Unveiling the quasiparticle behaviour in the pressure-induced high-$T_c$ phase of an iron-chalcogenide superconductor

TL;DR

This study investigates how pressure affects the electronic structure and superconducting properties of FeSe$_{0.82}$S$_{0.18}$, revealing enhanced superconductivity, Fermi surface expansion, and complex pairing mechanisms under high pressure.

Contribution

It provides the first quantum oscillations data on the high-$T_c$ phase of FeSe$_{0.82}$S$_{0.18}$ under pressure, linking Fermi surface changes to superconductivity enhancement.

Findings

Fermi surface expands with pressure

Superconductivity increases threefold under pressure

Unusual scattering indicates complex pairing mechanisms

Abstract

Superconductivity of iron chalocogenides is strongly enhanced under applied pressure yet its underlying pairing mechanism remains elusive. Here, we present a quantum oscillations study up to 45 T in the high-$T_c$ phase of tetragonal FeSe$_{0.82}$S$_{0.18}$ up to 22 kbar. Under applied pressure, the quasi-two dimensional multiband Fermi surface expands and the effective masses remain large, whereas the superconductivity displays a three-fold enhancement. Comparing with chemical pressure tuning of FeSe$_{1-x}$S$_x$, the Fermi surface enlarges in a similar manner but the effective masses and $T_c$ are suppressed. These differences may be attributed to the changes in the density of states influenced by the chalcogen height, which could promote stronger spin fluctuations pairing under pressure. Furthermore, our study also reveals unusual scattering and broadening of superconducting transitions in the high-pressure phase, indicating the presence of a complex pairing mechanism.