Charge order and suppression of superconductivity in HgBa2CuO4 at high pressures

New insight into the superconducting properties of HgBa2CuO4 (Hg-1201) cuprates is provided by combined measurements of the electrical resistivity and single crystal X-ray diffraction under pressure. The changes induced by increasing pressure up to 20GPa in optimally doped single crystals were investigated. The resistivity measurements as a function of temperature show a metallic behavior up to ~10GPa that gradually passes to an insulating state, typical of charge ordering, that totally suppresses superconductivity above 13GPa. 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 appearing at 10GPa of wavevector [0.25, 0, L]. As pressure induces a charge transfer of about 0.02 at 10GPa, our results are the first observation of charge order competing with superconductivity that develops in the over-doped region of the phase diagram of a cuprate.Comment: 9 pages, 3 figure

Magnetovolume and magnetocaloric effects in Er2Fe17

Combining different experimental techniques, investigations in hexagonal P63/mmc Er2Fe17 show remarkable magnetovolume anomalies below the Curie temperature, TC. The spontaneous magnetostriction reaches 1.6×10−2 at 5 K and falls to zero well above TC, owing to short-range magnetic correlations. Moreover, Er2Fe17 exhibits direct and inverse magnetocaloric effects (MCE) with moderate isothermal magnetic entropy ΔSM, and diabatic temperature ΔTad changes [ΔSM∼−4.7 J(kgK)−1 and ΔTad∼2.5 K near the TC, and ΔSM∼1.3 J(kgK)−1 and ΔTad∼−0.6 K at 40 K for ΔH=80 kOe, respectively, determined from magnetization measurements]. The existence of an inverse MCE seems to be related to a crystalline electric field-level crossover in the Er sublattice and the ferrimagnetic arrangement between the magnetic moments of the Er and Fe sublattice. The main trends found experimentally for the temperature dependence of ΔSM and ΔTad as well as for the atomic magnetic moments are qualitatively well described considering a mean-field Hamiltonian that incorporates both crystalline electric field and exchange interactions. ΔSM(T) and ΔTad(T) curves are essentially zero at ∼150 K, the temperature where the transition from direct to inverse MCE occurs. A possible interplay between the MCE and the magnetovolume anomalies is also discussed.Financial support from Spanish MICINN (MAT2011-27573-C04-02) and from the Basque Government (IT-347- 07) is acknowledged. J.L.S.Ll. acknowledges the support received from CONACYT, Mexico, under the project CB2010-01-156932, and Laboratorio Nacional de Investigaciones en Nanociencias y Nanotecnología (LINAN, IPICyT). J.A.R.V. acknowledges the support from the research project MAT2007-61621. We thank ILL and CRG-D1B for allocating neutron beamtime, and ESRF for synchrotron beamtime. The SCTs at the University of Oviedo and the technical support received from M.Sc. G. J. Labrada-Delgado and B. A. Rivera-Escoto (DMA, IPICyT) are also acknowledged

The magnetic structure of β-cobalt hydroxide and the effect of spin-orientation

Synchrotron X-ray and neutron diffraction experiments at various temperatures, down to 3 K, along with ab initio calculations, are carried out to elucidate the magnetic order of layered β-cobalt-hydroxide. This combination of techniques allows for the unambiguous assignment of the magnetic structure of this material. Our results confirm that below the Néel temperature high-spin cobalt centers are ferromagnetically coupled within a layer, and antiferromagnetically coupled across layers (magnetic propagation vector k = (0,0,½)), in agreement with the indirect interpretation based on magnetic susceptibility measurements. A paramagnetic/antiferromagnetic transition is observed at around 15 K. Moreover, the thermal expansion behavior along the c-lattice direction, perpendicular to the layers, shows an inflection slightly above this temperature, at around 30 K. The neutron diffraction patterns and the non-collinear DFT+U calculations indicate that the magnetization forms an angle of about 35° with the cobalt planes. In particular, for an isolated ferromagnetic layer, the electronic structure calculations reveal sharp cusps on the potential energy surface when the spins point parallel or perpendicular to the planes, suggesting that the ferromagnetic superexchange mechanism is strongly sensitive to the orientation of the magnetic moment.JARV acknowledges CSIC for a JAEdoc contract. This study has been supported by grants of ANPCYT/PICT 2012-2292 and UBACYT 20020120100333BA.Peer Reviewe

Pressure-induced amorphization of the Y3Ga5O12 garnet studied to 1 Mbar

We use micro-beam synchrotron x-ray diffraction to study the pressure-induced amorphization of nano-sized and single crystals of Y3Ga5O12 up to pressures exceeding 1 Mbar in static compression. The abrupt pressure-induced amorphization found for both 56 nm and bulk micrometric crystals at around 76 GPa independently of the pressure transmitting medium employed demonstrates its intrinsic nature, previously predicted at 79 GPa by ab initio calculations. The single crystal structural solution at 50 GPa shows that the contraction of the unit-cell, mostly accommodated by the compressible YO8 dodecahedra, gives rise to a regularization and tilting increase of the GaO6 polyhedra with the Y?O-Ga angle changing from 104.84° to 102.34° in 50 GPa. We obtain a bulk modulus of 178(3) GPa for the single crystal and 172(3) GPa for the nanocrystals in excellent agreement with previous calculations.V. M. and J.R–F. thank the Spanish Ministry of Science, Innovation and Universities for the Juan de la Cierva program (FJCI-2016-27921) and for project PGC-2018-097520-A-100, respectively

Investigation of pressure-induced magnetic transitions in Co xFe3-xO4 spinels

Under the terms of the Creative Commons Attribution License 3.0 (CC-BY).Room temperature Fe and Co K-edge x-ray magnetic circular dichroism, synchrotron x-ray powder diffraction, and magnetization measurements were carried out to investigate the stability of the ferrimagnetic ground state in CoxFe3-xO4 (x=0, 1, 1.5, and 2) ferrites under pressure up to about 30 GPa using diamond anvil cells. The x-ray magnetic circular dichroism at the Fe K-edge is observed to decrease continuously up to the highest reached pressure by ∼50% in Fe3O4 (at 25 GPa) and by ∼70% in Co1.5Fe1.5O4 (at 16 GPa) and Co2FeO4 (at 21 GPa). In CoFe2O 4, the suppression by ∼80% of the x-ray magnetic circular dichroic signal occurs simultaneously at both Fe and Co K-edges between 24 and 27 GPa. However, a continuous decrease of the dichroic signal with pressure is only observed at the Co K-edge, whereas the Fe K-edge dichroic intensity drops drastically. The synchrotron x-ray powder diffraction measurements indicate the occurrence of structural phase transitions at critical pressures in coincidence with the suppression of ferrimagnetism. In CoFe2O4, bulk magnetization measurements up to 26 GPa confirm the disappearance of ferrimagnetism and indicate an almost linear dependence of the magnetization with the magnetic field in the high pressure phase. We thus conclude that high-pressure CoFe2O4 is either paramagnetic or antiferromagnetic. © 2013 American Physical Society.This research was financed by the Spanish MICINN (Project No. MAT2012-38213-C02-01) and DGA (CAMRADS).Peer Reviewe

Structure of liquid ammonia at high pressures and temperatures

International audienceThe structure of liquid ammonia (NH3) is investigated from 1 to 6.3 GPa and up to 800 K by means of synchrotron x-ray diffraction (XRD) and ab initio molecular dynamics (AIMD) simulations. The XRD data are used to extract the molecular structure factor S mol (Q), pair distribution function (PDF) g mol (r), and the density of NH3. There is an excellent agreement between present S mol (Q) and g mol (r) at our lowest density and those reported in reference neutron experiments. Our densities agree better with the equation of state of Tillner-Roth et al. [1] than with more recent equation of state (EoS) models. The experimental structure factor and PDF are well reproduced by AIMD simulations using either the BLYP or the PBE exchange-correlation functional. The shapes of S mol (Q) and g mol (r) vary little over the investigated pressure range and suggest a compact liquid with weak orientational correlations between molecules, which is corroborated by the coordination number varying from 12.7 to ∼14. The simulations are used to study the evolution of the site-site pair distribution functions, which reveals that the number of H-bonds per molecule (between 1.5 and 2) do not evolve with density, and that the distribution of H atoms around N atoms becomes more and more anisotropic with pressure

Carbon enters silica forming a cristobalite-type CO2-SiO2 solid solution

International audienceRelaxor ferroelectric perovskites are highly polarizable and can exhibit giant coupling between elastic strain and an applied electric field. Here, we report an in situ extended X-ray absorption fine structure (EXAFS) study of a PbZn1/3Nb2/3O3 (PZN) single crystal as a function of the electric field. We show that the strong dipoles in the NbO6 octahedra bonds are aligned along the four directions close to the orientation of the electric field, while a small reversible polar shift occurs for Zn in the direction of the electric field, i.e., positive or negative. This reversible Zn-O polar shift is proposed to play an important role in both the "easy" switching of the ferroelectric polarization and the giant piezoelectric effect in PZN

Melting Curve and Liquid Structure of Nitrogen Probed by X-ray Diffraction to 120 GPa

International audienc

Polyamorphism in cerium based bulk metallic glasses: Electronic and structural properties under pressure and temperature by x-ray absorption techniques

High pressure and high temperature x-ray absorption near edge spectroscopy experiments have been carried out on Ce60Al20Cu20 bulk metallic glass showing an electronic delocalization of the 4f-electron of cerium under pressure. In parallel, high pressure extended x-ray absorption fine structure spectroscopy reveals large structural modifications of the cerium local environment. This study provides experimental evidence that an electronic driven structural transformation occurs in cerium based bulk metallic glasses (Ce-BMGs). The effect of temperature on the hysteresis of this amorphous-amorphous phase transition is also discussed, suggesting the existence of a critical point in the phase diagram of Ce-BMGs. This work will encourage further investigations on Ce-based metallic glasses phase diagrams in order to support, or refute, the actual theoretical understanding of polyamorphism