Emergence of an incipient ordering mode in FeSe

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) $T^{\ast }\approx$ 75 K denoted by an onset of electron-hole asymmetry in STS, enhanced spin fluctuations, and increased positive magnetoresistance; (ii) $T^{\ast \ast }\approx$ 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 $T^{\ast }$ and its nucleation below $T^{\ast \ast }$. 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.

5 pages, 4 figures