Orbital-Selective High-Temperature Cooper Pairing Developed in the Two-Dimensional Limit

Atomic-Site-Dependent Pairing Gap in Monolayer FeSe/SrTiO3(001)-(√13 × √13)

The interfacial FeSe/TiO2-δ coupling induces high-temperature superconductivity in monolayer FeSe films. Using cryogenic atomically resolved scanning tunneling microscopy/spectroscopy, we obtained atomic-site dependent surface density of states, work function, and the pairing gap in the monolayer FeSe on the SrTiO3(001)-(√13 × √13)-R33.7° surface. Our results disclosed the out-of-plane Se-Fe-Se triple layer gradient variation, switched DOS for Fe sites on and off TiO5□, and inequivalent Fe sublattices, which gives global spatial modulation of pairing gap contaminants with the (√13 × √13) pattern. Moreover, the coherent lattice coupling induces strong inversion asymmetry and in-plane anisotropy in the monolayer FeSe, which is demonstrated to correlate with the particle-hole asymmetry in coherence peaks. These results disclose delicate atomic-scale correlations between pairing and lattice-electronic coupling in the Bardeen-Cooper-Schrieffer to Bose-Einstein condensation crossover regime, providing insights into understanding the pairing mechanism of multiorbital superconductivity.

Iron Vacancy Tunable Superconductor-Insulator Transition in FeSe/SrTiO_{3} Monolayer

The Fe_{4}Se_{5} with a sqrt[5]×sqrt[5] Fe vacancy order is suggested to be a Mott insulator and the parent state of bulk FeSe superconductor. The iron vacancy ordered state has been considered as a Mott insulator and the parent compound of bulk FeSe-based superconductors. However, for the superconducting FeSe/SrTiO_{3} monolayer (FeSe/STO) with an interface-enhanced high transition temperature (T_{c}), the electronic evolution from its Fe vacancy ordered parent phase to the superconducting state, has not been explored due to the challenge to realize an Fe vacancy order in the FeSe/STO monolayer, even though important to the understanding of superconductivity mechanism. In this study, we developed a new method to generate Fe vacancies within the FeSe/STO monolayer in a tunable fashion, with the assistance of atomic hydrogen. As a consequence, an insulating sqrt[5]×sqrt[5] Fe vacancy ordered monolayer is realized as the parent state. By using scanning tunneling microscopy and scanning tunneling spectroscopy, the spectral evolution from superconductivity to insulator is fully characterized. Surprisingly, a prominent spectral weight transfer occurs, thus implying a strong electron correlation effect. Moreover, the Fe vacancy induced insulating gap exhibits no Mott gap-like features. This work provides new insights in understanding the high-T_{c} superconductivity in FeSe/STO monolayer.

Anisotropic Gap Structure and Sign Reversal Symmetry in Monolayer Fe(Se,Te)

The iron-based superconductors are an ideal platform to reveal the enigma of the unconventional superconductivity and potential topological superconductivity. Among them, the monolayer Fe(Se,Te)/SrTiO₃(001), which is proposed to be topological nontrivial, shows interface-enhanced high-temperature superconductivity in the two-dimensional limit. However, the experimental studies on the superconducting pairing mechanism of monolayer Fe(Se,Te) films are still limited. Here, by measuring the quasiparticle interference in monolayer Fe(Se,Te)/SrTiO₃(001), we report the observation of the anisotropic structure of the large superconducting gap and the sign change of the superconducting gap on different electron pockets. The results are well consistent with the "bonding-antibonding" s_±-wave pairing symmetry driven by spin fluctuations in conjunction with spin-orbit coupling. Our work is of basic significance not only for a unified superconducting formalism in the iron-based superconductors, but also for understanding of topological superconductivity in high-temperature superconductors.