Ordres cachés et magnétisme étudiés par spectroscopie Raman en conditions extrêmes

New electronic phases, the so-called "exotic phases", where the order parameter isn’tclearly identified, sometimes appear at low temperature and/or under high pressure in thematter. This thesis focuses on the study by Raman spectroscopy of these exotic orders inthe URu2Si2 and PrRu4P12 compounds and of the magnetism in the multiferroïc compoundBiFeO3 . For this purpose, we developed a new Raman spectroscopy set-up probing very lowenergy excitations (1 meV) under high pressure (20 GPa) and at low temperature (3 K). Inthe PrRu4P12 skutterudite, we followed crystal field excitations and phonon modes underpressure up to 17 GPa. Our results confirm that the lattice dynamic is a side effect of themetal-insulator transition. However, they refute current theories that explain the electronicorder’s evolution under pressure based on crystal field levels inversion. They underline thepossible existence of magnetism under pressure. The URu2Si2 compound has been studiedat room pressure and low temperature. We observe new signatures of the hidden orderphase consisting of a very narrow excitation and a low energy gap, both only seen in theA2g symmetry. The study of the electronic response and of the lattice dynamic shows theinfluence of the anisotropy on the Kondo physics. Finally, combining Raman measurementsunder pressure at room temperature, numerical simulations and theoretical calculations, weprovide a global understanding of BiFeO3’s magnetism through its various structural phasesup to 12 GPa.Outre le magnétisme classique, de nouvelles phases électroniques, dites "exotiques", dontle paramètre d’ordre n’est pas clairement identifié, apparaissent parfois dans la matière àbasse température et/ou sous pression. Cette thèse porte sur l’étude de ces ordres exotiquesdans les matériaux URu2Si2 et PrRu4P12 et du magnétisme dans le composé multiférroïqueBiFeO3 par spectroscopie Raman. Pour cela, nous avons développé un nouveau dispositifde spectroscopie Raman des excitations de très basses énergies (1 meV) sous haute pression(20 GPa) et à basse température (3 K). Dans la skutterudite PrRu4P12, nous avons suivil’évolution en pression des excitations de champ cristallin et des phonons jusqu’à 17 GPa.Nos résultats confirment le fait que la dynamique de réseau est un effet secondaire de latransition métal-isolant. En revanche, ils infirment les théories actuelles expliquant l’évolutionde l’ordre électronique sous pression, basées sur un croisement de niveaux de champ cristallin.Ils pointent une possible présence de magnétisme sous pression. Le composé URu2Si2 a étéétudié à pression ambiante et basse température. Nous observons de nouvelles signaturesde la phase d’ordre caché, à savoir une excitation étroite et un gap de basse énergie, et ce,uniquement dans la symétrie A2g. L’étude de la réponse électronique et de la dynamique deréseau montre l’influence de l’anisotropie de la physique Kondo. Enfin, grâce à la combinaisonde notre mesure Raman sous pression à température ambiante, d’une simulation numérique etd’un calcul théorique, nous proposons une compréhension globale du magnétisme de BiFeO3 àtravers ses différentes phases structurales jusqu’à 12 GPa

Clean-limit superconductivity in Im3¯m H3S synthesized from sulfur and hydrogen donor ammonia borane

We present detailed studies of the superconductivity in high-pressure H$_3$S. X-ray diffraction measurements show that cubic Im$\bar{3}$m H3S was synthesized from elemental sulfur and hydrogen donor ammonia borane (NH$_3$BH$_3$). Our electrical transport measurements confirm superconductivity with a transition temperature T$_c$=197K at 153 GPa. From the analysis of both the normal-state resistivity and the slope of the critical field, we conclude that the superconductivity is described by clean-limit behavior. A significant broadening of the resistive transition in finite magnetic field is found, as expected for superconductors. We identify a linear temperature-over-field scaling of the resistance at the superconducting transition which is not described by existing theories

Fermi surface reconstruction and electron dynamics at the charge-density-wave transition in TiSe2

The evolution of the charge carrier concentrations and mobilities are examined across the charge-density-wave (CDW) transition in TiSe2. Combined quantum oscillation and magnetotransport measurements show that a small electron pocket dominates the electronic properties at low temperatures whilst an electron and hole pocket contribute at room temperature. At the CDW transition, an abrupt Fermi surface reconstruction and a minimum in the electron and hole mobilities are extracted from two-band and Kohler analysis of magnetotransport measurements. The minimum in the mobilities is associated with the overseen role of scattering from the softening CDW mode. With the carrier concentrations and dynamics dominated by the CDW and the associated bosonic mode, our results highlight TiSe2 as a prototypical system to study the Fermi surface reconstruction at a density-wave transition.Comment: 15 pages, 3 figures, including supplementary informatio

Lifshitz transition enabling superconducting dome around the quantum critical point in TiSe2_2

Superconductivity often emerges as a dome around a quantum critical point (QCP) where long-range order is suppressed to zero temperature. So far, this has been mostly studied in magnetically ordered materials. By contrast, the interplay between charge order and superconductivity at a QCP is not fully understood. Here, we present resistance measurements proving that a dome of superconductivity surrounds the charge-density-wave (CDW) QCP in pristine samples of 1$T$-TiSe$_2$ tuned with hydrostatic pressure. Furthermore, we use quantum oscillation measurements to show that the superconductivity sets in at a Lifshitz transition in the electronic band structure. We use density functional theory to identify the Fermi pockets enabling superconductivity: large electron and hole pockets connected by the CDW wave vector $\vec{Q}$ which emerge upon partial suppression of the zero-pressure CDW gap. Hence, we conclude that superconductivity is of interband type enabled by the presence of hole and electron bands connected by the CDW $\vec{Q}$ vector. Earlier calculations show that interband interactions are repulsive, which suggests that unconventional s$_{\pm}$ superconductivity is realised in TiSe$_2$ - similar to the iron pnictides. These results highlight the importance of Lifshitz transitions in realising unconventional superconductivity and help understand its interaction with CDW order in numerous materials.Comment: 21 pages, 5 figure

Exciton-phonon coupling in InP quantum dots with ZnS and (Zn, Cd) Se shells

InP-based colloidal quantum dots are promising for optoelectronic devices such as light-emitting diodes and lasers. Understanding and optimizing their emission process is of scientific interest and essential for large-scale applications. Here we present a study of the exciton recombination dynamics in InP QDs with various shells: ZnS, ZnSe, and (Zn,Cd)Se with different amounts of Cd (5, 9, 12%). Phonon energies extracted from Raman spectroscopy measurements at cryogenic temperatures (4-5 K) are compared with exciton emission peaks observed in fluorescence line narrowing spectra. This allowed us to determine the position of both the bright F = +/- 1 state and the lowest dark F = +/- 2 state. We could identify the phonon modes involved in the radiative recombination of the dark state and found that acoustic and optical phonons of both the core and the shell are involved in this process. The Cd content in the shell increases electron wave-function delocalization, and thereby enhances the exciton-phonon coupling through the Frohlich interaction

Conceptualising Contemporary Antisemitism: How Debates About Immigration Have Shaped the Understanding of Jew-Hatred in Germany and Britain since 1945

Nanocrystalline InP quantum dots (QDs) hold promise for heavy-metal free opto-electronic applications due to their bright and size-tunable emission in the visible range. Photochemical stability and high photoluminescence (PL) quantum yield are obtained by a diversity of epitaxial shells around the InP core. To understand and optimize the emission line shapes, the exciton fine structure of InP core/shell QD systems needs be investigated. Here, we study the exciton fine structure of InP/ZnSe core/shell QDs with core diameters ranging from 2.9 to 3.6 nm (PL peak from 2.3 to 1.95 eV at 4 K). PL decay measurements as a function of temperature in the 10 mK to 300 K range show that the lowest exciton fine structure state is a dark state, from which radiative recombination is assisted by coupling to confined acoustic phonons with energies ranging from 4 to 7 meV, depending on the core diameter. Circularly polarized fluorescence line-narrowing (FLN) spectroscopy at 4 K under high magnetic fields (up to 30 T) demonstrates that radiative recombination from the dark F = ±2 state involves acoustic and optical phonons, both from the InP core and the ZnSe shell. Our data indicate that the highest-intensity FLN peak is an acoustic phonon replica rather than a zero-phonon line, implying that the energy separation observed between the F = ±1 state and the highest-intensity peak in the FLN spectra (6 to 16 meV, depending on the InP core size) is larger than the splitting between the dark and bright fine structure exciton states

Data for publication "Experimental evidence for orthorhombic Fddd crystal structure in elemental yttrium above 100 GPa"

Raw data for publication of manuscript titled "Experimental evidence for orthorhombic Fddd crystal structure in elemental yttrium above 100 GPa". Accepted in Physical Review B

Data for publication "Experimental evidence for orthorhombic Fddd crystal structure in elemental yttrium above 100 GPa"

Raw data for publication of manuscript titled "Experimental evidence for orthorhombic Fddd crystal structure in elemental yttrium above 100 GPa". Accepted in Physical Review B