The thesis is focused on the electrodeposition of tantalum from ionic solution 1-butyl-1- methylpyrrolidinium bis(trifluoromethylsulfonyl) imide, ([BMP]Tf2N) at 200°C and description of related transfer and kinetic phenomena on the interface between the ionic solution and working electrode. Furthermore, the electrodeposition of tantalum layers in different ionic solutions containing tantalum salts and especially in ([BMP]Tf2N) is introduced. In the last part of the thesis, results of cyclic voltammetry, surface analysis SEM and elemental analysis EDX are discussed. MIS (metal-insulator-semiconductor) structure was determined comprising porous anodic alumina as the insulator and semiconductive TaxOy on the top of the insulator. The electric conductivity of the MIS structures was studied by using amper-volt IV and CV characteristics. The results of the cyclic voltametry, impedance spectroscopy and chronoamperometry are discussed.

The localized/delocalized duality of 5f electrons plays an important role in understanding the complex physics of actinides. Band-structure calculations based on the ad hoc assumption that 5f electrons are simultaneously localized and delocalized explained the observed dHvA experiments very well. This ad hoc assumption also gives the correct equilibrium volume for delta-Pu. Experimentally, the duality of 5f electrons is observed by inelastic neutron scattering experiments, or by soft X-ray angle-resolved photoelectron spectroscopy. It is worth recalling that the origin of partial localization in the 3d and 5f systems is quite different. In compounds with 3d electrons, the large crystalline electric field set up by the surrounding environment of transition metal ions plays a major role. On the other hand, in 5f systems, the Hund's rule correlations play the key role whilst the crystalline electric field is less important. In this thesis we have studied the effect of intra-atomic correlations on anisotropies in hopping matrix elements of different 5f orbitals. For that purpose, we used the effective model that includes on-site interactions that are responsible for Hund's rules and effective hopping terms that result from the hybridization of different 5f orbitals with the environment. Two different approximations, namely, rotationally invariant slave-boson mean-field (RISBMF) and infinite time-evolving block decimation (iTEBD), have been used to investigate the ground-state properties of the Hamiltonian. We have demonstrated that Hund's rule correlations enhance strongly anisotropies in hopping matrix elements. For a certain range of 5f bandwidth parameters this effect may result in a complete suppression of hopping processes for some of 5f orbitals, i.e., the system is in a partially localized phase. Within the RISBMF method, we calculated the ground-state properties and the phase diagram of the system. The suppression of hopping processes in some of 5f orbitals due to Hund's rule correlations can be seen through orbital-dependent quasiparticle weights. In a mean-field theory, a quasiparticle weight of zero for an orbital means a complete suppression of hopping processes in this orbital. Thus, quasiparticle weights and occupation numbers were used to classify partially localized phases. In the calculated phase diagram we obtain four partially localized phases that can be separated into two different sets. In the first set electrons in two orbitals are localized. In the second, electrons in one orbital are localized. The difference between the two sets is not simply the number of localized orbitals but the mechanism for the partial localization. For the first set, the Hund's rule mechanism applies: only those 5f electrons that enable the remaining ones to form a Hund's rule state will delocalize. This mechanism requires to have at least two localized orbitals, therefore it is definitely not applicable to those phases with only one localized orbital. For the second set, a situation similar to a single-band Mott-Hubbard transition applies. The direct on-site Coulomb interaction between jz and -jz electrons plays the key role for understanding the partial localization transition. In order to assess the validity of the RISBMF results we have used the iTEBD method to calculate the ground-state properties of a 1D system. Qualitatively, the two approaches agree with each other. However, we found an area where the RISBMF yields an artificial ground-state. Note that the mean-field method is worst for a 1D system. Therefore one shoud not judge from it the quality of the RISBMF method for the more general case.

No description available.

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.

No description available.

Cette thèse présente une étude expérimentale de la transition de Mott dans V2O3 dopé au chrome par deux différentes techniques expérimentales entre la température ambiante à 500 K et sous pressions jusqu'à 6 kbar. La technique ultrasonore fournit une mesure directe de la compressibilité et a été utilisé pour estimer les effets de la réseau sur la transition de Mott. Nous avons été en mesure de mener la première étude systématique de l'influence des degrés de liberté du réseau sur la transition de Mott dans V2O3. On a vérifiée l'existence d'une température électronique critique Tel c qui diffère de l'ordre de 3% ou 4% par rapport à la température de transition effectivement observée Tc. L'observation du comportement du mode transverse de la vitesse du son a permis de conclure que la transition de Mott à haute température a lieu sans brisure de symétrie et le paramètre d'ordre est un scalaire. Dans la deuxième partie de la thèse l'évolution de coefficient du Seebeck a été étudiée dans les mêmes conditions expérimentales. Les études autour du point critique nous ont permis d'établir un diagramme de phase expérimentale. Nous avons montré que le changement de S à la transition est dominé par le changement de la résistivité plutôt que par le changement de la symétrie particule-trou. En conséquence, nous avons pu relier les lois d'échelle pour les paramètres critiques relatifs à la conductivité à nos données. Dans le voisinage de la transition nos données sont assez bien reproduites par les exposants de champ moyen observés dans les mesures de conductivité.

In this thesis we employ various mean-field approaches to study the shortrange interaction effects in topological insulators. We start with the Kane-Mele model on the decorated honeycomb lattice and study the stability of topological insulator phase against different perturbations. We establish an adiabatic connection between a noninteracting topological insulator and a strongly interacting spin liquid in its Majorana fermion representation. We use the Hartree-Fock mean-field approach, slave-rotor approach and slave-boson approach to study correlation effects related to topological insulators. With the spontaneous symmetry breaking mechanism, we can have an interaction driven topological insulator with extended Hubbard models on the kagome lattice and decorated honeycomb lattice. For the interplay among spin-orbit coupling, distortion and correlation effect in transition metal oxides, we use the slave-rotor mean-field approach to study its phase transition. We identify regimes where a strong topological Mott insulator and a weak topological insulator reside due to the strong Coulomb interaction and distortion. This is relevant to experiments with the transition metal oxides as they hold promise to realize topological insulators. To study the doping effects and a possible spin liquid in Kane-Mele-Hubbard model on the honeycomb lattice, we employ the slave-boson mean-field approach which is appropriate for the intermediate interaction strength. We compare our results with those obtained from other methods. / text

Cette thèse présente l'étude de la coexistence métal isolant dans deux systèmes très différents pour la communauté scientifique de la matière condensée : l'isolant topologique 3D Bi2Te3 et le composé prototype de la transition de Mott V2O3. Ces deux systèmes ont été étudiés par des techniques basées sur la spectroscopie de photoélectrons. La première technique utilisée est le TR-ARPES (time and angle resolved photoemission spectroscopy), avec une résolution temporelle de 80 fs, appliquée à l'isolant topologique 3D Bi2Te3 pour distinguer la dynamique ultra-rapide des états métalliques de la surface de celle des états isolants du volume. Cette mesure a permet de comprendre les différents mécanismes de diffusion entre la surface et le volume, ainsi que l'amélioration de la relaxation du cône de Dirac par la préexistence à la sous-surface d'une bande de flexion. La seconde technique utilisée dans cette thèse est le SPEM (scanning photoelectron microscopy), avec une résolution spatiale de 150 nm, permettant d'étudier la coexistence des domaines métalliques et isolants à la transition de Mott dans V2O3 ; cette coexistence a pour origine le caractère 1 er ordre de la transition. La mesure montre une coexistence métal-isolant dans le Cr-dopé : les domaines métalliques sont dus à des centres de nucléations < 150 nm et la forme des domaines est clairement liée à la forme des marches de clivage.

No description available.

Cette thèse vise à étudier la stabilité des phases métalliques et isolantes en compétition dans les systèmes 2D 1T-VS2 et composés dérivés, Cu⅔V⅓V2S4 et Sr3V5S11. Pour atteindre cet objectif, nous avons développé et optimisé des voies ad hoc de synthèse à hautes pressions pour stabiliser les nouvelles phases sous forme de monocristal de haute qualité, qui nous permettrait d’étudier les propriétés électroniques et de transport. Un important résultat de notre étude est le contrôle de la concentration, x, des atomes interstitiels V situés entre les plans adjacents VS2 dans le système V1+xS2, qui est obtenu en variant la pression de synthèse. Cela nous a permis d’explorer le diagramme de phase T-x du système. Le résultat principal de cette étude est que la phase CDW observée dans la phase stoichiométrique (x = 0) disparait rapidement avec x, alors que les propriétés métalliques sont augmentées. Dans Cu⅔V⅓V2S4, la substitution partielle du V par Cu dans le site interstitiel change complètement le système en un fermion semi-lourd aux caractéristiques prononcées du liquide de Fermi jusqu’à ~ 20 K, où la transition de Kondo apparait. Ce phénomène inattendu dans les sulfures suggère que la force des corrélations électroniques dans ces composés peut être pilotée en variant simplement la nature chimique et la concentration de l’atome intercalé. La force modérée des corrélations dans Cu⅔V⅓V2S4 ouvre le chemin vers une description théorique fiable de la disparition du régime de liquide de Fermi. Les corrélations électroniques apparaissent importantes aussi pour piloter une phase isolante dans Sr3V5S11, qui devrait être un métal d’après la théorie conventionnelle de bande. Dans ce cas, les corrélations peuvent être augmentées par la dimensionnalité réduite créée par un large écartement des couches VS2 et par une modulation structurale 1D des couches. Des études supplémentaires pourront clarifier s’il s’agit d’un mécanisme d’Anderson de faible localisation qui contribue à la stabilisation d’un état isolant dans les plans pristine métalliques VS2. / This thesis work aims at studying the stability of the metallic and insulating phases that compete in the two-dimensional 1T-VS2 system and related compounds, Cu⅔V⅓V2S4, and Sr3V5S11. We have developed and optimized ad hoc high-pressure synthesis routes in order to stabilize the above novel phases in the form of high-quality single crystals, which enabled us to reliably investigate their electronic and transport properties. An important achievement of our study is the control of the concentration, x, of interstitial V atoms located between adjacent VS2 planes in the V1+xS2 system, which is obtained by varying synthesis pressure. This has enabled us to explore the T-x phase diagram of the system. The main result of this study is that the CDW phase observed in the stoichiometric (x=0) phase quickly disappears with x, whilst the metallic properties are enhanced. In Cu⅔V⅓V2S4, the partial substitution of V for Cu in the interstitial site is found to completely change the system into a semi-heavy fermion with pronounced Fermi-liquid characteristics down to ~20 K, where a Kondo transition occurs. These unexpected phenomena in sulfides suggest that the strength of the electronic correlations in these compounds can be tuned by simply varying the chemical nature and concentration of the intercalant atom. The moderate strength of the correlations in Cu⅔V⅓V2S4 opens the way towards a reliable theoretical description of the breakdown of the Fermi liquid regime. Electronic correlations appear to be important also to drive an insulating phase in Sr3V5S11, which should be a metal within a conventional band picture. In this case, the correlations may be enhanced by the reduced dimensionality caused by a large spacing between VS2 layers and by a 1D structural modulation of the layers. Further studies may clarify whether the Anderson’s mechanism of weak localization contributes to the stabilization of an insulating state in the pristine metallic VS2 planes.