Polarization of spontaneous magnetic field and magnetic fluctuations in $s+is$ anisotropic multiband superconductors

TL;DR

This paper investigates how multiband superconductors with broken time-reversal symmetry, like certain iron pnictides, generate spontaneous magnetic fields and fluctuations, providing experimental ways to distinguish different pairing states.

Contribution

It demonstrates that $s+is$ and $s+id$ states produce similar magnetic fields, offering a method to differentiate these states through muon spin relaxation experiments.

Findings

Both $s+is$ and $s+id$ states generate comparable spontaneous magnetic fields.

The magnetic field polarization differs between $s+is$ and $s+id$ states.

Magnetic fluctuations near the phase transition can be detected experimentally.

Abstract

We show that multiband superconductors with broken time-reversal symmetry can produce spontaneous currents and magnetic fields in response to the local variations of pairing constants. Considering the iron pnictide superconductor Ba$_{1-x}$K$_x$Fe$_2$As$_2$ as an example we demonstrate that both the point-group symmetric $s+is$ state and the C$_4$-symmetry breaking $s+id$ states produce in general the same magnitudes of spontaneous magnetic fields. In the $s+is$ state these fields are polarized mainly in ab crystal plane, while in the $s+id$ state their ab-plane and c-axis components are of the same order. The same is true for the random magnetic fields which are produced by the order parameter fluctuations near the critical point of the time-reversal symmetry breaking phase transition. Our findings can be used as a direct test of the $s+is/s+id$ dichotomy and the additional discrete symmetry breaking phase transitions with the help of muon spin relaxation experiments.