We perform a scanning tunneling microscopy/spectroscopy study on the layered iron-based superconductor ${\mathrm{KCa}}_2{\mathrm{Fe}}_4{\mathrm{As}}_4{\mathrm{F}}_2$ 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 an intraband scattering pattern possibly due to the holelike $\alpha$ pocket. The Fermi energy of this band is only about $24\pm 6$ meV, as derived from the energy dispersion result. Meanwhile, impurity-induced bound-state peaks can be observed at about $\pm 2.2$ meV on some spectra, and the peak value seems to be independent of magnetic field. On the second type of surface, which is rarely obtained, the fully gapped feature can still be observed in the tunneling spectra, although multiple gaps are obtained from either a single spectrum or separate ones, and the gap values determined from coherence peaks locate mainly in the range from 4 to 7 meV. Our results clearly indicate the multiple and nodeless superconducting gap nature of the layered superconductor ${\mathrm{KCa}}_2{\mathrm{Fe}}_4{\mathrm{As}}_4{\mathrm{F}}_2$, and the superfluid is mainly contributed by the holelike Fermi surfaces near the $\mathrm{\Gamma }$ point. This result should inspire further consideration of the effect of the shallow and incipient bands near the $M$ point and help us to understand the pairing mechanism in this highly layered iron-based superconductor.

中文翻译：

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层状铁基超导体KCa2Fe4As4F2中的单粒子隧道光谱和超导间隙

我们对层状铁基超导体进行了扫描隧道显微镜/光谱研究 ${\mathrm{钾盐}}_2{铁}_4{\mathrm{作为}}_4{\mathrm{F}}_2$临界温度约为 33.5 K。在真空中切割样品后，通常会观察到两种类型的终止表面。在一个常见的表面上，我们观察到一个完整的间隙特征，其能隙值接近 4.6 meV。这种类型的光谱在空间中显示出干净且均匀的全间隙，这表明该超导体中不存在间隙节点。还测量了准粒子干涉图样，其显示可能由于孔状的带内散射图样$\alpha$口袋。该能带的费米能量仅为$24\pm 6$ meV，来自能量色散结果。同时，杂质诱导的束缚态峰可以在大约$\pm 2.2$ meV 在某些光谱上，峰值似乎与磁场无关。在很少获得的第二种表面上，尽管从单个光谱或单独的光谱中获得了多个间隙，但在隧道光谱中仍然可以观察到全间隙特征，并且由相干峰确定的间隙值主要位于范围从 4 到 7 meV。我们的结果清楚地表明层状超导体的多重和无节点超导间隙性质${\mathrm{钾盐}}_2{铁}_4{\mathrm{作为}}_4{\mathrm{F}}_2$, 超流体主要是由附近的孔状费米面贡献的 $\mathrm{\Gamma }$观点。这一结果应激发进一步考虑附近浅带和初期带的影响。$米$ 指出并帮助我们理解这种高度分层的铁基超导体中的配对机制。