Single particle tunneling spectroscopy and superconducting gaps in layered iron based superconductor KCa$_{2}$Fe$_{4}$As$_{4}$F$_{2}$

Abstract

We perform scanning tunneling microscopy/spectroscopy study on the layered iron based superconductor KCa2Fe4As4F2 with a critical temperature of about 33.5 K. Two types of terminated surfaces are generally observed after cleaving the samples in vacuum. On one commonly obtained surface, we observe a full gap feature with energy gap values close to 4.6 meV. This type of spectrum shows a clean and uniform full gap in space, which indicates the absence of gap nodes in this superconductor. Quasiparticle interference patterns have also been measured, which show no scattering patterns between the hole and tiny electron pockets, but rather an intra-band scattering pattern is observed possibly due to the hole-like α pocket. The Fermi energy of this band is only about 24 ± 6 meV as derived from the energy dispersion result. Meanwhile, impurity induced bound-state peaks can be observed at about ±2.2 meV on some spectra, and the peak value seems to be independent to magnetic field. On the second type of surface which is rarely obtained, the fully gapped feature can still be observed on the tunneling spectra, although multiple gaps are obtained either from a single spectrum or separate ones, and the gap values determined from coherence peaks locate mainly in the range from 4 to 8 meV. Our results clearly indicate multiple and nodeless superconducting gap nature in this layered superconductor KCa2Fe4As4F2, and the superfluid is mainly contributed by the hole-like Fermi surfaces near Γ point. This would inspire further consideration on the effect of the shallow and incipient bands near M point, and help to understand the pairing mechanism in this highly layered iron-based superconductor.

Common (π,π) Band Folding and Surface Reconstruction in FeAs-Based Superconductors

“…[31] In KCa 2 Fe 4 As 4 F 2 , the √ 2 × √ 2 reconstruction of the 057404-4 potassium or calcium atoms is observed by STM on each cleaved surface. [32] The √ 2 × √ 2 surface reconstruction has also been observed in many other FeAs-based superconductors. [33−38] The common occurrence of the √ 2 × √ 2 surface reconstruction provides a general scenario to understand the (𝜋,𝜋) band folding commonly observed in the FeAs-based superconductors.…”

Friedel Oscillations of Vortex Bound States Under Extreme Quantum Limit in KCa2Fe4As4F2

“…The flat area is the √ 2× √ 2 reconstructed surface by half amount of K or Ca atoms of the related layer after the cleavage, while there are many hollows with different sizes on the surface which may be the assembled vacancies [28]. However, the tunneling spectra measured in this kind of area show very homogeneous feature with coherence peak energies at about 4.3 ± 0.2 meV, and Fig.…”

“…Several Fermi surfaces with multiple and nodeless gaps were observed by the angle-resolved photoemission spectroscopy (ARPES) [27], and some of the bands are very shallow. It is found that the dominant contribution of density of states arises from the shallow α pocket based on STM/STS measurements [28]. Hence, the VBS should be interesting in this multiple and shallow band superconductor.…”

“…Figure 1(a) shows a typical topography which is commonly obtained on the cleaved surface of K12442 [28].…”