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

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 T-c approximate to 12 K. 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 300 mK showed a generally asymmetric V-shaped background, along with a sharp dip structure within similar to +/- 2-4 mV. These spectra appeared to vary over similar to 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 T-c, and with the spectral background having a non-superconducting origin. The persistence of finite zero-bias conductance down to 300 mK, well below T-c, 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. (C) 2010 Elsevier Ltd. All rights reserved.