Hidden Granular Superconductivity Above 500K in off-the-shelf graphite materials

It has been reported that graphite hosts room temperature superconductivity. Here we provide new results that confirm these claims on different samples of highly oriented pyrolytic graphite (HOPG) and commercial flexible graphite gaskets (FGG). After subtraction of the intrinsic graphite diamagnetism, magnetization measurements show convoluted ferromagnetism and superconducting-like hysteresis loops. The ferromagnetism is deconvoluted by fitting with a sigmoidal function and subtracting it from the data. The obtained superconducting-like hysteresis loops are followed to the highest available temperature, 400K. The extrapolation of the decrease of its moment width with temperature indicates a transition temperature T$_{c}$$\sim$ 550K$\pm$50K for all samples. Electrical resistance measurements confirm the existence at these temperatures of a transition in HOPG samples, albeit without percolation. Besides, the FGG show transitions at temperatures (70K, 270K) near to those reported previously on intercalated-deintercalated graphite, confirming the general character of these superconducting transitions. These results are the first steps in the unveiling of the above room temperature superconductivity of graphite

Lattice dynamics in the intermetallic LaFeSi and the derived superconducting compounds LaFeSiH and LaFeSiO

The intermetallic LaFeSi and the derived superconducting compounds LaFeSiH and LaFeSiO have been investigated by polarized Raman spectroscopy. The frequency and symmetry of the Raman phonons modes are well-reproduced by ab-initio calculations. The ionic character of the spacer in this series of compounds and its coupling with the FeSi layers as compared to As-based compounds are discussed. Already at room temperature, Fano-shape modes are reported in the A1g channel while an intriguing doubling of the Fe-based B1g phonon is measured in LaFeSiH. Origins of these observations are discussed based on electron diffraction data and the different scenarios for the origin of such splitting are explored. Furthermore, there is no signature of a structural transition nor long range magnetic ordering in LaFeSiH down to 9 K.Comment: 8 pages, 6 figures

Pressure-induced high-spin/low-spin disproportionated state in the Mott insulator FeBO3

The pressure-induced Mott insulator-to-metal transitions are often accompanied by a collapse of magnetic interactions associated with delocalization of 3d electrons and high-spin to low-spin (HS-LS) state transition. Here, we address a long-standing controversy regarding the high-pressure behavior of an archetypal Mott insulator FeBO3 and show the insufficiency of a standard theoretical approach assuming a conventional HS-LS transition for the description of the electronic properties of the Mott insulators at high pressures. Using high-resolution x-ray diffraction measurements supplemented by Mössbauer spectroscopy up to pressures ~ 150 GPa, we document an unusual electronic state characterized by a “mixed” HS/LS state with a stable abundance ratio realized in the R3 ¯ c crystal structure with a single Fe site within a wide pressure range of ~ 50–106 GPa. Our results imply an unconventional cooperative (and probably dynamical) nature of the ordering of the HS/LS Fe sites randomly distributed over the lattice, resulting in frustration of magnetic moments. © 2022, The Author(s).EAR-1634415; U.S. Department of Energy, USDOE: DE-FG02-94ER14466; Office of Science, SC: DE-AC02-06CH11357; Argonne National Laboratory, ANL; University of Chicago; Israel Science Foundation, ISF: 1189/14, 1552/18, 1748/20; Helmholtz Association; 122021000039-4The authors would like to thank Dr. A. Chumakov (ESRF, Grenoble, France, Kurchatov Institute, Moscow, Russia) and Dr. G. Smirnov (Kurchatov Institute, Moscow, Russia) who provided us by high-quality single crystals of FeBO, Prof. L. Dubrovinsky and Prof. D. I. Khomskii for valuable discussions, Dr. V. Prakapenka and Dr. I Kantor for experimental assistance with the facilities of the 13ID-D GSECARS beamline of APS and Dr. S. Clark for experimental assistance with the facilities of the beam line 12.2.2 at ALS, Berkeley. We are grateful also to the team of the ID-27 beamline of the European Synchrotron Radiation Facility, Grenoble, for assisting with the powder XRD measurements. A few Mössbauer spectrum at 115 and 140 GPa were collected at the ID-18 beamline of the European Synchrotron Radiation Facility. We are grateful to Dr. D. G. Merkel, Dr. R. Rüffer and Dr. A. Chumakov for their assistance in using beamline ID-18 and Dr. G. Hearne and Dr. E. Carleschi for assisting with the SMS measurements. This research was supported by Israeli Science Foundation (Grants No. 1189/14, No. 1552/18 and No. 1748/20). I.L. acknowledges support by the state assignment of Minobrnauki of Russia (theme “Electron” No. 122021000039-4). Portions of this work were performed at GeoSoilEnviroCARS (The University of Chicago, Sector 13), Advanced Photon Source (APS), Argonne National Laboratory. GeoSoilEnviroCARS is supported by the National Science Foundation–Earth Sciences (EAR-1634415) and Department of Energy– GeoSciences (DE-FG02-94ER14466). This research also used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. We acknowledge DESY (Hamburg, Germany), a member of the Helmholtz Association HGF, for the provision of experimental facilities. Parts of this research were carried out at P02.2 station of PETRA-III, DESY. 3The authors would like to thank Dr. A. Chumakov (ESRF, Grenoble, France, Kurchatov Institute, Moscow, Russia) and Dr. G. Smirnov (Kurchatov Institute, Moscow, Russia) who provided us by high-quality single crystals of FeBO3 , Prof. L. Dubrovinsky and Prof. D. I. Khomskii for valuable discussions, Dr. V. Prakapenka and Dr. I Kantor for experimental assistance with the facilities of the 13ID-D GSECARS beamline of APS and Dr. S. Clark for experimental assistance with the facilities of the beam line 12.2.2 at ALS, Berkeley. We are grateful also to the team of the ID-27 beamline of the European Synchrotron Radiation Facility, Grenoble, for assisting with the powder XRD measurements. A few Mössbauer spectrum at 115 and 140 GPa were collected at the ID-18 beamline of the European Synchrotron Radiation Facility. We are grateful to Dr. D. G. Merkel, Dr. R. Rüffer and Dr. A. Chumakov for their assistance in using beamline ID-18 and Dr. G. Hearne and Dr. E. Carleschi for assisting with the SMS measurements. This research was supported by Israeli Science Foundation (Grants No. 1189/14, No. 1552/18 and No. 1748/20). I.L. acknowledges support by the state assignment of Minobrnauki of Russia (theme “Electron” No. 122021000039-4). Portions of this work were performed at GeoSoilEnviroCARS (The University of Chicago, Sector 13), Advanced Photon Source (APS), Argonne National Laboratory. GeoSoilEnviroCARS is supported by the National Science Foundation–Earth Sciences (EAR-1634415) and Department of Energy– GeoSciences (DE-FG02-94ER14466). This research also used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. We acknowledge DESY (Hamburg, Germany), a member of the Helmholtz Association HGF, for the provision of experimental facilities. Parts of this research were carried out at P02.2 station of PETRA-III, DESY

Preparation, structural and magnetic studies on BiFe1-xCrxO3 (x = 0.0, 0.05 and 0.1) multiferroic nanoparticles

BiFe1-xCrxO3 (x = 0.0, 0.05 and 0.1) nanoparticles are prepared by the combustion method without using any solvent. All the synthesized nanoparticles are single phase in nature, nearly spherical in shape and crystallize in distorted perovskite structure (space group R3c) with an average crystallite size of the order of 40 nm. The room temperature magnetization observed in BiFeO3 nanoparticles is larger than that in the bulk. Saturation magnetization and coercive field increase with increasing Cr-doping. Strong superexchange interaction between Fe3+ and Cr3+ atoms is likely to give rise to such increase in magnetization with Cr-doping. Mössbauer data of these nanoparticles show ordered magnetic state in which Fe atoms are in 3+ oxidation states