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

Abstract

For multiband superconductors, the orbital multiplicity yields orbital differentiation in normal-state properties and can lead to orbital-selective spin-fluctuation Cooper pairing. The orbital-selective phenomenon has become increasingly pivotal in clarifying the pairing “enigma”, particularly for multiband high-temperature superconductors. Meanwhile, in one-unit-cell (1-UC) FeSe/SrTiO3, since the standard electron–hole Fermi pocket nesting scenario is inapplicable, the actual pairing mechanism is subject to intense debate. Here, by measuring high-resolution Bogoliubov quasiparticle interference, we report observations of highly anisotropic magnetic Cooper pairing in 1-UC FeSe. Theoretically, it is important to incorporate orbitally selective effects of electronic correlations within a spin-fluctuation pairing calculation, where the d xy orbital becomes coherence-suppressed. The resulting pairing gap is compatible with the experimental findings, which suggests that high-T c Cooper pairing with orbital selectivity applies to 2D-limit 1-UC FeSe. Our findings imply the general existence of orbital selectivity in iron-based superconductors and the universal significance of electron correlations in high-T c superconductors.

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

“…39 Recently, it was shown that the band dispersion extracted from the FT-QPI patterns in 1-UC FeSe/STO is heavily renormalized in the ΓX direction at E = 37 meV, coinciding in the bosonic mode energy detected in the tunneling spectrum, whereas this dispersion renormalization is moderate in the ΓM ̃direction. 46 This result once again confirms that the electron pairs in the ΓX ̃direction generate a larger SC gap and a stronger band renormalization at the same time. We noticed that upon increasing the band hybridization of the two elliptic-shaped electron pockets, the interplay between intrapocket and interpocket pairings will transform the system from a d-wave state to an s ± -wave state, where the location of the gap maximum simultaneously transforms from the ΓM ̃direction to the ΓX ̃direction.…”

“…In the microscopic theory of superconductivity, the formation of superconductivity not only opens a low-energy SC gap Δ­( k ), but also renormalizes the higher-energy band dispersion associated with the bosonic mode . Recently, it was shown that the band dispersion extracted from the FT-QPI patterns in 1-UC FeSe/STO is heavily renormalized in the Γ X ̃ direction at E = 37 meV, coinciding in the bosonic mode energy detected in the tunneling spectrum, whereas this dispersion renormalization is moderate in the Γ M ̃ direction . This result once again confirms that the electron pairs in the Γ X ̃ direction generate a larger SC gap and a stronger band renormalization at the same time.…”