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We have performed neutron diffraction measurements of tetragonal Sr2 CrO4 to study the crystallographic structure as a function of temperature and the magnetic properties developed below . We have measured the precise positions of the oxygen atoms of the CrO6 octahedra and observed that, with decreasing temperature, the octahedra pass from elongated to almost ideal, contradicting the intuitive idea of a Jahn-Teller effect. DFT calculations explain this apparent anomalous behaviour by establishing that energy level degeneracy only exists in a highly elongated octahedra. Our study agrees with the idea of a weak orbital ordering transition around 150 K followed by an itinerant antiferromagnetic ordering at 110 K with a wave vector determined both from neutron powder diffraction and theory.

BaFe2Se3 is a potential superconductor material exhibiting transition at 11 K and ambient pressure. Here we extended the structural and performed electrical resistivity measurements on this compound up to 51 GPa and 20 GPa, respectively, in order to distinguish if the superconductivity in this sample is intrinsic to the BaFe2Se3 phase or if it is originating from minor FeSe impurities that show a similar superconductive transition temperature. The electrical resistance measurements as a function of pressure show that at 5 GPa the superconducting transition is observed at around 10 K, similar to the one previously observed for this sample at ambient pressure. This indicates that the superconductivity in this sample is intrinsic to the BaFe2Se3 phase and not to FeSe with Tc > 20 K at these pressures. Further increase in pressure suppressed the superconductive signal and the sample remained in an insulating state up to the maximum achieved pressure of 20 GPa. Single-crystal and powder X-ray diffraction measurements revealed two structural transformations in BaFe2Se3: a second order transition above 3.5 GPa from Pnma (CsAg2I3-type structure) to Cmcm (CsCu2Cl3-type structure) and a first order transformation at 16.6 GPa. Here, {\gamma}-BaFe2Se3 transforms into {\delta}-BaFe2Se3 (Cmcm, CsCu2Cl3-type average structure) via a first order phase transition mechanism. This transitions is characterized by a significant shortening of the b lattice parameter of {\gamma}-BaFe2Se3 (17%) and accompanied by an anisotropic expansion in the orthogonal ac plane at the transition point.

New insight into the superconducting properties of HgBa2CuO4 (Hg-1201) cuprates is provided by combined measurements of electrical resistivity and single crystal X-ray diffraction under pressure. The changes induced by increasing pressure up to 20 GPa in optimally doped single crystals were investigated. The resistivity measurements as a function of temperature show a metallic behavior up to ~10 GPa that gradually passes into an insulating state, typical of charge ordering, which totally suppresses superconductivity above 13 GPa. The changes in resistivity are accompanied by the apparition of sharp Bragg peaks in the X-ray diffraction patterns, indicating that the charge ordering is accompanied by a 3D oxygen ordering. Considering that pressure induces a charge transfer of about 0.02 at 10 GPa, our results are the first observation of charge order competing with superconductivity developed in the overdoped region of the phase diagram of a Hg-based cuprate.

International audience; We report on the structure and physical properties of a bidimensional chromate, Sr 4 Cr 3 O 10 , the n=3 member of the Ruddlesden-Popper Sr n+1 Cr n O 3n+1 series. For the first time, using complementary x-ray powder diffraction and electron diffraction data, we have solved its layered crystallographic structure. Our study shows also that this high pressure phase is insulating and antiferromagnetic below T N =280K, a similar behavior already observed for n=1, 2 and n=+∞ members.

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

We report the synthesis and characterization of the novel silicide LaFeSiH displaying superconductivity with onset at 11 K. We find that this pnictogen-free compound is isostructural to LaFeAsO, with a similar low-temperature tetragonal to orthorhombic distortion. Using density functional theory we show that this system is also a multiband metal in which the orthorhombic distortion is likely related to single-stripe antiferromagnetic order. Electrical resistivity and magnetic susceptibility measurements reveal that these features occur side-by-side with superconductivity, which is suppressed by external pressure., Comment: 5 + epsilon pages, 7 figures, Supplemental Material on demand; v2 includes additional magnetization and resistivity data; v3 includes additional resistivity data from LaFeSiH single crystal

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.

International audience; High-pressure electrical resistivity ab measurements on intermetallic Pr(Co, Ni, Pt)2B2C compounds were performed down to 2K. At room pressure the ab(T ) curves for the non superconducting Pr(Co, Ni)2B2C compounds exhibit magnetic correlations at about 10 and 4 K, respectively. At low temperatures, PrCo2B2C shows a large spin-dependent electron scattering in comparison to PrNi2B2C. Under applied pressure the magnetic scattering tends to be suppressed more effectively in PrCo2B2C than in PrNi2B2C. The low temperature behavior of ab(T, P) for PrNi2B2C and PrCo2B2C suggests a spin fluctuations mechanism. In the other hand PrPt2B2C compound shows superconductivity at about 6 K and under pressure its superconducting transition temperature tends to be degraded at a rate dTC/dP = -0.34 K/GPa, as expected in compounds with transition metals. The experimental results in Co, Ni and Pt based compounds are analyzed from the point of view of the external and chemical internal pressure effects.

Quantum critical points (QCP) accompanied by superconductivity are ubiquitous in condensed matter physics. In general, the transition temperature T 0 of an ordered state, e.g. antiferromagnetic, goes to zero under the influence of an external parameter, e.g. pressure. Superconductivity appears before the disappearance of the ordered state, but reaches its maximum T c when T 0 = 0 . Presently, the implications of the QCP׳s on superconductivity are a subject of debate. It is proposed here that both transition temperatures satisfy the relation T ˜ 0 2 + T ˜ c 2 = 1 , where the tilde indicates normalization to the maximum values. Inspired from the basic postulate of SO ( N ) theories of superconductors, it is proposed as an extension from Bilbro–McMillan relation.

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 performed electrical resistivity measurements under pressures up to 20 GPa between 1 and 300 K on monocrystalline hexagonal Rb 0.19 WO 3 . For pressures lower than ∼5 GPa, we observe a decrease of the metallic-like resistivity at room temperature as well as a small decrease of T c . At this pressure, the resistivity starts to increase slowly up to 10 GPa accompanied by a sharper decrease of T c .The resistivity curves above 10 GPa denote an activated behaviour and a T c lower than 3 K indicating that there is a phase transition that takes place gradually between 5 and 10 GPa. We interpret our measurements as the result of structural transformations under high pressure.

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.

International audience; Many superconductors show a low temperature electrical resistivity of Fermi liquid type ρ =AT2. We show empirically that there exists a relationship between A and Tc when both vary under an external parameter, such as pressure. The more resistive the compound the higher the Tc. Through the analysis of Landau theory of FL, we find that it is a general feature of FL, due to the fact that the scattering that is the main cause of τ is the same one that bounds the pairs that condensed at Tc. We devise a method that allows the determination of the coupling constant λ, which is validated through application to 3He-'s superfluid transitions and τ's extracted from different properties. This method works for conventional superconductors, but fails with heavy fermions.

The antiferromagnetic order and structural distortion in the LaFe(As1-xSbx)O system have been investigated by powder neutron diffraction and physical properties measurements. Polycrystalline samples of LaFe(As1-xSbx)O (x, 21 pages, 6 figures

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 tetragonal FeSe under pressures up to 26GPa using synchrotron radiation and diamond anvil cells. The bulk modulus of the tetragonal phase is 28.5(3)GPa, much smaller than the rest of Fe based superconductors. At 12GPa we observe a phase transition from the tetragonal to an orthorhombic symmetry. The high pressure orthorhombic phase has a higher Tc reaching 34K at 22GPa., 15 pages, 4 figures

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 take advantage of the precise determination of the crystallographic structure of complex magnetic systems with localized moments to calculate their exchange interaction values using ab initio methods. The magnetic interactions are mapped onto a Heisenberg model whose exchange interaction terms are fitted to first-principles total energy calculations for different spin configurations. This method is well adapted to systems with well defined local moments, as is often the case in oxides. We discuss the delafossite YCuO2.5, with triangular layers of magnetic ions without magnetic ordering, and for which the magnetic interactions are unknown. Using these ab initio parameters, we perform exact diagonalization, which yields a valence bond solid ground state and shows excellent agreement with experimental magnetic susceptibility, provided that the different interactions between Cu1 and Cu2 atoms in the same triangle are taken into account.

We measure the pressure dependence of the electrical resistivity and the crystal structure of iron superconductor Sr2VO3FeAs. Below ~10 GPa the structure compresses but remains undeformed, with regular FeAs4 tetrahedrons, and a constant Tc. Beyond 10 GPa, the tetrahedron strongly distorts, while Tc goes gradually to zero. Band structure calculations of the undistorted structure show multiple-nesting features that hinder the development of an antiferromagnetic (AF) ground state, allowing the appearance of superconductivity. The deformation of the tetrahedra that breaks band degeneracy degrades multiple nesting, thus favouring one particular AF state at the expense of Tc.