Scrutinizing the double superconducting gaps and strong coupling pairing in (Li1-xFexOH)FeSe

TL;DR

This study investigates the pairing mechanism in (Li1-xFexOH)FeSe, revealing double anisotropic gaps and strong coupling, providing insights into high-temperature superconductivity in iron-based materials.

Contribution

The paper presents the first STS analysis of (Li1-xFexOH)FeSe single crystals, identifying double gaps and their orbital associations, and demonstrates strong coupling in this superconductor.

Findings

Identification of double anisotropic gaps with maximum values of 14.3 meV and 8.6 meV.

Assignment of larger gap to outer Fermi pockets and smaller to inner pockets.

Evidence of strong coupling mechanism with Delta_1/k_BT_c = 8.7.

Abstract

In the iron based superconductors, one of the on-going frontier studies is about the pairing mechanism. The recent interest concerns the high temperature superconductivity and its intimate reason in the monolayer FeSe thin films. The challenge here is how the double superconducting gaps seen by the scanning tunnelling spectroscopy (STS) associate however to only one set of Fermi pockets seen by the angle resolved photoemission spectroscopy (ARPES). The recently discovered (Li1-xFexOH)FeSe phase with Tc=40 K provides a good platform to check the fundamental problems. Here we report the STS study on the (Li1-xFexOH)FeSe single crystals. The STS spectrum clearly indicates the presence of double anisotropic gaps with maximum magnitudes of Delta_1=14.3 meV and Delta_2=8.6 meV, and mimics that of the monolayer FeSe thin film. Further analysis based on the quasiparticle interference (QPI) allows us to rule out the d-wave gap, and for the first time assign the larger (smaller) gap to the outer (inner) hybridized Fermi pockets associating with the dxy (dxz/dyz) orbitals, respectively. The huge value Delta_1/k_BT_c = 8.7 discovered here undoubtedly proves the strong coupling mechanism in the present superconducting system.