Coexistence of local magnetism and superconductivity in the heavy-fermion CeRh₂As₂ revealed by μSR studies

The superconducting (SC) state (Tc = 0.3 K) of the heavy-fermion compound CeRh₂As₂, which undergoes an unusual field-induced transition to another high-field SC state, emerges from an unknown ordered state below Tₒ = 0.55 K. While an electronic multipolar order of itinerant Ce-4f states was proposed to account for the Tₒ phase, the exact order parameter has not been known to date. Here, we report on muon spin relaxation (μSR) studies of the magnetic and SC properties in CeRh₂As₂ single crystals at low temperatures. We reveal a magnetic origin of the Tₒ order by identifying a spontaneous internal field below Tₒ = 0.55 K. Furthermore, we find evidence of a microscopic coexistence of local magnetism with bulk superconductivity. Our findings open the possibility that the Tₒ phase involves both dipole and higher order Ce-4f moment degrees of freedom and accounts for the unusual non-Fermi liquid behavior.