High Pressure Low Temperature Studies on 1-2-2 Iron-based Superconductors Using Designer Diamond Cells
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High Pressure Low Temperature Studies on 1-2-2 Iron-based Superconductors Using Designer Diamond Cells Walter O. Uhoya,1 Georgiy M. Tsoi,1 Yogesh K. Vohra,1 Jonathan, E. Mitchell,2 Athena Safa-Sefat,2 and Samuel Weir3 1 Department of Physics, University of Alabama at Birmingham (UAB), Birmingham, AL 35294, USA 2 Materials Science & Technology Division, Oak Ridge National Laboratory (ORNL), Oak Ridge, TN 37831, USA 3 Mail Stop L-041, Lawrence Livermore National Laboratory (LLNL), Livermore, CA 94550, USA ABSTRACT High pressure low temperature electrical resistance measurements were carried out on a series of 122 iron-based superconductors using a designer diamond anvil cell. These studies were complemented by image plate x-ray diffraction measurements under high pressures and low temperatures at beamline 16-BM-D, HPCAT, Advanced Photon Source. A common feature of the 1-2-2 iron-based materials is the observation of anomalous compressibility effects under pressure and a Tetragonal (T) to Collapsed Tetragonal (CT) phase transition under high pressures. Specific studies on antiferromagnetic spindensity-wave Ba0.5Sr0.5Fe2As2 and Ba(Fe0.9Ru0.1)2As2 samples are presented to 10 K and 41 GPa. The collapsed tetragonal phase was observed at a pressure of 14 GPa in Ba0.5Sr0.5Fe2As2 at ambient temperature. The highest superconducting transition temperature in Ba0.5Sr0.5Fe2As2 was observed to be at 32 K at a pressure of 4.7 GPa. The superconductivity was observed to be suppressed on transformation to the CT phase in 122 materials. INTRODUCTION The discovery of pressure enhanced superconductivity in chemically doped iron arsenide compounds in 2008 [1] has attracted extensive research activities on these materials in the field of high pressure physics. One of the most intensively investigated iron-based compound is BaFe2As2 with the tetragonal ThCr2Si2 (122) type crystal structure [2]. At ambient pressure, pure BaFe2As2 is known to undergo a crystallographic phase transition from a tetragonal phase (I4/mmm) to an orthorhombic phase (Fmmm) phase and a concurrent antiferromagnetic ordering (spin-density wave, SDW) at ~140 K [2]. The transition is suppressed by either chemical doping or application of high pressure resulting in the appearance of superconductivity [2, 3]. Recent experimental and theoretical studies have shown that BaFe2As2 interestingly undergoes an isostructural phase transition from tetragonal (T) phase to an isostructural collapsed tetragonal (CT) phase which is accompanied by anomalous compressibility effect and a sharp drop in axial ratio c/a [3-5]. In the present work, we present the electrical resistivity and high pressure x-ray diffraction measurements on single crystals of Ba0.5Sr0.5Fe2As2 (TN = 166 K) [6] and ruthenium-doped Ba(Fe0.9Ru0.1)2As2 (TN = 96 K) [7], to a pressure of 41 GPa and a temperature of 10 K. The studies are carried out using a synchrotron source and diamond anvil cell and investigated the influence of high pressure on the tetragonal crystal structure and the possibility of the occurrence of
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