Effect of Excess Fe on the Conductance Spectra of Iron Chalcogenides

We present point contact spectroscopy measurements on single crystal $Fe_{1+y} Te_{1-x} Se_x$ using a nanometer scale gold tip, as a function of applied magnetic field, temperature, doping and contact resistance. The superconducting samples exhibit a zero bias peak that persists up to 5K above $T_c$ while the $x=0$ compound shows a dip at zero bias, unaffected by applied field of up to 9T. For superconducting compounds, features observed above $T_c$ are also unaffected while superconducting features are diminished by the applied field. The sample surface is characterized by atomic force microscopy while the sample bulk is examined by x-ray diffraction, energy dispersive x-ray spectroscopy and transmission electron microscopy. Point-to-point reproducibility is limited by inhomogeneities in the crystal structure and composition.