Cryomagnetic Point-Contact Andreev Reflection Spectroscopy on Single Crystal Iron-Chalcogenide Superconductors

We report on cryomagnetic point-contact Andreev reflection spectroscopy performed on single crystals of superconducting FeTe$_{1-x}$S$_{x}$ and FeTe$_{1-x}$Se$_{x}$. The samples are cleaved in-situ and the measurements are carried out at temperatures down to 4.2K and in a field up to 9T. At base temperature and zero field, we observe a cone-shaped hump at lower voltages in the conductance spectra with no dips at zero bias and a linear background at higher voltages. The spectral evolution of gap size, zero-bias conductance, and excess spectral area are analyzed as a function of temperature and field. Further spectral analysis is carried out using theoretical models of conductance spectra in multiband superconductors [1,2] and of gap symmetry in Fe-based superconductors [3]. The role of interstitial iron is also considered, by comparison with atomically-resolved scanning tunneling spectroscopy data.\\[4pt] [1] V. Lukic and E.J. Nicol, PRB 76, 144508 (2007) [2] A. Golubov \emph{et al.}, PRL 103, 077003 (2009) [3] P.J. Hirschfeld \emph{et al.}, RPP 74, 124508 (2011)