Effect of multiple bands on point contact spectra in iron chalcogenide superconductors

We present point-contact spectroscopy (PCS) measurements on single crystal Fe$_{1+y}$Te$_{1-x}$Se$_{x}$ using a nanoscale Au tip contact. Analysis of PCS measurements using BTK theory[1] is well-established, and recent work[2] that extends BTK to two-band superconductors with a relative band phase may shed new light on the nature of the SC order parameter (OP) of these iron-based materials. Recent experiments suggests that they have $s_\pm$ symmetry, but $d$-wave and anisotropic $s$-wave OPs cannot yet be ruled out[3]. The two-band model predicts that $s_\pm$-wave leads to interference effects between bands that result in conductance profiles distinct from a $d$-wave OP. Preliminary fitting is discussed for the chalcogenides as well as the conventional multi-band $s_{++}$-wave superconductor, MgB$_2$. [1] G E Blonder, M Tinkham, T M Klapwijk, PRB \textbf{25}, 4515 (1982); [2] A A Golubov, \emph{et al.} PRL \textbf{103}, 077003 (2009); [3] M V Sadovskii, UFN \textbf{178} 1243 (2008)