Anomaly of longitudinal spin susceptibility at superconducting instability inside a magnetic phase

TL;DR

This paper investigates the unique anomaly in longitudinal spin susceptibility at the superconducting transition within a magnetic phase, highlighting a thermodynamic signature of coexistence and symmetry breaking.

Contribution

It reveals a linear magnetic field dependence of the superconducting gap and a susceptibility jump at the transition, indicating coexistence of superconductivity and magnetism.

Findings

Linear term in the superconducting gap under magnetic field.

Jump in longitudinal spin susceptibility at transition.

Signature of coexistence of superconductivity and magnetic order.

Abstract

We study the longitudinal spin susceptibility inside a magnetically ordered phase, which exhibits a superconducting instability leading to a coexistence of the two ordered phases. Inside the magnetic phase, the superconducting gap acquires a linear term in a magnetic field applied along the direction of the magnetic moment. We find that such a linear term generates a jump of the longitudinal spin susceptibility when the superconducting instability occurs via a continuous phase transition. This anomaly at the superconducting instability is a thermodynamic signature of the microscopic coexistence of superconductivity and magnetism, and can be a general feature associated with the breaking of spin rotational symmetry inside the magnetic phase.