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Fil: Jeanneau, Justin. Universite Grenoble Alpes; Francia. Centre National de la Recherche Scientifique; Francia

The high-pressure superconducting properties and the local structure of the Ba8Si46 clathrate have been studied using electrical resistance and x-ray absorption spectroscopy measurements up to more than 20 GPa. At pressures above 10–13 GPa, corresponding to a well-known volume collapse phase transformation, a sudden increase in the critical temperature leads to a maximum value of the superconducting critical temperature Tc of ∼8.5–9 K at 20 GPa. In the low-pressure clathrate phase, the superconducting critical temperature decreases as pressure is applied with an electron-phonon coupling constant λ=1.1 derived from the temperature evolution of the electrical resistance at different pressures. A progressive disorder in the Ba-Si correlations in the Ba@Si24 cages of the structure is observed for pressures beyond ∼5 GPa. These observations exclude a structural homothecy at the volume collapse transition. The high-pressure collapsed phase of Ba8Si46 is then associated with local structural changes and shows enhanced superconducting properties.

We present measurements of the superconducting and charge density wave critical temperatures (Tc and TCDW) as a function of pressure in the transition metal dichalchogenides 2H-TaSe2 and 2H-TaS2. Resistance and susceptibility measurements show that Tc increases from temperatures below 1 K up to 8.5 K at 9.5 GPa in 2H-TaS2 and 8.2 K at 23 GPa in 2H-TaSe2. We observe a kink in the pressure dependence of TCDW at about 4 GPa that we attribute to the lock-in transition from incommensurate CDW to commensurate CDW. Above this pressure, the commensurate TCDW slowly decreases coexisting with superconductivity within our full pressure range.

We have studied the resistance of $1T$-CrSe${}_{2}$, as the Cr atoms are substituted by V or Ti. The V replacement leads to a logarithmic increase in the resistance as the temperature is lowered, proportional to the V concentration. While this behavior is consistent with the Kondo effect, the weak dependence of the resistance with magnetic field and the fact that the system has antiferromagnetic order, rule out a Kondo effect due to spin degeneracy. In contrast to the case of V, Ti substitution does not increase the logarithmic term while application of pressure destroys it. Calculations of the electronic structure within the framework of density functional theory, maximally localized Wannier functions, and many-body calculations in a cluster containing a Cr or V atom and its six nearest-neighbor Se atoms, helped to reveal the existence of an orbital Kondo effect due to orbital degeneracy in the V substitutional impurities.

Recently, a golden colored, dense polymorph of titanium sesquioxide, Ti${}_{2}$O${}_{3}$ with a Th${}_{2}$S${}_{3}$-type structure, has been synthesized at high-pressure high-temperature conditions. In this paper, we present results of investigations of structural, optical, and electronic transport properties of this unusual golden polymorph of Ti${}_{2}$O${}_{3}$ under high pressure. Several experimental techniques were used, including x-ray diffraction studies using synchrotron radiation, Raman spectroscopy, electrical resistivity, and thermoelectric power. The structural studies showed that the Th${}_{2}$S${}_{3}$-type lattice is conserved under pressure, while it is subjected to an isostructural phase transition with a \ensuremath{\sim}0.7$%$ volume drop at 38.5 GPa. We speculated that this transition could be driven by the $s\ensuremath{\rightarrow}d$ electron transfer in the Ti atoms. For the Th${}_{2}$S${}_{3}$-type Ti${}_{2}$O${}_{3}$, we have established a bulk modulus value, ${B}_{0}$ $=$ 258.3 GPa at ${B}_{0}^{\ensuremath{'}}$ $=$ 4.1. A full profile analysis of the diffraction patterns allowed us to discover anomalies in the compression behavior of the Th${}_{2}$S${}_{3}$-type structure. The bond valence sums method suggested that at ambient conditions the Ti cations have predominantly Ti${}^{3+}$ oxidation state, but applied pressure stimulates a partial charge disproportionation between the Ti1 and Ti2 sites achieving the maximal effect---reduction of the Ti1 cations to \ensuremath{\sim}Ti${}^{2.5+}$ and oxidation of the Ti2 ones to \ensuremath{\sim}Ti${}^{3.5+}$ near 14 GPa. Pressure evolution of Raman spectra across the above crossovers showed distinct changes corroborating the above findings. The high-pressure electronic transport studies confirmed that the Th${}_{2}$S${}_{3}$-type Ti${}_{2}$O${}_{3}$ remains semiconducting up to 21 GPa at ambient and low temperatures down to 4.2 K. These studies found additional features, e.g., in the activation energy curve near 7 GPa, that is accompanied by inversion of the dominant conductivity type from electron to hole. The intriguing high-pressure behavior of Ti${}_{2}$O${}_{3}$ with the Th${}_{2}$S${}_{3}$-type structure can contribute to better understanding of high-pressure properties of transition-metal sesquioxides.

We measure the electrical resistance of $o$-TaS${}_{3}$ between 1 and 300 K under pressures up to 20 GPa. We observe a gradual decrease of the charge density wave transition temperature ${T}_{\mathrm{CDW}}$ with increasing pressure $P$ following a mean field quantum fluctuation power law ${T}_{\mathrm{CDW}}=215\phantom{\rule{0.28em}{0ex}}\mathrm{K}{[({P}_{C}\ensuremath{-}P)/{P}_{C}]}^{\ensuremath{\gamma}}$ with a quantum critical point at a pressure ${P}_{C}=11.5\phantom{\rule{0.28em}{0ex}}\mathrm{GPa}$ and $\ensuremath{\gamma}\ensuremath{\approx}0.5$. Around the quantum critical point (QCP) we observe a superconducting dome with a maximum superconducting transition temperature ${T}_{c}=3.1\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. Such dome is similar to superconducting domes around other types of order suggesting that the QCP is directly responsible for the enhancement of superconductivity through a universal mechanism still not well understood.

We have studied the occurrence of superconductivity under pressure in the ${\mathrm{CaSi}}_{2}$ family of compounds, namely the semimetallic trigonal phase and the $\ensuremath{\alpha}\ensuremath{-}{\mathrm{ThSi}}_{2}$-type tetragonal one. Although only the latter is superconducting ${(T}_{c}=1.56\mathrm{K})$ at ambient pressure, starting from the trigonal phase and applying high pressure (g12 GPa) we have found a new superconducting phase with ${T}_{c}^{\mathrm{onset}}\ensuremath{\sim}14\mathrm{K},$ among the highest ever found in silicon based materials.

We have studied the effect of substitution of Cr in metastable $1T$-CrSe${}_{2}$ by Ti and V on its structural and magnetic properties. The structural transitions observed between 165--180 K in the pure material are stomped by the doping. The pure compound has a magnetization corresponding to an antiferromagnetic (AF) ground state. On Ti substitution, we observe an increase of the lattice constants and a gradual passage towards a ferromagnetic state, while V replacement maintains AF order up to our highest doping, ${x}_{\text{V}}=0.5$. With our experimental results and the help of first-principles calculations, we construct the phase diagram of the system.

The electrical resistivity, crystalline structure, and electronic properties calculated from the experimentally measured atomic positions of the compound SmFeAsO${}_{0.81}$F${}_{0.19}$ have been studied up to pressures \ensuremath{\sim}20 GPa. The correlation between the pressure dependence of the superconducting transition temperature (${T}_{c}$) and crystallographic parameters on the same sample shows clearly that a regular FeAs${}_{4}$ tetrahedron maximizes ${T}_{c}$ through optimization of carrier transfer to the FeAs planes as indicated by the evolution of the electronic band structures.

We have studied the structural and superconducting properties of the compound ${\text{LaFeAsO}}_{0.9}{\text{F}}_{0.1}$ under pressures up to 32 GPa using synchrotron radiation and diamond anvil cells. We obtain an ambient pressure bulk modulus ${K}_{0}=78(2)\text{ }\text{GPa}$ compressibility comparable to some cuprates. At high pressures, the sample is in the overdoped region, with a linear decrease with pressure variation of the superconducting transition temperature.

We have measured transport properties as a function of temperature and pressure up to $30\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ in the ${\mathrm{Na}}_{x}\mathrm{Co}{\mathrm{O}}_{2}$ system. For the $x=0.5$ sample, the transition temperature at $53\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ increases with pressure, while paradoxically the sample passes from an insulating state to a metallic ground state. Compression on the $x=0.75$ sample transforms the sample from a metallic state to an insulating state. We discuss our results in terms of interactions between band structure effects and ${\mathrm{Na}}^{+}$ order.

We report measurements on ${\mathrm{MgB}}_{2}$ up to $\ensuremath{\sim}40 \mathrm{GPa}.$ Increasing pressure yields a monotonous decrease of the lattice parameters and of the $c/a$ ratio, but no structural transition down to parameters smaller than those of ${\mathrm{AlB}}_{2}.$ The transition superconducting temperature also decreases with temperature in a sample-dependent way. The results are explained by an increase of the filling of the two-dimensional ${p}_{\mathrm{xy}}$ bands with pressure, the Mg stoichiometry determining the starting position of the Fermi level. Our measurements indicate that these hole bands are the relevant ones for superconductivity.