Emergence of an incipient ordering mode in FeSe

TL;DR

This study uncovers an incipient electronic ordering mode in FeSe, characterized by new energy scales and symmetry breaking, indicating coupling between spin and charge degrees of freedom in this superconductor.

Contribution

It reveals the emergence of an incipient ordering mode in FeSe at specific temperature scales, highlighting coupling between spin and charge channels, which was not previously understood.

Findings

Identification of two new energy scales at T* and T**

Observation of electron-hole asymmetry and partial gap opening

Evidence of coupling between spin and charge degrees of freedom

Abstract

The structurally simplest Fe-based superconductor FeSe with a critical temperature $T_{c}\approx$ 8.5 K displays a breaking of the four-fold rotational symmetry at a temperature $T_{s}\approx 87$ K. We investigated the electronic properties of FeSe using scanning tunneling microscopy/spectroscopy (STM/S), magnetization, and electrical transport measurements. The results indicated two new energy scales (i) $T^{*} \approx$ 75 K denoted by an onset of electron-hole asymmetry in STS, enhanced spin fluctuations, and increased positive magnetoresistance; (ii) $T^{**} \approx$ 22 - 30 K, marked by opening up of a partial gap of about 8 meV in STS and a recovery of Kohler's rule. Our results reveal onset of an incipient ordering mode at $T^{*}$ and its nucleation below $T^{**}$. The ordering mode observed here, both in spin as well as charge channels, suggests a coupling between the spins with charge, orbital or pocket degrees of freedom.