Decoupled interband pairing in a bilayer iron-based superconductor evidenced by ultrahigh-resolution ARPES

TL;DR

This study provides direct ARPES evidence of weakly coupled multiband superconductivity in a bilayer iron-based superconductor, revealing distinct transition behaviors and emphasizing the role of interband pairing suppression.

Contribution

It demonstrates the existence of a weakly coupled multiband superconducting state with band-specific transition temperatures in a bilayer iron-based superconductor, supported by Eilenberger theory.

Findings

K-compound shows two distinct transition temperatures.

Cs-compound exhibits a single transition behavior.

Weak interband pairing interactions are key to the observed phenomena.

Abstract

We present direct experimental evidence of a weakly coupled multiband superconducting state in the bilayer iron-based superconductor ACa$_2$Fe$_4$As$_4$F$_2$ (A = K, Cs) via ultrahigh-resolution angle-resolved photoemission spectroscopy (ARPES). Remarkably, the K-containing compound exhibits two distinct transition temperatures, corresponding to two separate sets of bilayer-split bands, as evidenced by temperature-dependent superconducting gap and spectral weight near the Fermi energy, while its Cs counterpart displays conventional single transition behavior. These experimental observations are well described by the weakly coupled two-band model of Eilenberger theory, which identifies suppressed interband pairing interactions between the bilayer-split bands as the key mechanism. By exploring quantum phenomena in the weak-coupling limit within a multiband system, our findings pave the way for engineering exotic superconductivity via band-selective pairing control.