The structurally simplest Fe-based superconductor FeSe with a critical temperature $T_c\approx 8.5\mathrm{K}$ displays a breaking of the fourfold rotational symmetry at a temperature $T_s\approx 87\mathrm{K}$. We investigate the electronic properties of FeSe using scanning tunneling microscopy/spectroscopy (STM/STS), magnetization, and electrical transport measurements. The results indicate two energy scales: (i) $T^{*}\approx 75\mathrm{K}$, denoted by an onset of electron-hole asymmetry in STS, enhanced spin fluctuations, and increased positive magnetoresistance; and (ii) $T^{**}\approx 22–30 \mathrm{K}$, marked by the opening up of a partial gap of about 8 meV in STS and a recovery of Kohler's rule. Our results reveal the 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.

• Received 18 May 2015

DOI:https://doi.org/10.1103/PhysRevB.92.060505