Observation of Antiferromagnetic Order as Odd-Parity Multipoles inside the Superconducting Phase in CeRh₂As₂

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 CeRh₂As₂ (SC transition temperature T[SC]∼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[N] within an SC phase, with T[N]
縺（もつ）れ結晶に起因した「超伝導反強磁性」状態を発見. 京都大学プレスリリース. 2022-02-07.