Appearance of $c$-axis magnetic moment in odd-parity antiferromagnetic state in CeRh$_2$As$_2$ revealed by $^{75}$As-NMR

TL;DR

This study uses $^{75}$As-NMR to reveal that CeRh$_2$As$_2$ exhibits a $c$-axis magnetic moment in an odd-parity antiferromagnetic state within its superconducting phase, linking magnetic structure to unconventional superconductivity.

Contribution

It provides the first detailed NMR evidence of a $q=0$ $A$-type AFM with moments along the $c$ axis in CeRh$_2$As$_2$, clarifying the magnetic structure in the SC phase.

Findings

Internal magnetic field is oriented along the $c$ axis.

The AFM state is consistent with a $q=0$ $A$-type structure.

Spin susceptibility reduction indicates spin-singlet pairing.

Abstract

CeRh$_2$As$_2$ shows the superconducting (SC) multiphase under the $c$-axis magnetic field, which is considered to originate from local inversion symmetry breaking at the Ce site. We reported that the antiferromagnetic (AFM) order is inside the SC phase and that the AFM state disappears at the transition field to the high-field SC phase. However, the magnetic structure in the AFM state has not been clarified yet. In this study, we performed $^{75}$As-NMR measurements in the SC phase in $H\parallel [110]$ to identify the magnetic structure. Comparing the NMR linewidth with $H \parallel c$, we found that the internal magnetic field is oriented to the $c$ axis. This suggests a $q = 0$ $A$-type AFM with the moments parallel to the $c$ axis. We also observed the reduction of the spin susceptibility, which indicates spin-singlet superconductivity in the low-field SC phase. This study provides an important clue to clarify the correlation between the SC multiphase, magnetism, and local inversion symmetry breaking.