Prof. Jian Wang group reports superconducting pairing mechanism of the monolayer Fe(Se,Te) film
The iron-based superconductors are an ideal platform to reveal the enigma of the high-temperature superconductivity and potential topological superconductivity. Among them, the monolayer Fe(Se,Te)/SrTiO3(001) owns a large superconducting (SC) gap of 18 meV and SC transition temperature (Tc) of 62 K, which is much higher that of bulk Fe(Se,Te) (Tc ≈14.5 K). Furthermore, monolayer Fe(Se,Te) has been predicted to be topological nontrivial. By scanning tunneling spectroscopy (STS) measurements, Jian Wang group has observed robust zero-energy bound states at both ends of atomic line defects in monolayer Fe(Se,Te)/SrTiO3 as a signature of Majorana bound states [Nat. Phys. 16, 536-540 (2020)], indicating the potential topological superconductivity in monolayer Fe(Se,Te).
The research on SC pairing mechanism is of basic significance not only for uncovering the fundamental ingredients of the high-temperature superconductivity, but also for further understanding of topological superconductivity. However, the researches on pairing mechanism of monolayer Fe(Se,Te)/SrTiO3 are limited. The hole pocket in the center (Γ) of the Brillouin zone (BZ) sinks below the Fermi energy in monolayer Fe(Se,Te)/SrTiO3 due to the interface electron doping, which challenges the widely accepted s±-wave pairing symmetry within the electron-hole Fermi pocket nesting picture in iron-based superconductors. The measurements of SC gap structure and gap sign are crucial to determine the pairing symmetry and deduce the microscopic pairing mechanism.
By analyzing quasiparticle interference (QPI) data taken from the scanning tunneling microscopy/spectroscopy (STM/S) measurements, Prof. Jian Wang group reports the anisotropic SC gap structure and gap sign change in monolayer Fe(Se,Te)/SrTiO3. The results are well consistent with ‘bonding-antibonding’ s±-wave pairing symmetry. The high-quality monolayer Fe(Se,Te)/SrTiO3 was grew by molecular beam epitaxy (MBE) technique (Fig. 1a). The two-band Dynes model fitting of the scanning tunneling spectroscopy (STS) spectra at 4.2 K shows the SC gap anisotropy (Fig. 1b). The quantitative SC gap structure was further extracted by Fourier transformed QPI (FT-QPI) patterns (Fig. 1c). The anisotropic gap structure with the gap maximum in the ΓX ̃ direction and the gap minimum in the ΓM ̃ direction (Fig. 1d,e) agrees with ‘bonding-antibonding’ s±-wave. In addition, the gap sign change was directly visualized by the phase-referenced QPI (PR-QPI) pattern (Fig. 1f), which is consistent with the simulated PR-QPI pattern according to the ‘bonding-antibonding’ s±-wave (Fig. 1g). The ‘bonding-antibonding’ s±-wave pairing symmetry can be driven by spin fluctuations in conjunction with hybridization of the electron pockets from spin-orbit coupling in monolayer Fe(Se,Te)/SrTiO3. The widely discussed ingredients for topological superconductors are unconventional pairing in multiband systems and spin-orbit coupling. Therefore, this work not only reveals the pairing symmetry and pairing mechanism of the monolayer Fe(Se,Te)/SrTiO3, but also gives valuable input for further understanding of whether and how topological superconductivity can be realized in iron-based superconductors.
The paper was online published in Nano Letters on Nov. 30, 2022. Yu Li is the first author, and Prof. Jian Wang is the corresponding author. Other authors are Dingyu Shen, Andreas Kreisel, Cheng Chen, Tianheng Wei, and Xiaotong Xu. This work was financially supported by the Beijing Natural Science Foundation, National Natural Science Foundation of China, the National Key Research and Development Program of China, and Strategic Priority Research Program of Chinese Academy of Sciences.
Paper link: https://pubs.acs.org/doi/10.1021/acs.nanolett.2c03735
Figure 1. (a) STM topographic image of the monolayer Fe(Se,Te) film. (b) Two-band anisotropic Dynes fitting of a typical dI/dV spectrum. (c) Fourier transformed quasiparticle interference (FT-QPI) pattern of |g(q,E=18meV)|. (d) Intensity of |g(q,E)| where line cuts are along ΓM ̃ directions. The red dashed curve shows the anisotropic gap structure. The orange dashed curve shows the Bogoliubov spectrum. (e) The k-space structure of the SC gap, obtained from the red dashed curve in (d). (f) Phase-referenced quasiparticle interference (PR-QPI) pattern of gpr (q,E=18meV). (g) Simulated PR-QPI patterns according to ‘bonding-antibonding’ s±-wave.