Local density of states and superconducting gap in the iron chalcogenide superconductorFe1+δSe1−xTe

Abstract

We report on the investigation of the quasiparticle local density of states and superconducting gap in the iron chalcogenide superconductor Fe 1+␦ Se 1−x Te x ͑T c ϳ 14 K͒. The surface of a cleaved crystal revealed an atomic square lattice, superimposed on the inhomogeneous background, with a lattice constant of ϳ3.8 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 ϳ2.3 meV, corresponding to 2⌬ / k B T 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 c shows spatial variation over a wide energy range of more than 1 eV, probably due to the excess iron present in the crystal.

Nanoscale chemical phase separation in FeTe0.55Se0.45as seen via scanning tunneling spectroscopy

“…Figure 1(c) and (d) show the atomically-resolved STM images with opposite-polarity bias on the same location of a cleaved FeTe surface. They are similar to previous STM studies of Fe-chalcogenides [16][17][18] , where no reconstruction is observed. The lattice constant estimated from Fourier transform of the image is ∼ 3.8Å, in good agreement with that obtained from the bulk 14 .…”

“…Note that similar images were obtained in previous STM studies on the doped compounds 17,18 . Both filled-state or empty-state images show very similar contrast of two groups of atoms.…”

“…What has been expected for many doped correlated electron materials is just the opposite: chemical homogeneity but electronic and magnetic phase separation 13 . Electronic homogeneity and very small corrugation in the STM topograph coupled with the nearly homogeneous superconducting gap 17,18 indicate that this compound is closer to an itinerant metal than to a highly correlated material. Superconductivity in this compound with nanscale chemical phase separation must be a consequence of the fact that the sizes of chemical patches are smaller than the superconducting coherence length resulting in a homogenous superconducting gap.…”

“…We note few "big bright spots" that are randomly distributed on the surface. These have been assumed to be excess Fe atoms 16,17 . Nevertheless, these "big bright spots" are located at the bridge site of the surface lattice, which is consistent the results of X-ray diffraction refinement 14 and density functional investigation 19 .…”

Pauli-limiting effects in the upper critical fields of a clean LiFeAs single crystal

“…In the '11' system, the multi-gap feature was recently found to form 2Δ L / k B T c ≈ 4 -8.3 and 2Δ S / k B T c ≈ 0.8 -4 in optics, 49 μSR, 50 STS, 51 and specific heat studies. 52 It is thus important to note that the 2Δ L enhancement factor, corresponding to ~ 1 -2.4 times of 2Δ BCS , is generally larger than that of LiFeAs, indicating that the strong-coupling effect in the 2Δ L is more pronounced than the '111' case.…”