Phase diagram of CeRh₂As₂ for out-of-plane magnetic field

The heavy-fermion superconductor (SC) CeRh_2⁢As₂ (𝑇_c = 0.35 K) shows two SC phases SC1 and SC2 when a magnetic field is applied parallel to the 𝑐 axis of the tetragonal unit cell. All experiments to date have indicated that the change in SC order parameter detected at 𝜇₀⁢𝐻* ≈ 4T is due to strong Rashba spin-orbit coupling at the Ce sites caused by the locally noncentrosymmetric environments of the otherwise globally centrosymmetric crystalline structure. Another phase (phase I) exists in this material below 𝑇₀ = 0.54 K. In a previous specific heat study [K. Semeniuk et al., Phys. Rev. B 107, L220504 (2023)], we have shown that phase I persists up to a field 𝜇₀⁢𝐻₀ ≈ 6T, larger than 𝐻*. From thermodynamic arguments, we expected the phase-I boundary line to cross phase SC2 at a tetracritical point. However, we could not find any signature of the phase-I line inside the SC2 phase and speculated that this was because the 𝑇_0⁡(𝐻) line is almost perpendicular to the 𝐻 axis and therefore invisible to 𝑇-dependent measurements. This would imply a weak competition between the two order parameters. Here, we report magnetic-field-dependent measurements of the magnetostriction and ac susceptibility on high-quality single crystals. We see clear evidence of the singularity at 𝐻₀ inside the SC2 phase and confirm our previous prediction. Furthermore, we observe the transition across the 𝑇*⁡(𝐻) line in 𝑇-dependent specific heat measurements, which show that the 𝑇*⁡(𝐻) line is not perpendicular to the field axis but has a positive slope. Our findings support recent muon spin resonance results which suggest coexistence of phase I with SC.