Decoupling multi-phase superconductivity from normal state ordering in CeRh${}_2$As${}_2$

Abstract

CeRh${}_2$As${}_2$ is a multi-phase superconductor with $T_{\text{c}}=0.26\text{K}$. 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 $\hat{I}{¼}_{\text{0}}{H}^{\ast }=4\text{T}$. Such parity switching is possible due to a strong Rashba spin-orbit coupling at the Ce sites located in locally non-centrosymmetric environments of the globally centrosymmetric lattice. Existence of another ordered state (Phase I) below a temperature $T_{\text{0}}\approx 0.4\text{K}$ suggests an alternative interpretation of the $H^{\ast }$ 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 CeRh${}_2$As${}_2$, showing much sharper signatures at $T_{\text{c}}=0.31\text{K}$ and $T_{\text{0}}=0.48\text{K}$. We refine the $T$-$H$ phase diagram of CeRh${}_2$As${}_2$ and demonstrate that $T_0(H)$ and $T_{\text{c}}(H)$ lines meet at $\hat{I}{¼}_{\text{0}}H\approx 6\text{T}$, well above $H^{\ast }$, 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.

Includes Supplemental Material