Pressure evolution of the low-temperature crystal structure and bonding of the superconductor FeSe(Tc=37 K)

Abstract

FeSe with the PbO structure is a key member of the family of new high-Tc iron pnictide and chalcogenide superconductors, as while it possesses the basic layered structural motif of edge-sharing distorted FeSe4 tetrahedra, it lacks interleaved ion spacers or charge-reservoir layers. We find that application of hydrostatic pressure first rapidly increases Tc which attains a broad maximum of 37 K at ∼7 GPa (this is one of the highest Tc ever reported for a binary solid) before decreasing to 6 K upon further compression to ∼14 GPa. Complementary synchrotron X-ray diffraction at 16 K was used to measure the low-temperature isothermal compressibility of α-FeSe, revealing an extremely soft solid with a bulk modulus, K0 = 30.7(1.1) GPa and strong bonding anisotropy between inter-and intra-layer directions that transforms to the more densely packed β-polymorph above ∼9 GPa. The non-monotonic Tc(P ) behavior of FeSe coincides with drastic anomalies in the pressure evolution of the interlayer spacing, pointing to the key role of this structural feature in modulating the electronic properties. PACS numbers: 74.70.Dd, 74.25.Ha, 61.05.C-The α-polymorph of the simple binary FeSe phase has recently emerged as a superconductor with an ambient P T c of ∼8-13 K. 1,2 Its structure comprises stacks of edgesharing FeSe 4 tetrahedra with a packing motif essentially identical to that of the FeAs layers in the families of the FeAs-based high-T c superconductors 3,4,5,6,7 but lacking any interleaved ion spacers or insulating layers. The structural analogy is reinforced by the observation that below 70 K the high-temperature crystal structure becomes metrically orthorhombic (space group Cmma), 8 displaying an identical distortion of the FeSe layers to that observed in the iron oxyarsenide family. 9,10 Theoretical calculations also find a very similar 2D electronic structure to that of the FeAs-based superconductors with cylindrical electron sections at the zone corner and cylindrical hole surface sections. 11 Moreover, superconductivity in FeSe is very sensitive to defects and disorder and occurs over a limited range of FeSe 1−δ nonstoichiometry. 12The effect of applied pressure on T c provides crucial information in differentiating between competing models of superconductivity and in the FeSe binary, T c is initially extremely sensitive to P and rises rapidly to 27 K at 1.48 GPa. 2 At the same time, antiferromagnetic spin fluctuations present above T c are strongly enhanced by pressure. 13 In the FeAs-based superconductors, the response of T c to pressurization is complex and sensitively depends on the composition of the materials and their doping level. Both positive and negative initial pressure coefficients, dT c /dP have been measured. Typically for the REFeAsO 1−x F x families, dT c /dP is positive at low doping levels and switches over to a negative value as x increases. 14,15,16,17,18 Moreover, for systems where the initial dT c /dP is positive, there is a critical value of P above which the trend is reversed and T c then decreases...

Fermi Surface, Pressure-Induced Antiferromagnetic Order, and Superconductivity in FeSe

“…55,56) To minimize the lattice energy from first principles, we start with the structural optimization of the internal coordinate h Se and c/a ratio in each pressure with the space group P 4/nmm and the experimental crystal parameters of FeSe. 17,59) We have optimized the structure requiring the Se atomic force to be less than 0.5 mRy/bohr. The optimized lattice parameter a is 2% smaller (and c is 2% larger) than those in experiments at ambient pressure.…”

New Superconducting Phase of Lix(C6H16N2)yFe2−zSe2 with Tc = 41 K Obtained through the Post-Annealing

“…2,3) In single-layer FeSe films grown on SrTiO 3 substrates, recently, the opening of a possible superconducting gap has been observed at low temperatures below ~ 43 K and 65 K in the scanning tunneling microscopy/spectroscopy 4) and angle-resolved photoemission spectroscopy 5) measurements, respectively. Moreover, a superconducting transition has been detected at a high temperature above 100 K in the electrical transport measurements of the single-layer FeSe film.…”

Increase in T c and Change of Crystal Structure by High-Pressure Annealing in BiS2-Based Superconductor CeO0.3 F 0.7BiS2

“…2,9) Electrical resistivity and magnetic susceptibility measurements under high pressure revealed that the transition temperature (T c ) of the BiS 2 -based family was sensitive to application of external pressure and can be significantly increased [11][12][13] as observed in the Fe-based superconductor family. [14][15][16][17] In addition, superconducting properties of the BiS 2 -based superconductors strongly depend on the sample preparation method. Solid-state-reacted (As-grown) LaO 1-x F x BiS 2 shows filamentary superconductivity: in other words, the superconducting volume fraction is obviously low, whereas superconducting states are evidently generated.…”