosecond Electron Diffraction” (FED) en ’n nuwe benadering, naamlik, “Femtosecond Streaked Electron Diffraction” (FSED). Die meer gevestigde FED tegniek, wat gebaseer is op femtosekonde “pump-probe” spektroskopie, word gebruik as die hoof ondersoek metode terwyl die FSED tegniek, wat gebaseer is op die ultra vinnige “streak camera” tegnologie, ’n poging is om beskikbare tegnieke uit te brei wat gebruik kan word om strukturele dinamika in materie te bestudeer op die sub-picosekonde tydskaal. Met behulp van hierdie twee tegnieke, word die strukturele dinamika tydens die fase oorgang van die ooreenkomstige tot nie-ooreenkomstige LDG fase in 4Hb-TaSe2 deur diffraksie patrone met ’n tydresolusie van minder as 500 fs waargeneem. Die studie toon ’n sterk korrelasie tussen die elektroniese sisteem en kristalrooster. Verskeie tydkonstantes van onder en bo ’n picosekonde kon ook uit die data onttrek word en gebruik word om die strukturele veranderinge beter te verstaan. Hierdie nuwe kennis het ons in staat gestel om ’n model van al die betrokke prosesse voor te stel.

Стилі APA, Harvard, Vancouver, ISO та ін.

23

Miclea, Corneliu Florin. "Investigation of superconducting order parameters in heavy-fermion and low-dimensional metallic systems under pressure." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2006. http://nbn-resolving.de/urn:nbn:de:swb:14-1153315149468-90260.

Повний текст джерела

Анотація:

The understanding of new emerging unconventional ground states is a great challenge for experimental and theoretical solid-state physicists. New ground states are developing, where different energy scales compete, leading to a high sensitivity of the system to external tuning parameters like doping, pressure or magnetic field. The exploration of superconductivity proved to be a fascinating and challenging scientific undertaking. Discovered by H. Kammerlingh Onnes in 1911, prior to the development of the quantum theory of matter, superconductivity was defying a microscopic theory for more than four decades until the BCS theory was formulated in 1957 by J. Bardeen, L. N. Cooper and J. R. Schrieffer. Superconductivity of most of the simple metals or metallic alloys is well described within the frame of the BCS scenario, however, in the last thirty years numerous new superconducting materials were found to exhibit exotic properties not accounted for by the BCS theory. Among them are included the high-Tc compounds, the heavy-fermion superconductors and as well the organic superconductors. It was the purpose of this work to probe different facets of superconductivity in heavy-fermion and in low-dimensional metallic compounds. This dissertation is divided into six chapters. After this introduction, in Chapter 1 we will outline the basic theoretical concepts later needed for the analysis of the experimental results. In Chapter 2 we briefly introduce the experimental techniques with a special focus on the new pressure cells developed during this thesis and used for the measurements presented in Chapters 3 to 5. In Chapter 3 the possible realization of the inhomogeneous superconducting FFLO state in CeCoIn5 is studied by specific heat measurements under hydrostatic pressure, while in Chapter 4 the results of AC specific heat experiments on UBe13 under uniaxial pressure are presented. The ambient pressure properties as well as results obtained by resistivity measurements under hydrostatic pressure on the one-dimensional metallic compounds TlxV6S8 are discussed in Chapter 5. At the end, Chapter 6 summarizes and concludes this thesis.

Стилі APA, Harvard, Vancouver, ISO та ін.

24

Nichols, John A. "The development and implementation of electromechanical devices to study the physical properties of Sr2IrO4 and TaS3." UKnowledge, 2012. http://uknowledge.uky.edu/physastron_etds/3.

Повний текст джерела

Анотація:

Transition metal oxides (TMO) have proven to exhibit novel properties such as high temperature superconductivity, magnetic ordering, charge and spin density waves, metal to insulator transitions and colossal magnetoresistance. Among these are a spin-orbit coupling (SOC) induced Mott insulator Sr2IrO4. The electric transport properties of this material remain finite even at cryogenic temperatures enabling its complex electronic structure to be investigated by a scanning tunneling microscope. At T = 77 K, we observed two features which represent the Mott gap with a value of 2D ~ 615 meV. Additionally an inelastic loss feature was observed inside this gap due to a single magnon excitation at an energy of ~ 125 meV. These features are consistent with similar measurements with other probes. In addition to these features, at T = 4.2 K lower energy features appear which are believed to be due to additional magnetic ordering. Another material that exhibits a unique physical behavior is the sliding charge density wave (CDW) material TaS3. It is a quasi-one dimensional material that forms long narrow ribbon shaped crystals. It exhibits anomalies including non-ohmic conductivity, a decrease in the Young’s modulus, a decrease in the shear modulus and voltage induced changes in the crystal’s overall length. In addition, we have observed the torsional piezo-like response, voltage induced torsional strain (VITS), in TaS3 which was first discovered by Pokrovskii et. al. in 2007. Our measurements were conducted with a helical resonator. The VITS response has a huge effective piezoelectric coefficient of ~ 104 cm/V. In addition we have concluded that the VITS is a very slow response with time constants of ~ 1 s near the CDW depinning threshold, that these time constants are dependent on the CDW current, and we suggest that the VITS is due to residual twists being initially present in the crystal.

Стилі APA, Harvard, Vancouver, ISO та ін.

25

Zhang, Hao. "THE DEVELOPMENT AND IMPLEMENTATION OF SYSTEMS TO STUDY THE PHYSICAL PROPERITES OF TANTALUM TRISULFIDE AND SMALL-MOLECULE ORGANIC SEMICONDUCTORS." UKnowledge, 2015. http://uknowledge.uky.edu/physastron_etds/26.

Повний текст джерела

Анотація:

The charge-density-wave (CDW) material orthorhombic tantalum trisulfide (TaS3) is a quasi-one dimensional material that forms long ribbon shaped crystals, and exhibits unique physical behavior. We have measured the dependence of the hysteretic voltage-induced torsional strain (VITS) in TaS3, which was first discovered by Pokrovskii et. al. in 2007, on temperature and applied torque. Our experimental results shows that the application of torque to the crystal could also change the VITS time constant, magnitude, and sign. This suggests that the VITS is a consequence of residual torsional strain originally present in the sample which twists the polarizations of the CDW when voltage is applied. This polarization twist then results in torque on the crystal. Another group of materials that may attract interest is that of small-molecule soluble organic semiconductors. Due to their assumed small phonon thermal conductivities and higher charge carrier mobilities, which will increase their seebeck coefficients with doping as compared to polymers, the small-molecule organic materials are promising for thermoelectric applications. In our experiments, we have measured the interlayer thermal conductivity of rubrene (C42H28), using ac-calorimetry. For rubrene, we find that the interlayer thermal conductivity, ≈ 0.7 mW/cm·K, is several times smaller than the (previously measured) in-plane value. Also, we have measured the interlayer and in-plane thermal conductivities of 6,13-bis((triisopropylsilyl)ethynyl) pentacene (TIPS-Pn). The in-plane value is comparable to that of organic metals with excellent π-orbital overlap. The interlayer (c-axis) thermal diffusivity is at least an order of magnitude larger than the in-plane, and this unusual anisotropy implies very strong dispersion of optical modes in the interlayer direction, presumably due to interactions between the silyl-containing side groups. Similar values for both in-plane and interlayer conductivities have been observed for several other functionalized pentacene semiconductors with related structures.

Стилі APA, Harvard, Vancouver, ISO та ін.

26

Ritschel, Tobias. "Electronic self-organization in layered transition metal dichalcogenides." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2015. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-188265.

Повний текст джерела

Анотація:

The interplay between different self-organized electronically ordered states and their relation to unconventional electronic properties like superconductivity constitutes one of the most exciting challenges of modern condensed matter physics. In the present thesis this issue is thoroughly investigated for the prototypical layered material 1T-TaS2 both experimentally and theoretically. At first the static charge density wave order in 1T-TaS2 is investigated as a function of pressure and temperature by means of X-ray diffraction. These data indeed reveal that the superconductivity in this material coexists with an inhomogeneous charge density wave on a macroscopic scale in real space. This result is fundamentally different from a previously proposed separation of superconducting and insulating regions in real space. Furthermore, the X-ray diffraction data uncover the important role of interlayer correlations in 1T-TaS2. Based on the detailed insights into the charge density wave structure obtained by the X-ray diffraction experiments, density functional theory models are deduced in order to describe the electronic structure of 1T-TaS2 in the second part of this thesis. As opposed to most previous studies, these calculations take the three-dimensional character of the charge density wave into account. Indeed the electronic structure calculations uncover complex orbital textures, which are interwoven with the charge density wave order and cause dramatic differences in the electronic structure depending on the alignment of the orbitals between neighboring layers. Furthermore, it is demonstrated that these orbital-mediated effects provide a route to drive semiconductor-to-metal transitions with technologically pertinent gaps and on ultrafast timescales. These results are particularly relevant for the ongoing development of novel, miniaturized and ultrafast devices based on layered transition metal dichalcogenides. The discovery of orbital textures also helps to explain a number of long-standing puzzles concerning the electronic self-organization in 1T-TaS2 : the ultrafast response to optical excitations, the high sensitivity to pressure as well as a mysterious commensurate phase that is commonly thought to be a special phase a so-called “Mott phase” and that is not found in any other isostructural modification.

Стилі APA, Harvard, Vancouver, ISO та ін.

27

Kalinowski, Lech Stanisław. "Niskotemperaturowe badania własności fizycznych oraz wpływ domieszek na stan podstawowy Ce3Ru4Sn13 z silnymi korelacjami elektronowymi." Doctoral thesis, Katowice: Uniwersytet Śląski, 2017. http://hdl.handle.net/20.500.12128/5840.

Повний текст джерела

Анотація:

Experimental and theoretical studies of strong electronic correlations have led to the discovery of fascinating collective quantum phenomena, what still motivates to perform researches in solid state physics and generally induces the development of modern science and technology. Emergent quantum states appear in macroscopic physical experiments, and led to the discovery and explanation of Kondo effect [1], heavy-fermion states [2], superconductivity [3], and even high-temperature superconductivity [4]. The doctoral dissertation is a guidebook through the three scientific publications. Dissertation proves the truth of the thesis: "Atomic disorder, local electronic disorder, inhomogeneity and structural distortion of heavy fermionic compounds with strongly correlated electrons determine the occurrence and dynamics of phase states and ground state properties.", through careful analysis of experimental data. To confirm thesis data from advanced physical experiments such as x-ray diffraction (XRD), magnetic susceptibility and temperature magnetization studies, thermal and electronic transport studies at low and ultra-low temperature (up to 350 mK) also in strong magnetic fields, and electronic structure research has been analyzed. The results description and conclusions were based on advanced calculation methods for the electronic structure simulations based on the density functional theory (DFT) and on Anderson's theoretical model. For the purposes of research, the computer program "DISTorX" [5] was created to simulate charge order of the Charge Density Wave state and the structural distortion; directly visible on X-ray diffraction patterns. Generally the created program is universal enough that it can be successfully applied to a variety of structures. Ce3Ru4Sn13 compound allows us to study surprisingly many physical phenomena’s such as Kondo effect, quantum phase transition, superstructure occurrence, superconductivity, electron correlations or rattling effect.

Стилі APA, Harvard, Vancouver, ISO та ін.

28

Leroux, Maxime. "Supraconductivité, Onde de Densité de Charge et Phonons Mous dans les dichalcogénures 2H-NbSe2 et 2H-NbS2, et le composé intermétallique Lu5Ir4Si10." Thesis, Grenoble, 2012. http://www.theses.fr/2012GRENY069/document.

Повний текст джерела

Анотація:

Cette thèse présente une étude expérimentale de l'interaction entre la supraconductivité et une onde de densité de charge (ODC). Dans la théorie standard, la température critique d’un matériau supraconducteur est favorisée principalement par deux paramètres : une grande densité d’états au niveau de Fermi (nF), et un fort couplage électron-phonon. Cependant, un fort couplage électron-phonon favorise aussi l’apparition d’une ODC, ce qui réduit nF et rivalise ainsi avec la supraconductivité.Notre démarche a consisté à étudier deux composés où supraconductivité et ODC coexistent, et dans lesquels on peut faire disparaître l’ODC grâce à un paramètre externe : pression ou substitution. Le premier composé, 2H-NbSe2, présente une ODC en dessous de 33 K à pression ambiante. Celle-ci coexiste avec la supraconductivité en dessous de 7 K. Sous pression, l’ODC disparaît au-dessus de 4.6 GPa, sans que la température critique varie notablement. L’ODC disparaît aussi en remplaçant le sélénium par du soufre : 2H-NbS2 est ainsi un supraconducteur sans ODC (Tc = 6 K), et peut donc servir de composé témoin pour une étude comparative. Dans le second composé, Lu5Ir4Si10, une ODC est présente en dessous de 77 K à pression ambiante. Celle-ci disparaît sous pression au-dessus de 2 GPa, tandis que la température critique saute simultanément de 4 à 9 K. Pour étudier ces composés, j’ai utilisé trois techniques expérimentales : la mesure de la dispersion des phonons à basse température (300-2 K) et sous pression (0-16 GPa) par diffusion inélastique des rayons X, la mesure de la dépendance en température de la longueur de pénétration magnétique grâce à un oscillateur à diode tunnel et la mesure des champs critiques via des microsondes Hall.Dans la première partie, je présente la dépendance en température de la dispersion des phonons dans 2H-NbS2. Nous observons la présence d’un phonon mou dont l’énergie reste toujours positive, même extrapolée à température nulle. Ce composé est ainsi à la limite d'une instabilité ODC. De plus, nous montrons qu’il est relativement unique, car seuls les effets anharmoniques empêchent l’amollissement complet des phonons. Je présente ensuite la dépendance en température et en pression de la dispersion des phonons dans 2H-NbSe2. Ces expériences montrent qu’un mode de phonon mou persiste jusqu’à 16 GPa, même quand l'état à température nulle n'est pas l’ODC. La dépendance en température de ce phonon mou est alors similaire à celle de 2H-NbS2. Dans les deux composés, ces phonons mous semblent liés à la présence d'un couplage électron-phonon à la fois fort et anisotrope. Nous suggérons qu’il s’agit d’un élément essentiel pour expliquer leurs propriétés supraconductrices.Dans la seconde partie, je mesure l'anisotropie et la dépendance en température de la longueur de pénétration magnétique dans l’état supraconducteur de 2H-NbS2 et Lu5Ir4Si10. La dépendance en température de la densité superfluide dans 2H-NbS2 confirme la présence d'un gap supraconducteur réduit dont l'amplitude est très proche de celle mesurée dans 2H-NbSe2. Les phonons mous et le gap réduit étant présents dans 2H-NbS2 et 2H-NbSe2, nous prouvons expérimentalement qu'il faut raisonner en termes de renforcement de la supraconductivité par les phonons mous plutôt qu'en termes d’interaction avec l'état fondamental (ODC ou métal). Nous proposons que ce renforcement soit lié à l'anisotropie du couplage électron-phonon.En revanche, cet effet n’est pas général aux composés où supraconductivité et ODC coexistent. Les propriétés supraconductrices de Lu5Ir4Si10 sont en effet bien décrites par le modèle BCS couplage faible. Ceci est peut être lié aux caractéristiques de l’ODC : la présence d’une hystérésis montre que la transition ODC est du premier ordre. D’autre part, les mesures de diffraction X sous pression et à basse température révèlent que cette ODC est multiple : en plus de la périodicité 1/7, nous observons une seconde périodicité de 1/20
This thesis presents an experimental study of the interaction between superconductivity and a charge density wave (CDW). In the standard theory, the critical temperature of a superconductor is principally enhanced by two parameters: a large density of states at the Fermi level (nF) and a strong electron-phonon coupling. However, a strong electron-phonon coupling also favors the appearance of a CDW, which reduces nF and therefore competes with superconductivity.Our strategy was to study two compounds in which superconductivity and CDW coexist, and in which the CDW can be suppressed through an external parameter: pressure or substitution. The first compound is 2H-NbSe2, it presents a CDW below 33 K at ambient pressure. This CDW coexists with superconductivity below 7 K. Under pressure, the CDW disappears above 4.6 GPa, meanwhile the critical temperature slowly changes. The CDW also disappears when replacing selenium by sulfur: 2H-NbS2 is a superconductor without CDW (Tc=6 K), it can therefore serve as a “test compound” for a comparative study. The second compound is Lu5Ir4Si10, it presents a CDW below 77 K at ambient pressure. Under pressure, this CDW disappears above 2 GPa, meanwhile the critical temperature abruptly jumps from 4 to 9 K.For this study, I used three experimental techniques: inelastic x-ray scattering at low temperature (300-2 K) and under pressure (0-16 GPa) to measure the dispersion of phonons, a tunnel diode oscillator to measure the temperature dependence of the magnetic penetration depth, and Hall microprobes to measure the first and second critical fields. In the first part, I present the temperature dependence of the phonon dispersion in 2H-NbS2. We observe a soft phonon that always remains at positive energies, even extrapolated to zero temperature. Thus, this compound is on the verge of CDW instability. It is also relatively unique, since we show anharmonicity is the only effect that prevents the complete softening of the phonons.Then I present the temperature and pressure dependence of the phonon dispersion in 2H-NbSe2. These experiments show that a soft phonon persists up to 16 GPa, even if the ground state is not a CDW. The temperature dependence of this soft phonon is then similar to that of 2H-NbS2. In both compounds, these soft modes seem to be related to the strength and anisotropy of the electron-phonon coupling. We suggest this is a fundamental element to explain their superconducting properties.In the second part, I measure the anisotropy and temperature dependence of the magnetic penetration depth in the superconducting state of 2H-NbS2 and Lu5Ir4Si10. The temperature dependence of the superfluid density in 2H-NbS2 confirms the presence of a reduced superconducting gap. Its amplitude is very similar to the one measured in 2H-NbSe2. The soft modes and the reduced gap being present in both 2H-NbSe2 and 2H-NbS2, we prove experimentally that the enhancement of superconductivity is related to the soft modes rather than to the nature of the ground state (CDW or metal). We suggest this enhancement is due the anisotropy of the electron-phonon coupling.However, this effect is not general to all compounds where superconductivity and CDW coexist. The superconducting properties of Lu5Ir4Si10 are indeed well fitted by the BCS model in the weak coupling limit. This may be related to the characteristics of the CDW: the presence of hysteresis shows that the CDW transition is first order. In addition, under pressure and at low temperature, x-ray diffraction measurements indicate that the CDW is multiple: aside from the periodicity of 1/7, we observe a second periodicity of 1/20

Стилі APA, Harvard, Vancouver, ISO та ін.

29

Miclea, Corneliu Florin. "Investigation of superconducting order parameters in heavy fermion and low dimensional metallic systems under pressure." Doctoral thesis, Göttingen Cuvillier, 2005. http://d-nb.info/990426440/04.

Повний текст джерела

Стилі APA, Harvard, Vancouver, ISO та ін.

30

Kaddour, Wafa. "Coexistence Onde de Densité de Charge / Supraconductivité dans TTF[Ni(dmit)2]2." Phd thesis, Université Paris Sud - Paris XI, 2013. http://tel.archives-ouvertes.fr/tel-00881509.

Повний текст джерела

Анотація:

Au cours de cette thèse, nous étudions la compétition entre les états onde de densité de charge (ODC) et supraconducteur dans le composé multi-bandes à deux chaines TTF[Ni(dmit)2]2. Nous avons réalisé des mesures de résistivité, de pouvoir thermoélectrique et de conductivité thermique sous pression hydrostatique jusqu'aux basses températures. A basse pression, deux ondes de densité de charge détectées par mesures de rayons X et observées par nos mesures de résistivité transverse sont associées aux bandes 1D LUMO et HOMOI de la chaine Ni(dmit)2. A 12kbar, la fusion de ces deux instabilités est associée à l'emboîtement des bandes LUMO avec les bandes HOMOI à travers le point Γ de la première zone de Brillouin. A 18kbar et sur un intervalle de 5-6kbar, on observe un pic de commensurabilité attribué au vecteur d'emboîtement commensurable 2kF = 1/3b*.La supraconductivité est observée à partir de 2kbar et jusqu'à 22kbar avec une température critique 0.6K qui augmente avec la pression et est corrélée à la variation des températures de transition ODC. La supraconductivité est associée à la bande 2D HOMOII du Ni(dmit)2. Les mesures de champ magnétique critique ont permis de donner des informations sur l'évolution de la texture de coexistence métal- ODC en fonction de la pression. Elles mettent aussi en évidence une supraconductivité non conventionnelle avec des nœuds dans le gap.Ces résultats nous ont permis de revisiter le diagramme de phase Température-Pression de ce composé qui s'est révélé beaucoup plus riche que ce qui avait été rapporté jusqu'à maintenant.

Стилі APA, Harvard, Vancouver, ISO та ін.

31

Duverger-Nédellec, Elen. "Transitions vers des états électroniques complexes et des structures super périodiques dans les bronzes mono phosphates de tungstènes." Thesis, Normandie, 2017. http://www.theses.fr/2017NORMC239/document.

Повний текст джерела

Анотація:

Les matériaux conducteurs de basse dimensionnalité électronique peuvent présenter des transitions vers des états électroniques complexes tels que la supraconduction, les Ondes de Densité de Spins (ODS) et les Ondes de Densité de Charges (ODC). La coexistence de plusieurs de ces instabilités au sein d’un même matériau a donné lieu à une recherche active de nouveaux systèmes conducteurs quasi-bidimensionnels. Dans cette thèse nous avons choisi de nous intéresser à la famille des Bronzes MonoPhosphate de Tungstène à tunnels pentagonaux (MPTBp), de formule chimique (PO2)4(WO3)2m ; l’un des principaux intérêts réside dans la possibilité de contrôler directement la dimensionnalité du matériau et sa densité de porteurs en faisant varier la valeur de m (2 ≤ m ≤ 14). Dans la littérature il a été montré que certains termes de cette famille (m=4, 5, 6) admettent des états ODC successifs alors que d’autres (m=10) présentent des ordres de type ferro électrique ; l’objectif de cette thèse est alors de montrer l’effet de la dimensionnalité du matériau sur l’apparition et la stabilité de ces états électroniques dans les MPTBp. Pour cela, des mesures de transport électronique, des études des structures modulées incommensurables et commensurables par diffraction des rayons X avant et après chaque transition et des mesures de diffusion inélastique ont été effectuées sur plusieurs termes à valeur de m pair. Dans ce travail nous avons pu montrer l’existence d’un état ODC pour les termes m=2, 4, 6, 8 et 10, caractérisé par la formation d’amas de tungstène dans certaines régions du matériau, accompagné, pour les m=8 et 10, de l’installation progressive d’une mise en ordre de type anti-ferroélectrique des déplacements des atomes de tungstène. Un phénomène de dépiégeage d’ODC a été observé pour le terme quasi-unidimensionnel m=2, ce qui n’avait encore jamais été reporté chez les MPTBp. Un couplage électron-phonon fort a pu être mis en évidence pour les hauts termes (m ≥ 8) à la fois via les analyses structurales, l’observation de transitions résistives du premier ordre mais également par des mesures de diffusion inélastique des rayons X. Pour ces hauts termes, une transition de type ordre-désordre est alors à envisager
Conductive materials with low electronic dimensionality can present some transitions toward complex electronic states as superconductivity, Spin Density Waves (SDW) and Charge Density Waves (CDW). The coexistence of several of these instabilities in the same material leads to bustling investigations on new quasi-bidimensional conductors. In this thesis, we chose to study the MonoPhosphate Tungsten Bronzes with pentagonal channels family (MPTBp), of chemical formula (PO2)4(WO3)2m ; one of the main interests of this family is the possibility to directly control the compound’s dimensionality and its carriers density by varying m value (2 ≤ m ≤ 14). In the literature, it’s been shown that some MPTBp members (m=4, 5, 6) present successive CDW states whereas others (m=10) show ferroelectric-type orders. The aim of this thesis is thus to bring into light the effect of the material’s dimensionality on the appearance and the stability of these electronic states in the MPTBp family. In this way, transport measurements, X-Rays diffraction studies of the commensurate and incommensurate modulated structures below and above each transition and inelastic scattering measurements were done on several members with even value of m. In this work we reveal the existence of a CDW state for the m=2, 4, 6, 8 and 10 members, characterized by the formation of tungsten clusters in some areas of the crystal. For the m=8 and 10 members, the CDW is accompanied by a gradual installation of an anti-ferroelectric-like ordering of the tungsten atoms displacements. A CDW depinning phenomenon was observed for the quasi-unidimensional m=2 member, which has never been reported before in the MPTBp family. A strong electron-phonon coupling was evidenced for the high terms of the family (m ≥ 8) by structural analysis, first order resistive transitions observation and by X-Rays inelastic scattering measurements. For these high terms, an order-disorder transition must be considered

Стилі APA, Harvard, Vancouver, ISO та ін.

32

Federico, Mazza. "First Principle Calculations & Inelastic Neutron Scattering on the Single-Crystalline Superconductor LaPt2Si2." Thesis, KTH, Materialvetenskap, 2020. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-278071.

Повний текст джерела

Анотація:

This work presents a comprehensive study on single crystalline LaPt2Si2, in which superconductivity and a charge density wave (CDW) coexist. The usage of density functional theory (DFT) modeling and Inelastic Neutron Scattering has been the primary form of investigation, in order to determine all the characteristic features of the sample taken under consideration. From the results one can observe that the Fermi surface nesting is the primary contributor for the CDW wavevector ~qCDW = (1/3, 0, 0). In addition, the phonon density of states present two typical energy levels, with soft modes in the Pt3-Pt4 layer coherent with the presence of a CDW. The superconducting temperature has been estimated at Tc = 1.6 K. The experimental data from the inelastic instrument High Resolution Chopper Spectrometer (HRC) at the J-PARC neutron source are in good agreement with the theoretical simulations, showing the same energy levels for the polarization phonon modes (from 4 to 18 meV and from 32 to 42 meV).
Denna rapport presenterar en omfattande studie av enkristalls LaPt2Si2 i vilken supraledning och en laddningsdensitetsvåg (CDW) samexisterar. Användandet av DFT-modellering och neutronspridning har varit de huvudsakliga undersökningsmetoderna, för att bestämma alla karakteristiska drag hos det undersökta provet. Från resultaten kan observeras att den inneslutna Fermiytan är den huvudsakliga bidragaren till CDW-vågvektorn~qCDW = (1/3, 0, 0). Vidare visar den närvarande fonontillståndsdensiteten två typiska energinivåer, med mjuka lägen i Pt3-Pt4-skiktet, som stämmer överens med närvaron av en CDW. Den supraledande temperaturen har uppskattats till Tc = 1.6 K. Experimentella data från det inelastiska instrumentet HRC vid J-PARCs neutronkälla stämmer väl överens med teoretiska simuleringar, som visar samma energinivåer för polarisationsfononlägena (från 4 till 18 meV och från 32 till 42 meV).

Стилі APA, Harvard, Vancouver, ISO та ін.

33

Butaud, Patrick. "Etude par résonance magnétique nucléaire des ondes de densité de charge dans les conducteurs quasi-unidimensionnels inorganiques." Grenoble 1, 1987. http://www.theses.fr/1987GRE10018.

Повний текст джерела

Анотація:

Etude plus particuliere du bronze bleu de rubidium rb::(0,3)moo::(3). Analyse du profil de raie spectrale, de relaxation magnetique en presence d'une onde de densite de charge statique, puis dynamique. Mise en evidence d'effets pretransitionnels tant statiques que dynamiques. Observation d'une mise en ordre de defauts charges par l'onde de densite de charge. L'etude met en evidence l'absence de zones commensurables bien definies a basse temperature malgre un changement dans la dynamique du systeme. Observation directe par rmn du mouvement d'une onde de densite de charge responsable du transport non lineaire

Стилі APA, Harvard, Vancouver, ISO та ін.

34

Britel, Mohamed Reda. "Etude des propriétés élastiques du système unidimensionnel à onde de densité de charge (TaSe4)2I." Université Joseph Fourier (Grenoble), 1994. http://www.theses.fr/1994GRE10087.

Повний текст джерела

Анотація:

L'etude des proprietes elastiques du systeme unidimensionnel a onde de densite de charge (tase4)2i, par la technique ultrasonore a conduit a la mise en evidence, dans un intervalle de temperatures de 1 a 300 k et de frequences de 10 a 100 mhz, des proprietes tres importantes, par la mesure de l'attenuation et de la vitesse d'ondes longitudinales et transverses ultra-sonores: ramollissement critique de la constante c#4#4 a la transition de peierls. Cette anomalie elastique, en accord avec les proprietes electriques, peut etre relie au deplacement atomiques perpendiculaires a l'axe c, mis en evidence dans les experiences de diffraction aux rayons x aux neutrons. Un processus de relaxation existant dans l'intervalle de temperature situe au voisinage de la transition de peierls, en accord avec les experiences de diffusion de neutrons. L'activation thermique du mecanisme est du type vogel-fulcher et non pas arrhenius comme l'ont confirme les mesures, dans un grand intervalle de frequences, entre 1 hz et 100 mhz. Il semble que ce processus active thermiquement puisse etre associe aux fluctuations de la structure en domaine qui existent au voisinage de la transition de peierls. Il y a gel de ces fluctuations a une temperature voisine de 200 k. Une forte diminution de la constante transverse c#4#4 est observee au dessous de 50 k. Elle s'explique par le fait que ce mode est fortement dispersif (mode plat). La diminution de c#4#4 et l'augmentation de l'attenuation sont analysees en considerant que la frequence de la branche acoustique est constante, egale a 0,15 thz, pour les grandes valeurs du vecteur d'onde

Стилі APA, Harvard, Vancouver, ISO та ін.

35

Gonzalez, Vallejo Isabel. "Study of the structural dynamics of phase transitions using time resolved electron and X-ray diffraction." Thesis, Université Paris-Saclay (ComUE), 2019. http://www.theses.fr/2019SACLS496.

Повний текст джерела

Анотація:

L'application d'une perturbation externe à certains matériaux (tels que la température, la pression ou la lumière) provoque souvent l'émergence de nouvelles propriétés macroscopiques dont l'origine se situe au niveau atomique. Par conséquent, une étude détaillée de la dynamique atomique devient essentielle pour la compréhension de processus tels que les transformations chimiques ou les transitions de phase. Dans le cas particulier des transitions de phase structurales, la symétrie du cristal est soumise à une transformation entre deux états différents à une valeur critique. Les échelles de temps typiques de la dynamique structurale sont de l'ordre de quelques centaines de femtosecondes à plusieurs picosecondes. Le développement des impulsions laser femtosecondes a permis aux chercheurs d'accéder aux échelles de temps nécessaires pour explorer la dynamique structurale ultra-rapide. Plus précisément, la diffraction résolue en temps s'est révélée être une technique idéale pour suivre et dévoiler les voies hors équilibre suivies par le réseau après une courte impulsion laser. Cette thèse présente des études expérimentales réalisées par des techniques de diffraction d'électrons et de rayons X résolue en temps sur deux transitions de phase structurelles différentes. Dans une première partie, nous démontrons les capacités du montage expérimental de diffraction d'électrons ultra-rapide (UED) développé au Laboratoire d'Optique Appliquée. Nos résultats expérimentaux réalisés sur des échantillons monocristallins de haute qualité sont présentés ainsi qu'une étude quantitative des implications des effets de diffraction dynamique dans les expériences UED. Dans une deuxième partie, nous présentons un ensemble d'expériences de diffraction d'électrons pompe-sonde réalisée sur GdTe₃, un composé appartenant à la famille des Tritellurides de terres rares qui présente un état d'onde de densité de charge. L'arrivée d'une excitation optique déclenche la transition de phase de manière non thermique. La dynamique de relaxation est caractérisée par un ralentissement qui augmente avec la fluence incidente ainsi qu'avec la température initiale de l'échantillon. Ces résultats éclairent les interprétations controversées actuelles concernant l’ émergence de défauts topologiques photo-induits.Dans une troisième partie, nous présentons des expériences de diffraction des rayons X en temps résolu réalisées à la ligne CRISTAL du synchrotron SOLEIL. Dans ce cas, nous avons étudié le composé A15 Nb₃Sn, qui présente une transition de phase d'une symétrie cubique à une symétrie tétragonale à l'équilibre thermique. Les résultats de notre expérience pompe-sonde mettent en évidence une réponse différente du réseau par rapport à la transition d’ équilibre thermique, ce qui encourage les futures recherches sur la dynamique structural de ce matériau
The application of an external perturbation in certain materials (such as temperature, pressure or light) often gives rise to the emergence of new macroscopic properties with their origin at the atomic level. Therefore, a detailed study of the atomic dynamics becomes essential to the understanding of processes such as chemical transformations or phase transitions. In the particular case of structural phase transitions, the symmetry of the crystal undergoes a transformation between two different states at a given critical value. The typical time scales of the structural dynamics occur on the order of few hundreds of femtoseconds to several picoseconds. The development of femtosecond laser pulses has enabled scientists to access the required time scales to explore the ultrafast dynamics of the lattice structure in the relevant time scales. More precisely, time-resolved diffraction has proven to be an ideal technique to track and unveil the out-of-equilibrium pathways followed by the lattice after a short laser pulse.This thesis presents experimental studies performed by time-resolved electron and X-ray diffraction techniques on two different structural phase transitions. In a first part, we demonstrate the capabilities of the ultrafast electron diffraction (UED) experimental setup developed at Laboratoire d’Optique Apliquée. Our UED experimental results performed on high quality single crystal samples are presented along with a quantitative study of the implications of dynamical diffraction effects in UED experiments. In a second part, we present a set of pump-probe electron diffraction experiments performed on GdTe₃, a compound belonging to the rare-earth Tritellurides family which presents a charge density wave state. The arrival of an optical excitation triggers the phase transition non-thermally with the out-of-equilibrium relaxation dynamics of the charge density wave state characterized by slowing down that increases with incident fluence as well as with initial sample temperature. These results shed more light on current controversial interpretations involving the emergence of photoinduced topological defects. In a third part, we present time resolved X-ray diffraction experiments performed at CRISTAL beamline at SOLEIL synchrotron. In this case we have studied the A15 compound Nb₃Sn, which displays a displacive phase transition evolving from cubic to tetragonal symmetry at thermal equilibrium. Our pump-probe results present evidence of a different lattice response with respect to the thermal equilibrium transition, encouragingf uture investigations on the dynamics of this material

Стилі APA, Harvard, Vancouver, ISO та ін.

36

Chapeau-Blondeau, François. "Etude des mecanismes de polarisation electrique du polyethylene basse densite." Paris 6, 1987. http://www.theses.fr/1987PA066301.

Повний текст джерела

Анотація:

Cette these presente une etude des mecanismes de polarisation electrique du polyethylene basse densite, realisee au moyen d'une methode experimentale qui met en oeuvre la propagation d'une onde de pression induite par laser. Cette methode permet de mesurer, de facon non destructive, la repartition spatiale des charges ou des champs electriques dans un materiau dielectrique

Стилі APA, Harvard, Vancouver, ISO та ін.

37

Guyot, Hervé. "Etude des transitions d'onde de densité de charge et des propriétés de transport des oxydes de molybdène [êta]-Mo4O11 et [gamma]-Mo4O11." Grenoble 1, 1986. http://www.theses.fr/1986GRE10122.

Повний текст джерела

Стилі APA, Harvard, Vancouver, ISO та ін.

38

Rötger, Antonia. "Etude par magnétotransport de l'état d'onde de densité de charge dans les bronzes oxygénés quasi-bidimensionnels KMo6O17 et (PO2)4(WO3)2m." Grenoble 1, 1993. http://www.theses.fr/1993GRE10036.

Повний текст джерела

Анотація:

Nous avons etudie la magnetoresistance des bronzes oxygenes de metaux de transition, le bronze violet kmo#6o#1#7 et les composes m=4, 6, 7 de la famille (po#2)#4(wo#3)#2#m. Ces composes ont des proprietes quasi-bidimensionnelles. Ils subissent des transitions de peierls a une temperature t#p. La magnetoresistance de kmo#6o#1#7 a ete mesuree a des temperatures comprises entre 1. 8 k et 300 k en champ magnetique jusqu'a 37 t. Des oscillations quantiques ont ete observees. Base sur le concept du nesting cache, nous proposons un modele de la surface de fermi dans l'etat d'onde de densite de charge. Elle serait constituee de petites poches lesquelles seraient a l'origine de la magnetoresistance elevee au-dessous de t#p. La temperature de peierls, observee par des mesures de resistivite electrique sous pression hydrostatique, augmente avec la pression. La compressibilite des parametres du reseau est anisotrope. La resistivite et la magnetoresistivite ont ete etudiees a des temperatures comprises entre 4. 2 k et 300 k en champ magnetique jusqu'a 6 t dans les composes m=4, 6, 7 de la famille (po#2)#4(wo#3)#2#m. La temperature de peierls augmente avec m. La magnetoresistivite diminue, ainsi que l'anisotropie quand m augmente. Nous discutons ce comportement en considerant le caractere de basse dimensionnalite, la densite de porteurs et la densite de defauts cristallins

Стилі APA, Harvard, Vancouver, ISO та ін.

39

Lorenzo-Diaz, José-Emilio. "Etude par diffusion de neutrons des transitions de phase dans les composés quasi-unidimensionnels (TaSe4)2I et (NbSe4)3I." Grenoble 1, 1992. http://www.theses.fr/1992GRE10184.

Повний текст джерела

Анотація:

On a étudié la dynamique de réseau associée à la transition de phase de Peierls dans le composé quasi unidimensionnel à onde de densité de charge (odc) (tase4)2i, par diffusion de neutrons. Cette étude nous a permis de distinguer ce qui est caractéristique de l'état odc. En particulier, nous proposons d'interpréter la résonance observée en conductivité ac (45 ghz) comme des modes optiquement actives par l'état odc, plutôt que comme des modes caractéristiques de cet état. De même, la nature isotrope des fluctuations critiques ainsi que le caractère transverse acoustique des déplacements atomiques en phase modulée, amènent à placer cette transition de Peierls dans une catégorie à part. Un modèle phénoménologique qui prend en compte les deux aspects ci-dessus a été également proposé

Стилі APA, Harvard, Vancouver, ISO та ін.

40

Leriche, Raphaël. "Unconventional superconductivity in quasi-2D materials with strong spin-orbit coupling." Thesis, Sorbonne université, 2019. http://www.theses.fr/2019SORUS577.

Повний текст джерела

Анотація:

La réalisation de supraconducteurs topologiques constitue un des principaux enjeux actuels de la physique de la matière condensée. Il a en effet été prédit que ces systèmes devaient abriter des fermions de Majorana. Ces fermions de Majorana disposent à la fois d’une statistique non-abélienne et, du fait de leur origine topologique, d’une robustesse face au désordre local, ce qui les rend très attrayants pour des applications en informatique quantique. Une approche susceptible de conduire à de la supraconductivité topologique consiste à considérer des systèmes supraconducteurs à fort couplage spin-orbite et brisant la symétrie d’inversion. C’est dans cette optique que, dans le cadre de cette thèse, j’ai effectué des mesures de microscopie et spectroscopie par effet tunnel sur des matériaux quasi-bidimensionnels : (LaSe)1,14(NbSe2)2 et Sr2IrO4.J’ai tout d’abord étudié les propriétés électroniques du matériau incommensurable (LaSe)1,14(NbSe2)2, proche parent du composé dichalcogénure de métaux de transition 2HNbSe2. (LaSe)1,14(NbSe2)2 est une hétérostructure faite d’alternances de biplans NbSe2 à géométrie prismatique trigonale et de biplans de LaSe avec une structure de sel de roche. Le fort couplage spin-orbite ainsi que la non-centrosymétrie présents dans les plans NbSe2 font de (LaSe)1,14(NbSe2)2 un potentiel candidat pour de la supraconductivité topologique. Dans cette thèse, je présente des résultats de spectroscopie montrant que la structure électronique de (LaSe)1,14(NbSe2)2 est similaire à celle de la monocouche de NbSe2 avec un dopage de type électron accompagné par un déplacement du potentiel chimique de 0,3 eV, jusqu’alors inégalé. J’ai également pu démontrer la nature quasi–bidimensionnelle de (LaSe)1,14(NbSe2)2 et notamment la présence d’un fort couplage spin-orbite de type Ising. De plus, la faible robustesse de la supraconductivité vis à vis du désordre non magnétique couplée à des mesures d’interférences de quasiparticules m’a permis de mettre en avant le caractère non conventionnel du paramètre d’ordre supraconducteur dans (LaSe)1,14(NbSe2)2. Cette étude permet d’envisager l’utilisationd’hétérostructures incommensurables telles que (LaSe)1,14(NbSe2)2 pour explorer la physique des dichalcogénures de métaux de transition dans la limite bidimensionnelle, pour laquelle de nombreuses études théoriques ont prédit une supraconductivité topologique. Dans cette thèse, je présente également une étude des effets du dopage sur les propriétés électroniques de l’oxyde d’iridium Sr2IrO4. Sr2IrO4 est un isolant de Mott non conventionnel puisqu’il doit cette propriété à la présence d’un fort couplage spin-orbite. Du fait d’une brisure locale de la symétrie d’inversion, certaines prédictions théoriques ont pu montrer que Sr2IrO4 devrait devenir un supraconducteur topologique une fois dopé. Ici, je montre qu’avec le dopage, Sr2IrO4 subit une transition de phase inhomogène à l’échelle nanométrique entre un état isolant de Mott et un état pseudo-métallique. Ce travail justifie la pertinence d’utiliser une sonde locale telle que le microscope à effet tunnel afin de venir compléter des résultats sur la physique de Mott obtenus par des méthodes intégratives comme la spectroscopie électronique résolue en angle
The realization of topological superconductors is one of the main current goals of condensed matter physics. It was indeed predicted that such systems should host Majorana fermions. These Majorana fermions possess both a non-Abelian statistics and, because of their topological origin, a certain robustness against local disorder, which makes them attractive for quantum computing applications. One approach likely to lead to topological superconductivity consists in considering superconducting systems with strong spin-orbit coupling and with broken inversion symmetry. It is in this framework that, during this thesis, I performed scanning tunneling microscopy and spectroscopy measurements on quasi-2D materials : (LaSe)1,14(NbSe2)2 and Sr2IrO4. I first studied the electronic properties of misfit compound LaNb2Se5, which is a parent of transition metal dichalcogenide 2H-NbSe2. (LaSe)1,14(NbSe2)2 is a heterostructure made out of alternations of NbSe2 bilayers with trigonal prismatic geometry and LaSe bilayers with rocksalt structure. (LaSe)1,14(NbSe2)2 is a potential candidate for topological superconductivity because of the presence of both a strong spin-orbit coupling and of broken inversion symmetry in NbSe2 planes. Here, I present spectroscopic results showing that the electronic structure of(LaSe)1,14(NbSe2)2 is very similar to the one of electron-doped monolayer NbSe2 with a shift of the chemical potential of 0,3 eV, priorly never reached. I could also demonstrate the quasi- 2D nature of (LaSe)1,14(NbSe2)2 and more particularly the presence of a strong Ising spinorbit coupling. Moreover, the observed weakness of superconductivity against non-magnetic disorder combined with quasiparticle interferences measurements allowed me to exhibit the unconventional nature of (LaSe)1,14(NbSe2)2 superconducting order parameter. This study opens the possibility to use misfit heterostructures such as (LaSe)1,14(NbSe2)2 to study thephysics of transition metal dichalcogenides in the 2D limit, for which many theoretical studies predict topological superconductivity. In this thesis, I also present a study on the effects of doping on the electronic properties of iridate compound Sr2IrO4. Sr2IrO4 is a spin-orbit induced Mott insulator. Because inversion symmetry is locally broken in Sr2IrO4, some theoretical predictions suggest that Sr2IrO4 should turn into a topological superconductor once doped. Here, I exhibit a nanometer-scaleinhomogeneous doping-driven Mott insulator to pseudo-metallic phase transition. This work further justifies the importance of using a local probe such as scanning tunnelling microscopy in order to complete results on Mott physics obtained by integrative methods like angle-resolved photoemission spectroscopy

Стилі APA, Harvard, Vancouver, ISO та ін.

41

Veuillen, Jean-Yves. "Etude par spectroscopie Mössbauer et de photoémission en rayonnement ultraviolet des bronzes bleus de molybdène A(0. 3)MoO3 (A = K, Rb)." Grenoble 1, 1986. http://www.theses.fr/1986GRE10146.

Повний текст джерела

Анотація:

Le bronze bleu a::(0,3)moo::(3) (a = k, rb) est un metal quasi-unidimensionnel a temperature ambiante qui presente une transition isolant-metal de type peierls a 183k. Etude de la structure de bandes dans la direction metallique par photoemission angulaire. Mesure de l'energie de liaison et du vecteur de fermi. Evolution thermique de l'interaction onde de densite de charge-impuretes, au voisinage de 100k, etudiee par spectrometrie moessbauer

Стилі APA, Harvard, Vancouver, ISO та ін.

42

Boujida, Mohamed. "Contribution à l'étude des propriétés de transport de quelques oxydes métalliques et supraconducteurs de basse dimensionnalité." Grenoble 1, 1988. http://www.theses.fr/1988GRE10157.

Повний текст джерела

Анотація:

Les proprietes de transport (magnetoconductivite, effet hall, ondes de densite de charge, ondes de densite de spin, transitions supraconductrices) des oxydes metalliques et supraconducteurs quasi-bidimensionnels sont etudiees a basse temperature

Стилі APA, Harvard, Vancouver, ISO та ін.

43

Ben, Salem Azzedine. "Synthese et caracterisation physique et structurale des conducteurs unidimensionnels de type fe : :(1+x)nb::(3-x)se::(10)." Nantes, 1987. http://www.theses.fr/1987NANT2018.

Повний текст джерела

Анотація:

Ce compose est le prototype d'une serie de formulation (mm')nb::(2)s::(10)(mm' = fe,v,cr,nb,ta) presentant une chaine trigonale prismatique |nbse::(3)| et une double chaine oelaedrique |(mm')se::(6)|. Ces composes ont une transition metal isolant vers 140 k. L'observation d'une onde de densite de charge aux caracteristiques identiques a celles de nbse::(3) indique que la chaine conductrice est la chaine trigonale prismatique |nbse::(3)|. La variation de la conductivite au voisinage de la transition indique un mecanisme de localisation d'anderson. Les proprietes physiques du compose ou se a ete substitue par s ont ete etudiees. L'etude de l'insertion par li a ete effectuee

Стилі APA, Harvard, Vancouver, ISO та ін.

44

Kundu, Hemanta Kumar. "Probing charge-density waves and superconductivity in low-dimensional systems." Thesis, 2019. https://etd.iisc.ac.in/handle/2005/5042.

Повний текст джерела

Анотація:

The recent advent of two-dimensional materials has led to the discovery of materials with fascinating phase diagrams. These systems have exciting electronic phases which coexist or compete with each other. Here in this thesis, we have looked into two such electron-phonon mediated many-body states { charge density wave and superconductivity. We start with a novel, dynamically modulated quantum phase transition between two distinct charge density wave (CDW) phases in 2-dimensional 2H-NbSe2. There is recent spectroscopic evidence for the presence of these two quantum phases, but its evidence in bulk measurements remained elusive [1]. We studied suspended, ultra-thin NbSe2 devices fabricated on piezoelectric substrates - with tunable akes thickness, disorder level, and strain. We find that over a certain range of temperature, the conductance fluctuates between two precise values separated by the quantum of conductance, e2=h. These observations could be explained as arising from strain-induced dynamical phase transition between the two CDW states [2]. To affirm this, we vary the lateral strain across the device and map out the phase diagram near the critical point

Стилі APA, Harvard, Vancouver, ISO та ін.

45

Chen, Yu-Yuan, and 陳裕元. "Physical Properties of Y5Ir4Si10 near Charge-Density-Wave Transitions." Thesis, 2004. http://ndltd.ncl.edu.tw/handle/18323753624780141643.

Повний текст джерела

Анотація:

碩士
國立東華大學
應用物理研究所
92
We present the results of electrical resistivity, Hall coefficient, magnetic susceptibility, specific heat, thermal conductivity, and thermoelectric power on the rare-earth-transition-metal compound Y5Ir4Si10 as a function of temperature. For all measured transport and thermodynamic properties, successive anomalies due to charge-density-wave (CDW) formation were observed. In contrast to the strong-coupled nature of the phase transitions found in that of its isostructural compounds R5Ir4Si10 (R = Dy - Lu), Y5Ir4Si10 could be described as a weak-coupled CDW system. Most interestingly, the complex temperature-dependent thermoelectric power and sign reversal of Hall coefficient in this material indicate a drastic modification of Fermi surface and a sudden change of the band structure, associated with the electron-hole asymmetry at the transitions.

Стилі APA, Harvard, Vancouver, ISO та ін.

46

Mahajan, Mehak. "Charge Density Wave-driven Carrier Transport in Layered Heterostructures." Thesis, 2022. https://etd.iisc.ac.in/handle/2005/5850.

Повний текст джерела

Анотація:

Metal-based electronics remain one of the longstanding goals of researchers to achieve ultra-fast and radiation-hard electronic circuits. Generally, metals are primarily used as passive conductors in modern electronics and do not play an active role. Nanoscale materials with distinctive size-dependent properties provide opportunities to achieve new device functionalities. Ta-based di-chalcogenides, particularly 1T-TaS2 and 2H-TaSe2, which form layered structures and exhibit charge density waves (CDW), are promising in this context. CDW is a macroscopic state shown by materials with reduced dimensions, for example, one-dimensional and layered two-dimensional crystals. It results from the modulation in the electronic charge arising due to a periodic modulation in the crystal lattice. 1T-TaS2 exhibits one of the strongest known CDW characteristics enabling temperature-dependent distinct resistivity phases. The nearly commensurate (NC) to the incommensurate (IC) CDW phase transition that usually occurs at 353 K and can be driven electrically at room temperature is of high practical interest. However, resistivity switching during this phase transition is weak (< 2) and cannot be modulated by an external gate voltage – limiting its widespread usage. Using a back-gated 1T-TaS2/2H-MoS2 heterojunction, we show resistivity switching up to 17.3, which is ~14.5-fold higher than standalone TaS2. We demonstrate a low barrier electrical contact between a TaS2 source and a MoS2 channel, promising “all-2D” flexible electronics. Additionally, we show that the usual resistivity switching in TaS2 due to different phase transitions is accompanied by a surprisingly strong modulation in the Schottky barrier height (SBH) at the TaS2/MoS2 interface – providing an additional knob to control the degree of the phase-transition-driven resistivity switching by an external gate voltage. In particular, the commensurate (C) to triclinic (T) CDW phase transition increases the SBH owing to a collapse of the Mott gap in TaS2. The change in SBH allows us to estimate an electrical Mott gap opening of ~71 ± 7 meV in the C phase of TaS2. The results show a promising pathway to externally control and amplify the CDW induced resistivity switching. Further, we achieve gate- and light-controlled negative differential resistance (NDR) characteristics in an asymmetric 1T-TaS2/2H-MoS2 T-junction by exploiting the electrically driven CDW phase transition of TaS2. The device operation is purely governed by majority charge carriers, making it distinct from typical tunneling-based NDR devices, thus avoiding the bottleneck of weak tunneling efficiency in van der Waals heterojunctions. Consequently, we achieve a peak current density over 10^5 nA μm^(-2), which is about two orders of magnitude higher than that obtained in typical layered material-based NDR implementations. An external gate voltage and photo-gating can effectively tune the peak current density. The device characteristics show a peak-to-valley current ratio (PVCR) of 1.06 at 290 K, increasing to 1.59 at 180 K. To exploit the low thermal conductivity of 1T-TaS2 and 2H-TaSe2 in a local heater structure, we insert 2H-TaSe2 in between TaS2 and MoS2 layers, thereby forming a triple-layered 1T-TaS2/2H-TaSe2/2H-MoS2 T-junction. TaSe2 acts as a buffer layer preventing the CDW-induced SBH modulation at TaS2/MoS2 interface. This will allow efficient thermionic switching of carriers resulting from sharp temperature rise in the junction due to electrically driven TaS2 phase transitions. Interestingly, the device can toggle between the current increment and NDR characteristics by simply changing the biasing conditions. At TaS2 biasing, the heterostructure device shows a current increment by a factor of 3 at 300 K, which gets enhanced up to ~10^3 at 77 K, beneficial for various switching circuits and sensing applications. However, under TaSe2 biasing, the device exhibits NDR characteristics with a PVCR of 1.04 and 1.10 at 300 K and 77 K, respectively. The external back-gate voltage can effectively tune the current enhancement factor and NDR. The devices mentioned above are robust against ambiance-induced degradation, and the characteristics repeat in multiple measurements over more than six months. Conventional metals, in general, do not exhibit strong photoluminescence. However, we found that 2H-TaSe2 exhibits a surprisingly strong optical absorption and photoluminescence resulting from inter-band transitions. We use this perfect combination of electrical and optical properties in several optoelectronic applications. We show a seven-fold enhancement in the photoluminescence intensity of otherwise weakly luminescent multi-layer MoS2 through non-radiative resonant energy transfer from TaSe2 transition dipoles. Using a combination of scanning photocurrent and time-resolved photoluminescence measurements, we also show that the hot electrons generated by light absorption in TaSe2 have a relatively long lifetime, unlike conventional metals, making TaSe2 an excellent hot-electron injector. Finally, we show a vertical TaSe2/MoS2/graphene photodetector demonstrating a responsivity greater than 10 AW^(-1) at 0.1 MHz - one of the fastest reported photodetectors using MoS2. The findings will boost device applications that exploit CDW phase transitions, such as ultra-broadband photodetection, negative differential conductance, thermal sensors, fast oscillator, and threshold switching in neuromorphic chips. These functionalities will enable the implementation of active metal-based circuits.

Стилі APA, Harvard, Vancouver, ISO та ін.

47

Tsai, Huang-Ming, and 蔡煌銘. "Electronic Structures of RuO2, IrO2 Nanorods, Charge-density-wave K0.3MoO3 and High-density SiO2." Thesis, 2008. http://ndltd.ncl.edu.tw/handle/84171405827453437893.

Повний текст джерела

Анотація:

博士
淡江大學
物理學系博士班
96
In this thesis, it is used the X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS) and scanning photoelectron microscopy (SPEM) to discuss the electronic and atomic structure of different condensed materials including RuO2 and IrO2 nanorods, quasi-one-dimension (1-D) blue bronze K0.3MoO3 and high-density SiO2 thin films. In RuO2 and IrO2 nanorods research, the results reveal that the hybridization between O 2p and metal t2g obitals is weaker in IrO2 than in RuO2. The tip-region enhancement of the SPEM intensity is greater for RuO2 than for IrO2, which suggests that RuO2 be a better material for field emission application. In 1-D blue bronze K0.3MoO3 material, the existence of the K+ ion has play an important role to connect the MoO3 octahedral structure and maintain the anisotropy structure in the charge-density-wave (CDW) transition. In the high density SiO2 thin films, we discovered that the different growth processes will cause the vary Si–O bonding, therefore we can control the dielectric parameter by changing the growth method.

Стилі APA, Harvard, Vancouver, ISO та ін.

48

Luo, Jhong-You, and 羅中佑. "X-Ray Studies of Quasi-One-dimensional Charge Density Wave Dynamics." Thesis, 2004. http://ndltd.ncl.edu.tw/handle/36620573307114404700.

Повний текст джерела

Стилі APA, Harvard, Vancouver, ISO та ін.

49

Liao, Yin-Jiun, and 廖尹駿. "Resonant Soft X ray Scattering of Charge-density-wave Study in Chromium." Thesis, 2009. http://ndltd.ncl.edu.tw/handle/43694837093473885960.

Повний текст джерела

Анотація:

碩士
淡江大學
物理學系碩士班
97
In this study, we have demonstrated that CDW modulation in Cr can be probed using soft x-ray scattering. Because of the very weak signal from the satellite reflections caused by the formation of CDWs, it is difficult to undertake the detailed studies of CDWs with non-resonant x-ray scattering. However, with the help of resonant x-ray scattering, diffraction signals from weak satellite reflections could be largely enhanced. The wavelength λCDW was observed to have ~10a (a：lattice parameter of Cr) and Q_CDW≈(0.09 0 0) along [100] direction, and we obtained resonant enhancement of the signal near L2 and L3 edges. We studied the CDW peak at both of π and σ channels at T=77K and the temperature dependence between 70K and 300K at π channel.We also studied the temperature dependence of the Bragg peak.

Стилі APA, Harvard, Vancouver, ISO та ін.

50

Hsu, Fung-Hsueh, and 許芳雪. "Specific Heat Studies on the Charge-Density-Wave Transition of Lu5Ir4Si10 and Lu5Rh4Si10." Thesis, 2001. http://ndltd.ncl.edu.tw/handle/48546944451382290399.

Повний текст джерела

Анотація:

碩士
國立中山大學
物理學系研究所
89
Recently, the formation of charge density wave in 3D structure, Lu5Ir4Si10, had been observed in the X-ray diffraction experiment. At the same time, the transition in Lu5Ir4Si10 was thought to be first-order due to the spike-shaped anomaly in specific heat. The first-order transition usually accompanies with thermal hysteresis. In order to clarify this problem, we measure and analyze the specific heat result of Lu5Ir4Si10. As a matter of fact, we don’t observe the thermal hysteresis behavior within the resolution of our apparatus, and we think the formation of CDW in Lu5Ir4Si10 is strong interchain coupling. In addition, we also perform the resistivity, magnetic susceptibility and specific heat measurements under zero and external field on the isostructure component Lu5Rh4Si10 for comparison, which has also been thought to undergo a CDW transition. We indeed observe the thermal hysteresis behavior no matter on resistivity, susceptibility or on specific heat results, and this phenomenon doesn’t have magnetic effects. The thermal hysteresis features in Lu5Rh4Si10 are attributed to the presence of metastable states due to the pinning of the CDW phase to impurities, and we also discuss some possibilities about it. The specific heat measurement in our research is performed with an ac calorimetry, using chopped light as a heat source. The details of this technique are also discussed.

Стилі APA, Harvard, Vancouver, ISO та ін.

Дисертації з теми "Distinct charge density wave"

Оформте джерело за APA, MLA, Chicago, Harvard та іншими стилями