Pressure evolution of low-temperature crystal structure and bonding of 37 K $T_c$ FeSe superconductor

TL;DR

This study investigates how hydrostatic pressure affects the crystal structure and superconducting transition temperature of FeSe, revealing a maximum $T_c$ of 37 K at 7 GPa linked to structural changes.

Contribution

It provides detailed insights into the pressure-induced structural and electronic changes in FeSe, highlighting the role of interlayer spacing in superconductivity.

Findings

$T_c$ peaks at 37 K around 7 GPa

FeSe exhibits an extremely soft lattice with a bulk modulus of 30.7 GPa

Structural transition from $ extalpha$-FeSe to $eta$-polymorph occurs above 9 GPa

Abstract

FeSe with the PbO structure is a key member of the family of new high-$T_c$ iron pnictide and chalcogenide superconductors, as while it possesses the basic layered structural motif of edge-sharing distorted FeSe$_4$ tetrahedra, it lacks interleaved ion spacers or charge-reservoir layers. We find that application of hydrostatic pressure first rapidly increases $T_c$ which attains a broad maximum of 37 K at $\sim$7 GPa (this is one of the highest $T_c$ ever reported for a binary solid) before decreasing to 6 K upon further compression to $\sim$14 GPa. Complementary synchrotron X-ray diffraction at 16 K was used to measure the low-temperature isothermal compressibility of $\alpha$-FeSe, revealing an extremely soft solid with a bulk modulus, $K_0$ = 30.7(1.1) GPa and strong bonding anisotropy between inter- and intra-layer directions that transforms to the more densely packed $\beta$-polymorph above $\sim$9 GPa. The non-monotonic $T_c$($P$) 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.