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

摘要

We report on the first investigation of the quasiparticle local density ofstates and superconducting gap in the iron chalcogenide superconductorFe$_{1+\delta}$Se$_{1-x}$Te$_{x}$ ($T_{\mathrm{c}} \sim 14$ K). The surface ofa cleaved crystal revealed an atomic square lattice, superimposed on theinhomogeneous background, with a lattice constant of $\sim 3.8$ \AA without anyreconstruction. Tunneling spectra measured at 4.2 K exhibit the superconductinggap, 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 contrastto the cuprate superconductors, the value of the observed superconducting gapis relatively homogeneous, following a sharp distribution with a small standarddeviation of 0.23 meV. Conversely, the normal-state local density of statesobserved above $T_{\mathrm{c}}$ shows spatial variation over a wide energyrange of more than 1 eV, probably due to the excess iron present in thecrystal.