STM spectroscopy on superconducting FeSe1−xTex single crystals at 300 mK

Abstract

We report cryogenic scanning tunneling spectroscopy measurements on single crystals of superconducting FeSe1−xTex, at doping levels of x=0.5 and 0.7, with critical temperatures Tc≈12K. Atomically resolved topographic images were obtained, showing large-scale density-of-state clustering which appears to have no periodicity and to vary with the doping. Conductance spectra taken at 300mK showed a generally asymmetric V-shaped background, along with a sharp dip structure within ∼±2–4mV. These spectra appeared to vary over ∼nm length scale, and not correlated with the topography. The overall spectral evolution versus temperature is consistent with the dip structure arising from a superconducting energy gap which closes above Tc, and with the spectral background having a non-superconducting origin. The persistence of finite zero-bias conductance down to 300mK, well below Tc, indicates the presence of low-energy quasiparticles on parts of the Fermi surface. We discuss our data in light of some other recent spectroscopic measurements of FeSe1−xTex, and in terms of its characteristic band structure.