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

TL;DR

This study uses scanning tunneling spectroscopy to analyze the local density of states and superconducting gap in Fe$_{1+ ext{delta}}$Se$_{1-x}$Te$_{x}$, revealing homogeneous gaps and inhomogeneous normal-state spectra.

Contribution

First detailed measurement of quasiparticle local density of states and superconducting gap in Fe$_{1+ ext{delta}}$Se$_{1-x}$Te$_{x}$ using STM, highlighting gap homogeneity and spatial variation in normal state.

Findings

Superconducting gap of ~2.3 meV observed at 4.2 K.

Gap disappears above 18 K, consistent with $T_c \\sim 14$ K.

Normal-state LDOS shows wide spatial variation over 1 eV.

Abstract

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.