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