Cryogenic Scanning Tunneling Spectroscopy of Superconducting Iron Chalcogenide Single Crystals*

We report scanning tunneling spectroscopy measurements on the iron-based superconductors of the ``11'' family including Fe$_{1-y}$Te$_{1-x}$Se$_{x}$ and Fe$_{1-y}$Te$_{1-x}$S$_{x}$. Conductance spectra and atomically-resolved images are obtained on single crystals down to 300 mK. A gap-like structure is observed, showing an asymmetric spectral background, non-trivial spatial variation and temperature dependence. We discuss our data in terms of possible gap anisotropy and doping inhomogeneities, and in relation to other recent spectroscopic measurements on iron-based superconductors.