Cooper Pairing and Phase Coherence in Iron Superconductor Fe1+x(Te,Se)

TL;DR

This study investigates how interstitial Fe impurities influence superconductivity in Fe1+x(Te,Se), revealing that they cause decoherence of Cooper pairs without significantly affecting pairing strength, thus transforming the ground state.

Contribution

It provides a detailed microscopic understanding of impurity effects on pairing and phase coherence in Fe-based superconductors using STM/S techniques.

Findings

IFI hardly affect electron pairing strength

IFI cause significant decoherence of Cooper pairs

Ground state shifts from strong-coupling superconductor to diffusive metal

Abstract

The Cooper pairing and phase coherence are two fundamental aspects of superconductivity. Due to breaking time reversal symmetry, magnetic impurities are detrimental to superconductivity, yet microscopically how they affect the pairing strength and phase coherence in a real material is less understood. Recently we observed a robust zero-energy bound state at an interstitial Fe impurity (IFI) in superconducting Fe1+x(Te,Se), signifying intense impurity scattering. Here we report a comprehensive study, using scanning tunnelling microscopy/spectroscopy (STM/S) technique, of the global effects of IFIs on the ground state of Fe1+x(Te,Se) over a wide range of IFI concentration x. Our high resolution tunnelling spectroscopy and quasi-particle interference data at very low temperature demonstrate that IFIs hardly affect the electron pairing strength, while they cause significant decoherence of Cooper pairs in precedence of the Coulomb correlation, eventually driving the ground state of the system from strong-coupling-superconductor to diffusive-metal with incoherent electron pairs.