Transport evidence for the surface state and spin-phonon interaction in FeTe$_{0.5}$Se$_{0.5}$

TL;DR

This study provides electrical transport evidence for topological surface states and spin-phonon interactions in FeTe$_{0.5}$Se$_{0.5}$, elucidating their role in superconductivity and nematic transitions.

Contribution

It offers the first transport-based evidence of topological surface states and spin-phonon interactions in FeTe$_{0.5}$Se$_{0.5}$, advancing understanding of its superconducting mechanism.

Findings

Observation of a topological transition via nonlinear Hall conductivity.

Detection of phonon softening and nematic transition around 40 K.

Evidence of spin-phonon interaction influencing superconductivity.

Abstract

The iron chalcogenides have been proved to be intrinsic topological superconductors to implement quantum computation because of their unique electronic structures. The topologically nontrivial surface states of FeTe$_{0.5}$Se$_{0.5}$ have been predicted by several calculations and then confirmed by high-resolution photoemission and scanning tunneling experiments. However, so far, the shreds of the electrical transport evidence for topological surface states are still in absence. By carrying out electrical transport experiments, we observe a topological transition with a nonlinear Hall conductivity and simultaneous linear magnetoresistance near the superconducting transition temperature. Furthermore, we observe a sign reversal of the Hall coefficient accompanied by a concurrently softening of the ${A}_{1g}$ phonon mode at about 40 K, indicating a nematic transition. The synchronized phonon softening with nematicity manifests an enhanced fluctuation state through spin-phonon interaction. Our results solidly corroborate the topological surface states of FeTe$_{0.5}$Se$_{0.5}$ and provide an understanding of the mechanism of the superconductivity in iron chalcogenides.