The phase diagram of CeRh$_{2}$As$_{2}$ for out-of-plane magnetic field
The heavy-fermion superconductor CeRh$_{2}$As$_{2}$ ($T_{\textrm{c}} = 0.35\, \textrm{K}$) shows two superconducting (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 indicate that the change in SC order parameter detected at $\mu_{\textrm{0}}H^{*} \approx 4\, \textrm{T}$ is due to strong Rashba spin-orbit coupling at the Ce sites caused by the locally non-centrosymmetric environments of the otherwise globally centrosymmetric crystalline structure. Another phase (phase I) exists in this material below $T_{\textrm{0}} = 0.54\, \textrm{K}$. 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 $\mu_{\textrm{0}}H_{0} \approx 6\, \textrm{T}$, 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 due to the fact that 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 $H_{0}$ 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 work supports recent $\mu$SR results which suggest coexistence of phase I with superconductivity.