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The recent discovery of magnetic van der Waals (vdW) materials triggered a wealth of investigations in materials science and now offers genuinely new prospects for both fundamental and applied research. Although the catalog of vdW ferromagnets is rapidly expanding, most of them have a Curie temperature below 300 K, a notable disadvantage for potential applications. Combining element-selective X-ray magnetic imaging and magnetic force microscopy, we resolve at room temperature the magnetic domains and domain walls in micron-sized flakes of the CrTe

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.

We synthesize single-crystals of ScPdBi, a Half-Heusler compound by self-flux growth technique and report its physical properties such as magneto-transport and specific heat down to 2 K. Resistivity measurements were performed on these single-crystals at ambient and high-pressure conditions. Temperature-dependent resistivity measurements reveal that ScPdBi shows the metallic character at ambient pressure and without applied magnetic field. The metallic character of ScPdBi was un-altered even in extreme conditions such as high pressure (up to 19 GPa) and magnetic field (up to 9 T). We observe an upturn in the resistivity which persists even at high pressure. We rule-out the presence of the Kondo effect by performing the specific heat measurements down to 2 K which resulted in a low Sommerfeld coefficient (γ ≈ 2.6 ± 0.9 mJ mol−1 K−2). This anomaly in resistivity below 30 K could be attributed to an electron-hole scattering process or a carrier imbalance effect.

Due to the high degree of improvement and prediction success of ab initio calculations, the advance in Materials Science has been partially shifted from the empirical chemical quest of new compounds to the systematic calculation of putative structures that can yield compounds with promising properties. In particular, two dimensional (2D) materials are at present a central target, as many potential devices rely on particular characteristics of a single layer of atoms or low dimensional electronic active sheets in bulk materials. Several works have focused on this paradigm, yielding a large list of new interesting 2D candidates. Here, we introduce a technique based on an algorithmic filter for searching prospective candidates in very large electronic structure databases. It is designed to detect electronic bands with low energy dispersion along particular directions in reciprocal space, i.e. those perpendicular to certain plane or surface. We propose it as an absolute criterion for obtaining all possible two dimensional compounds. With it we show that previous explorations have missed out a whole group of possible two dimensional materials, namely only electronically 2D. Fil: Nuñez, Matias. Comision Nacional de Energia Atomica. Gerencia de Area de Aplicaciones de la Tecnología Nuclear. Gerencia de Investigación Aplicada. Grupo de Materiales Nucleares; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte. Instituto de Investigaciones en Biodiversidad y Medioambiente. Universidad Nacional del Comahue. Centro Regional Universidad Bariloche. Instituto de Investigaciones en Biodiversidad y Medioambiente; Argentina Fil: Weht, Ruben Oscar. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comisión Nacional de Energía Atómica. Gerencia de Área Investigaciones y Aplicaciones No Nucleares. Gerencia Física (CAC). Departamento de Física de la Materia Condensada; Argentina. Universidad Nacional de San Martín. Instituto Sabato; Argentina Fil: Nunez Regueiro, Manuel Daniel. Institut Politechnique de Grenoble; Francia

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.

We report on the discovery of a Ba-based chromate obtained by high pressure–high temperature treatment of the low pressure orthorhombic Ba2CrO4 phase. By combining transmission electron microscopy and powder X-ray diffraction measurements, we have determined its crystallographic structure. This new Cr-oxide has a cubic lattice with a = 13.3106(6) A built from a three-dimensional network of two Cr sites, Cr1 and Cr2, both in octahedral environments, with face sharing between Cr1 and Cr2 octahedra and corner-sharing between two Cr1 octahedra. The resulting chemical composition Ba19Cr12O45 and bond valence sum analysis suggest a possible charge disproportion between Cr4+ in the Cr1 site and Cr5+ in the Cr2 site. Finally analysis of magnetization measurements indicates antiferromagnetic correlations between Cr cations and also points toward a probable charge disproportion between Cr sites.

We report on the discovery of a Ba-based chromate obtained by high pressure-high temperature treatment of the low pressure orthorhombic Ba

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

We perform an extensive study of Sr_{3}Cr_{2}O_{7}, the n=2 member of the Ruddlesden-Popper Sr_{n+1}Cr_{n}O_{3n+1} system. An antiferromagnetic ordering is clearly visible in the magnetization and the specific heat, which yields a huge transition entropy, Rln(6). By neutron diffraction as a function of temperature we have determined the antiferromagnetic structure that coincides with the one obtained from density functional theory calculations. It is accompanied by anomalous asymmetric distortions of the CrO_{6} octahedra. Strong coupling and Lanczos calculations on a derived Kugel-Khomskii Hamiltonian yield a simultaneous orbital and moment ordering. Our results favor an exotic ordered phase of orbital singlets not originated by frustration.

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 studied the pressure and temperature dependence of the electrical resistivity of the superconducting compound magnesium diboride (MgB 2 ). The superconducting transition temperature decreases monotonically with pressure, being parabolic or linear, depending on samples. The rate of decrease under pressure is higher than in conventional superconductors. We discuss our results in terms of the semimetallic character of the electronic band structure of MgB 2 .

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.

We report in-plane resistivity and elastic constant C33 measurements on the Na0.8CoO2 and Na0.5CoO2 systems. An ordering transition is found at T0=280 K for Na0.8CoO2. The temperature dependence of the elastic constant C33 propagating perpendicular to the CoO2 layers is interpreted, in a phenomenological approach, as being due to the anharmonicity of atomic vibrations. The effective Gruneisen parameter γeff deduced directly from the temperature dependence of C33 exhibits important changes at the charge ordering, magnetic and metal-insulator phase transitions.

We have synthesized for the first time the metastable compound 1T-CrTe2. We have done its complete structural characterization and measured its magnetization, specific heat and electrical resistivity between 4 and 330 K. We have also performed detailed band structure calculations. We have found that it crystallizes in the CdI2 structure type and that its electrical resistance follows a metallic behaviour below room temperature. Its magnetization and specific heat curves show that the compound has a transition to a ferromagnetic state at TC = 310 K, with the magnetic moments ordered parallel to the basal plane. From the specific heat measurements and the ferromagnetic solutions obtained from our DFT calculations, we conclude that the ferromagnetism is of itinerant nature. Fil: Freitas, Daniele C.. Centre National de la Recherche Scientifique; Francia. Universitè Grenoble Alpes. Institut Néel; Francia. Centro Brasileiro de Pesquisas Fisicas; Brasil Fil: Weht, Ruben Oscar. Comisión Nacional de Energía Atómica. Centro Atómico Constituyentes. Gerencia de Investigación y Aplicaciones; Argentina. Universidad Nacional de San Martín. Instituto Sabato; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Sulpice, André. Centre National de la Recherche Scientifique; Francia. Universitè Grenoble Alpes. Institut Néel; Francia Fil: Remenyi, Gyorgy. Centre National de la Recherche Scientifique; Francia. Universitè Grenoble Alpes. Institut Néel; Francia Fil: Strobel, Pierre. Universitè Grenoble Alpes. Institut Néel; Francia. Centre National de la Recherche Scientifique; Francia Fil: Gay, Frédéric. Centre National de la Recherche Scientifique; Francia. Universitè Grenoble Alpes. Institut Néel; Francia Fil: Marcus, Jacques. Universitè Grenoble Alpes. Institut Néel; Francia. Centre National de la Recherche Scientifique; Francia Fil: Núñez-Regueiro, Manuel. Universitè Grenoble Alpes. Institut Néel; Francia. Centre National de la Recherche Scientifique; Francia

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 measured the pressure sensitivity of Tc in fluorinated HgBa2Ca2Cu3O(8+d) (Hg-1223) ceramic samples with different F contents, applying pressures up to 30 GPa. We obtained that Tc increases with increasing pressure, reaching different maximum values, depending on the F doping level, and decreases for a further increase of pressure. A new high Tc record (166 K +/- 1 K) was achieved by applying pressure (23 GPa) in a fluorinated Hg-1223 sample near the optimum doping level. Our results show that all our samples are at the optimal doping, and that fluorine incorporation decreases the crystallographic $a$-parameter concomitantly increasing the maximum attainable Tc. This effect reveals that the compression of the $a$ axes is one of the keys that controls the Tc of high temperature superconductors., 4 pages, 4 figures, submitted to Phys. Rev. B

We determine the pressure phase diagram of the 1111 compounds CaFeAsF and SrFeAsF, up to 20 GPa and down to 4 K by electrical resistivity measurements and the change of structure up to 40 GPa at room temperature. The antiferromagnetic transition temperature, as determined by the derivative peak, shows a minimum at ~5 GPa (10 GPa) for the Ca (Sr) compound. For CaFeAsF, superconductivity appears at this minimum, coincident with the development of a previously reported monoclinic phase. For SrFeAsF, where the orthorhombic and the monoclinic phase were reported to coexist, superconductivity exists above P≥1 GPa. Both phase diagrams can be scaled by a shift of ~10 GPa pressure at which the volume of SrFeAsF and that of CaFeAsF at ambient pressure coincide. The difference of our phase diagram with that of electron-doped 1111 samples is accounted for by hole doping under pressure, which we verified through electron band structure calculations. Fil: Freitas, Daniele C.. Centre National de la Recherche Scientifique; Francia. Universite Joseph Fourier; Francia Fil: Garbarino, Gaston Leonel. European Synchrotron Radiation Facility; Francia Fil: Weht, Ruben Oscar. Comisión Nacional de Energía Atómica. Centro Atómico Constituyentes. Gerencia de Investigación y Aplicaciones; Argentina. Universidad Nacional de San Martín. Instituto Sabato; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Sow, Amadou. Universite Joseph Fourier; Francia. Centre National de la Recherche Scientifique; Francia Fil: Zhu, Xiyu. Chinese Academy of Sciences; República de China Fil: Han, Fei. Chinese Academy of Sciences; República de China Fil: Cheng, Peng. Chinese Academy of Sciences; República de China Fil: Ju, Jing. Chinese Academy of Sciences; República de China Fil: Wen, Hai Hu. Chinese Academy of Sciences; República de China Fil: Nunez Regueiro, Manuel Daniel. Universite Joseph Fourier; Francia. Centre National de la Recherche Scientifique; Francia

CaO–CuO system has been investigated at high oxygen pressure. Only two ternary compounds, Ca 0.83 CuO 2 and Ca 14 Cu 24 O 41 , were found to be stable at P (O 2 )=500 bar. Ca 14 Cu 24 O 41 has been synthesized as a bulk phase for the first time. The formation of this phase occurs only in a very narrow range of temperature and oxygen pressure. Resistivity measurements at high pressure from 3 to 8.5 GPa show superconducting transition in the whole range of pressures. Magnetic properties of this compound at ambient pressure have been also studied.

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 been able to synthesize a new bismuth-based oxide, SrBiO3, and to induce superconductivity by K doping performed under high pressure. Tc max is about 12K (for 45%–60% of K). These new compounds have a distorted perovskite-based structure. Powder synchrotron and neutron diffraction studies have shown that, as a function of K doping, the symmetry changes from monoclinic (space group P21/n) to tetragonal (space group I 4/mcm) and remains the same down to 4K. Above room temperature, SrBiO3 keeps the same structure until decomposition around 700°C, whereas the K-doped phases decompose around 300°C with a progressive loss of oxygen until the (Sr,K)BiO2.5 stoichiometry is reached. The oxygen loss is accompanied by a decrease of Tc. An increase of Tc was observed when resistivity was measured as a function of pressure. Powder synchrotron studies indicate that, for K-doped samples, pressure induces an increase of the tetragonal distortion, as well as a change of the tilt angle of the octahedra until the pressure corresponding to Tcmax is reached. The (Sr,K) site is more affected by the pressure effect than the Bi one. In the case of SrBiO3, no noticeable change was observed under pressure.

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.

High-pressure studies on fullerenes have previously shown the existence of one- and two-dimensional (2D) polymerized C 60 structures. Synchrotron radiation measurements, performed on C 60 samples quenched from 13 gigapascals and 820 kelvin, yield unambiguous proof for the existence of a three-dimensional (3D) polymerized C 60 derivative. Moreover, unusual ellipsoidal Debye-Scherrer diffraction patterns are observed, which shows that the giant anisotropic deformation induced by the nonhydrostatic compression is retained in the quenched samples. The multiple bonding possibilities of the highly symmetrical C 60 allow the retention (down to ambient pressure) of the deformation, a phenomenon reported previously only under high pressure.

The superconducting properties of mercury cuprates are extremely sensitive to compression. The highest transition temperatures obtained up to date have been measured on Hg-1223 at 25–30 Gpa ( T c ∼ 164 K at the onset) and imply an increase of about 20 K with respect to the ambient pressure T c 0 . We report here the largest total pressure effect (> 50 K over a span of 20 GPa) ever observed for several samples of bi-mercury layer Hg 2 Ba 2 (Y 1− x Ca x )Cu 2 O 8−δ compositions. Different Ca doping levels give different initial T c s but retain the same strong pressure dependence. The large T c increase with pressure for all the Hg-based compounds, independently of their doping level, is revealing the existence of a common structural parameter, which identification will be a guide for future synthesis efforts of record-high T c materials.

polymers. Pure and mixed phase polymeric samples were synthesized by simultaneously subjecting microcrystalline C60 powder or pellets to various pressures ( \valunit{1}{GPa} < P < \valunit{4.8}{GPa} ) and temperatures ( \valunit{250}{\celsius} < T < \valunit{600}{\celsius} ). The optical spectra of the orthorhombic, tetragonal, and rhombohedral C60 polymer phases are observed to be quite distinct and rich. These spectra exhibit numerous lines and an overall downshift in frequency relative to C60 is observed, consistent with a loss of double bonds from the fullerene cage. The LDMS spectra of a sample synthesized at \valunit{2}{GPa} under hydrostatic conditions and \valunit{350}{\celsius} , exhibited a succession of \sim 12 clear peaks at mass numbers corresponding to (\chem{C_{60}})_{{N} = 1, 2\ldots 12} , similar to the LDMS data on the C60 photopolymer. This is taken as further evidence for interfullerene bonds in these high-pressure polymers. The XRD pattern of this sample indicates the presence of a strong texture in the sample.

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 wrapping of graphite planes into fullerenes induces, among other interesting properties, an increase of the superconducting temperature in their corresponding intercalation compounds that has been attributed to the cluster structure. This motivates the search for similar behaviour in other systems. Here we explore the clusters that may be obtained by the folding of CuO2 planes and study their magnetic and electronic properties. Natural murdochite mineral proves that, at least in an interconnected structure, the CuO2 layers fold into cages, while the BaCuO2 phase can be viewed as a first step towards their separation within a cationic matrix.

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 measured the electrical resistivity of several quasi 1-D chalcogenides between 1.2 and 300K and from ambient pressure up to 25GPa. On NbSe 3 we find that the high temperature charge density wave (CDW) transition T 1 disappears at a pressure of about 3.5GPa, where the superconducting transition temperature T c jumps to ∼5K. The similar pressure dependence of the logarithmic pressure derivative of both T 1 and T c above this pressure suggests the existence of independent superconducting states. On m-TaS 3 we observe a maximum on the T 1 -P variation at ∼3.5GPa, that we attribute to a CDW transition temperature strongly diminished by 1-D fluctuations that are reduced at low applied pressures, with T 2 behaving in a monotonous decreasing way with pressure. While on o-TaS 3 the CDW that develops at 210K at ambient pressure, gives way to a superconducting state of T c ∼3K at a pressure of 6.5GPa. The behaviour of (TaSe 4 ) 2 I is scaled to the precedent, with a higher critical pressure of 15GPa that induces the appearance of a superconducting state with T c =2.5K.

We studied the pressure and temperature dependence of the electrical resistivity of the superconducting compound magnesium diboride (MgB 2 ). The superconducting transition temperature decreases monotonically with pressure, being parabolic or linear, depending on samples. The rate of decrease under pressure is higher than in conventional superconductors. We discuss our results in terms of the semimetallic character of the electronic band structure of MgB 2 .

The high pressure experiments done on fullerenes are reviewed. C 60 has found to be stable up to about 20 GPa at room temperature and hydrostatic conditions. Application of stronger, or non-hydrostatic, pressures at room temperature can induce the formation of a partially sp3 bonded phase, that apparently conserves the fullerene cage. Extreme non-hydrostatic compressions above about 15 GPa can, though, break down the cage and produce amorphous or cubic diamond. Destruction of the cage at high temperatures has also been observed, but the resulting product is amorphous sp2 material. A preliminary pressure-temperature phase diagram for C 60 is proposed.

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 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 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.

The structure of transition metal trichalcogenides is made up of infinite chains of atoms in the center of trigonal prisms stacked on the top of each other. Depending on the effective coupling between chains, this family of compounds is well suited to exhibit properties which extend from pseudo one-dimensionality to anisotropic three dimensionality. Nb-Nb pairs makes NbS3 a semiconductor ; TaS3 exhibits a metal-insulator transition due to a Peierls distorsion at 210 K. TaSe3 is superconductor below 2.1 K and NbSe3 undergoes two independant CDW transitions at 145 K and 59 K. The resistivity anomalies associated with the CDWs formation in NbSe3 have been shown to be strongly nonlinear with the applied current. We summarize several theoretical models to explain these striking properties. NbSe3 exhibits also two others transitions at 2.2 K and around 0.6 K. The resistivity drops below 2.2 K by 30 to 75 % of its residual value at 4.2 K and reaches a plateau. Below 0.4 K the resistivity decreases linearly down to 7 mK. Initial susceptibility measurements down to 50 mK do not show any increase in diamagnetism indicating flux expulsion. However NbSe3 is a bulk superconductor under pressure. We discuss the nature of these low temperature transitions with the eventual occurence of superconductivity.

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.

We present synchrotron X-ray diffraction measurements performed on C60 samples quenched from 13 GPa and 820 K. The analysis of all the reciprocal space of a quenched sample shows that it consists of a 3-D arrangement of polymerised fullerene molecules. Moreover, it is shown that the intensity distribution in reciprocal space occurs in ellipsoidal surfaces instead of the normal spherical surfaces characteristic of a powder sample. This implies that the applied anisotropic stress is imprinted in the quenched transformed samples.

Attenuation of acoustical surface waves in the range 100–500 MHz has been measured in sublimed C 60 films. A large attenuation governed by a thermally activated relaxation mechanism having an activation energy of 0.07 eV is observed around 180 K. The ultrasonic results are compared with the NMR and low frequency elastic measurements.