We report on the first investigation of the quasiparticle local density of states and superconducting gap in the iron chalcogenide superconductor Fe$_{ 1+\delta}$Se$_{ 1-x}$Te$_{ x}$ ($T_{ \mathrm{ c}} \sim 14$ K). The surface of a cleaved crystal revealed an atomic square lattice, superimposed on the inhomogeneous background, with a lattice constant of $\sim 3.8$ \AA without any reconstruction. Tunneling spectra measured at 4.2 K exhibit the superconducting gap, which completely disappears at 18 K, with a magnitude of $\sim 2.3$ meV, corresponding to $2\Delta / k_{ \mathrm{ B}}T_{ \mathrm{ c}}=3.8$.In stark contrast to the cuprate superconductors, the value of the observed superconducting gap is relatively homogeneous, following a sharp distribution with a small standard deviation of 0.23 meV. Conversely, the normal-state local density of states observed above $T_{ \mathrm{ c}}$ shows spatial variation over a wide energy range of more than 1 eV, probably due to the excess iron present in the crystal.
Categories: cond-mat.supr-con