Emergence of an incipient ordering mode in FeSe

The structurally simplest Fe-based superconductor FeSe with a critical temperature T sub(s) approximately 8.5 K displays a breaking of the fourfold rotational symmetry at a temperature T sub(s) approximately 87 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* approximately 75 K, denoted by an onset of electron-hole asymmetry in STS, enhanced spin fluctuations, and increased positive magnetoresistance; and (ii) T** approximately 22-30 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.