Emerging superconductivity with broken time reversal symmetry inside a superconducting $s$-wave state

TL;DR

This paper reports the discovery of a new $s+is'$ superconducting phase with broken time-reversal symmetry inside a broad $s$-wave region of Ba$_{ m 1-x}$K$_{ m x}$Fe$_2$As$_2$, linked to a Fermi surface topology change.

Contribution

It provides the first experimental evidence of a BTRS superconducting dome induced by a Lifshitz transition in a centrosymmetric multiband superconductor.

Findings

Detection of spontaneous magnetic fields indicating BTRS using $bc$SR.

Identification of a narrow $s+is'$ phase dome near a Lifshitz transition.

Correlation between topological Fermi surface change and emergence of BTRS state.

Abstract

In general, magnetism and superconductivity are antagonistic to each other. However, there are several families of superconductors, in which superconductivity may coexist with magnetism, and only a few examples are known, when superconductivity itself induces spontaneous magnetism. The most known compounds are Sr$_2$RuO$_4$ and some noncentrosymmetric superconductors. Here, we report the finding of a narrow dome of a novel $s+is'$ superconducting (SC) phase with broken time-reversal symmetry (BTRS) inside the broad $s$-wave SC region of the centrosymmetric multiband superconductor Ba$_{\rm 1-x}$K$_{\rm x}$Fe$_2$As$_2$ ($0.7 \lesssim x \lesssim 0.85$). We observe spontaneous magnetic fields inside this dome using the muon spin relaxation ($\mu$SR) technique. Furthermore, our detailed specific heat study reveals that the BTRS dome appears very close to a change in the topology of the Fermi surface (Lifshitz transition). With this, we experimentally demonstrate the emergence of a novel quantum state due to topological changes of the electronic system.