Decoupling multiphase superconductivity from normal state ordering in CeRh2As2

CeRh2As2 is a multiphase superconductor with Tc=0.26 K. The two superconducting (SC) phases, SC1 and SC2, observed for a magnetic field H parallel to the c axis of the tetragonal unit cell, have been interpreted as even- and odd-parity SC states, separated by a phase boundary at μ0H∗=4 T. Such parity switching is possible due to a strong Rashba spin-orbit coupling at the Ce sites located in locally noncentrosymmetric environments of the globally centrosymmetric lattice. The existence of another ordered state (phase I) below a temperature T0≈0.4 K suggests an alternative interpretation of the H∗ transition: It separates a mixed SC+I (SC1) and a pure SC (SC2) state. Here, we present a detailed study of higher-quality single crystals of CeRh2As2, showing much sharper signatures at Tc=0.31 K and T0=0.48 K. We refine the T-H phase diagram of CeRh2As2 and demonstrate that T0(H) and Tc(H) lines meet at μ0H≈6 T, well above H∗, implying no influence of phase I on the SC phase switching. A basic analysis with the Ginzburg-Landau theory indicates a weak competition between the two orders.