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

We report on the investigation of the quasiparticle local density of states and superconducting gap in the iron chalcogenide superconductor ${\text{Fe}}_{1+\delta }{\text{Se}}_{1-x}{\text{Te}}_x\left(T_{\text{c}}\sim 14\text{K}\right)$. The surface of a cleaved crystal revealed an atomic square lattice, superimposed on the inhomogeneous background, with a lattice constant of $\sim 3.8\text{Å}$ without any reconstruction. Tunneling spectra measured at 4.2 K exhibit the superconducting gap, which completely disappears at 18 K, with a magnitude of $\sim 2.3\text{meV}$, corresponding to $2\Delta /k_{\text{B}}{T}_{\text{c}}=3.8$. 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 $T_{\text{c}}$ shows spatial variation over a wide energy range of more than 1 eV, probably due to the excess iron present in the crystal.

Figure 1

(Color online) (a) Large-scale surface topography of the cleaved surface at $4.2\text{K}\left(72\times 72{\text{nm}}^2\right)$. (b) Magnified topography of the region outlined by the box in panel (a), revealing a square atomic lattice with $a_0\sim 3.8\text{Å}\left(V=1.0\text{V},I=40\text{pA}\right)$. [(c) and (d)] Tunneling conductance dependence of the STM image in the same region ($6.8\times 6.8{\text{nm}}^2$ obtained at 18 K) for (c) low conductance (500 mV, 0.5 nA) and (d) high conductance (50 mV, 0.5 nA).

Figure 2

(Color online) (a) Spatially averaged tunneling conductance spectrum in the high-energy range measured at 4.2 K. (b) Low-energy part of the spatially averaged spectrum measured below (4.2 K) and above (18 K) $T_{\text{c}}$, indicating that the gap feature at low temperature corresponds to the superconducting gap. The dotted line on the 4.2 K spectrum indicates the calculated V-shaped background conductance (see text). The 18 K spectrum has been shifted vertically for clarity.

Figure 3

(Color online) Spatially averaged spectrum at 4.2 K normalized to the background conductance represented by the dotted line in Fig. 2b. The curve indicates a fit of the calculated DOS for an $s$-wave superconductor to the data.

Figure 4

(Color online) Normalized spectra at 4.2 K taken along a 10-nm-long linecut across the sample surface. All of the raw spectra were normalized to the background conductance shown by the dotted line in Fig. 2b.

Figure 5

(Color online) Spatial evolution of the high-energy background electronic states measured at 18 K. (a) Surface topography obtained at $V=20\text{mV}$ and $I=0.4\text{nA}$. (b) Differential conductance map at 50 mV for the same area as (a). (c) Tunneling spectra obtained along a linecut denoted by the arrows in (a) and (b); intensity is shown by a logarithmic scale. (d) Typical spectra, labeled 1–7, extracted from the linecut (shifted vertically for clarity). The setup parameters for the STS measurements were $V_{\text{set}}=400\text{mV}$ and $I_{\text{set}}=0.4\text{nA}$.