Local density of states and superconducting gap in the iron chalcogenide superconductor Fe${}_{1+\delta }$Se${}_{1-x}$Te${}_x$ observed by scanning tunneling spectroscopy

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 $2\mathrm{\Delta }\mathrm{/}{k}_{\mathrm{B}}{T}_{\mathrm{c}}=3.8$.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.