Field-angle dependence reveals odd-parity superconductivity in CeRh$_2$As$_2$

TL;DR

This study uses angle-dependent measurements to confirm that the high-field superconducting phase in CeRh₂As₂ is an odd-parity state, supporting the idea of unconventional superconductivity with complex symmetry properties.

Contribution

The paper provides experimental evidence linking the high-field phase to odd-parity superconductivity through detailed angle-dependent critical field measurements.

Findings

SC2 is suppressed when rotating the magnetic field away from the c axis.

The behavior of SC2 matches a model of a pseudospin triplet state with in-plane d-vector.

SC2 is confirmed as an odd-parity superconducting state.

Abstract

CeRh$_2$As$_2$ is an unconventional superconductor with multiple superconducting phases and $T_\mathrm{c} = 0.26$ K. When $H\parallel c$, it shows a field-induced transition at $\mu_0H^* = 4$ T from a low-field superconducting state SC1 to a high-field state SC2 with a large critical field of $\mu_0H_\mathrm{c2} = 14$ T. In contrast, for $H\perp c$, only the SC1 with $\mu_0H_\mathrm{c2} = 2$ T is observed. A simple model based on the crystal symmetry was able to reproduce the phase-diagrams and their anisotropy, identifying SC1 and SC2 with even and odd parity superconducting states, respectively. However, additional orders were observed in the normal state which might have an influence on the change of the superconducting state at $H^*$. Here, we present a comprehensive study of the angle dependence of the upper critical fields using magnetic ac-susceptibility, specific heat and torque on single crystals of CeRh$_2$As$_2$. The experiments show that the state SC2 is strongly suppressed when rotating the magnetic field away from the $c$ axis and it disappears for an angle of 35$^{\circ}$. This behavior agrees perfectly with our extended model of a pseudospin triplet state with $\vec{d}$ vector in the plane and hence allows to nail down that SC2 is indeed the suggested odd-parity state.