Observation of antiferromagnetic order as odd-parity multipoles inside the superconducting phase in CeRh$_{2}$As$_{2}$

TL;DR

This study provides experimental evidence of antiferromagnetic order with odd-parity multipoles inside the superconducting phase of CeRh₂As₂, highlighting the role of local inversion symmetry breaking in unconventional magnetic and superconducting states.

Contribution

First experimental validation of antiferromagnetic order with odd-parity multipoles inside the superconducting phase in CeRh₂As₂, linked to local inversion symmetry breaking.

Findings

Detection of antiferromagnetic order via NQR spectrum broadening.

Inversion symmetry breaking activates odd-parity magnetic multipoles.

Antiferromagnetism appears within the superconducting phase, with T_N < T_SC.

Abstract

Spatial inversion symmetry in crystal structures is closely related to the superconducting (SC) and magnetic properties of materials. Recently, several theoretical proposals that predict various interesting phenomena caused by the breaking of the local inversion symmetry have been presented. However, experimental validation has not yet progressed owing to the lack of model materials. Here we present evidence for antiferromagnetic (AFM) order in CeRh$_{2}$As$_{2}$ (SC transition temperature $T_{\rm SC} \sim 0.37$~K), wherein the Ce site breaks the local inversion symmetry. The evidence is based on the observation of different extents of broadening of the nuclear quadrupole resonance spectrum at two crystallographically inequivalent As sites. This AFM ordering breaks the inversion symmetry of this system, resulting in the activation of an odd-parity magnetic multipole. Moreover, the onset of antiferromagnetism $T_{\rm N}$ within an SC phase, with $T_{\rm N} < T_{\rm SC}$, is quite unusual in systems wherein superconductivity coexists or competes with magnetism. Our observations show that CeRh$_{2}$As$_{2}$ is a promising system to study how the absence of local inversion symmetry induces or influences unconventional magnetic and SC states, as well as their interaction.