Incommensurate modulation with $Q=0$ A-type Antiferromagnetic Order in CeRh${}_2$As${}_2$ revealed by NQR studies

Abstract

We performed ${}^{75}$As nuclear quadrupole resonance (NQR) and nuclear magnetic resonance (NMR) measurements on a higher-quality single-crystalline CeRh${}_2$As${}_2$, a heavy-fermion superconductor exhibiting multiple superconducting (SC) phases under magnetic fields along the $c$ axis. This SC multiphase is believed to originate from staggered Rashba spin-orbit coupling associated with locally broken inversion symmetry. In addition to superconductivity, CeRh${}_2$As${}_2$ exhibits phase I below $T_0\sim 0.5$ K and an antiferromagnetic (AFM) state below $T_N\sim 0.25$ K in the early-stage samples. In the higher-quality sample, the AFM transition becomes more pronounced, and $T_N$ increases to nearly coincide with $T_{SC}$. The NQR spectra at the As(1) site imply an internal field with an incommensurate distribution, indicating a two-dimensional incommensurate modulation of the magnetic structure superimposed on a $Q=0$ A-type AFM component. Moreover, a pronounced decrease in the NQR intensity at $T_0$ well-above $T_N$ and an abrupt increase in the internal field at $T_N$ suggest the emergence of a slowly fluctuating AFM order at $T_0$ which becomes static at $T_N$.