Quantum phase transition of correlated iron-based superconductivity in LiFe$_{1-x}$Co$_x$As

TL;DR

This study uses STM to investigate how Co impurities affect superconductivity in LiFe$_{1-x}$Co$_x$As, revealing a quantum phase transition driven by impurity scattering that transforms the superconducting gap into a nodal state.

Contribution

It provides the first detailed experimental and theoretical analysis of impurity-induced quantum phase transition in iron-based superconductors.

Findings

Impurities suppress the superconducting gap and induce low energy states.

Superconductivity persists in the strong-coupling limit despite impurity effects.

A transition from a fully gapped to a nodal state occurs before superconductivity is destroyed.

Abstract

The interplay between unconventional Cooper pairing and quantum states associated with atomic scale defects is a frontier of research with many open questions. So far, only a few of the high-temperature superconductors allow this intricate physics to be studied in a widely tunable way. We use scanning tunneling microscopy (STM) to image the electronic impact of Co atoms on the ground state of the LiFe$_{1-x}$Co$_x$As system. We observe that impurities progressively suppress the global superconducting gap and introduce low energy states near the gap edge, with the superconductivity remaining in the strong-coupling limit. Unexpectedly, the fully opened gap evolves into a nodal state before the Cooper pair coherence is fully destroyed. Our systematic theoretical analysis shows that these new observations can be quantitatively understood by the nonmagnetic Born-limit scattering effect in a s$\pm$-wave superconductor, unveiling the driving force of the superconductor to metal quantum phase transition.