Field-Angle Dependence Reveals Odd-Parity Superconductivity in CeRh2As2

Abstract

CeRh2As2is an unconventional superconductor with multiple superconducting phases andTc=0.26K. WhenH∥c, it shows a field-induced transition atμ0H*=4Tfrom a low-field superconducting state SC1 to a high-field state SC2 with a large critical field ofμ0Hc2=14T. In contrast, forH⊥c, only the SC1 withμ0Hc2=2Tis 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 atH*. 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 ofCeRh2As2. The experiments show that the state SC2 is strongly suppressed when rotating the magnetic field away from thecaxis and it disappears for an angle of 35°. This behavior agrees perfectly with our extended model of a pseudospin triplet state withd→vector in the plane and hence allows us to conclude that SC2 is indeed the suggested odd-parity state.

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Plain Language Summary

Superconductivity is a fascinating state of matter in which an electrical current can flow without any resistance. Usually, it can exist in two forms: one which is destroyed easily with a magnetic field (“even parity”), and one which is stable in magnetic fields applied in certain directions (“odd parity”). Consequently, the latter should present a characteristic angle dependence of the critical field where superconductivity disappears. But odd-parity superconductivity is rare in nature; only a few materials support this state, and in none of them has the expected angle dependence been observed. Here, we show that the angle dependence in the superconductorCeRh2As2is exactly that expected of an odd-parity state.

CeRh2As2was recently found to exhibit two superconducting states: A low-field state changes into a high-field state at 4 T when a magnetic field is applied along one axis. For varying field directions, we measured the specific heat, magnetic susceptibility, and magnetic torque of this material to obtain the angle dependence of the critical fields. We find that the high-field state quickly disappears when the magnetic field is turned away from the initial axis. These results are in excellent agreement with our model identifying the two states with even- and odd-parity states.

CeRh2As2presents an extraordinary opportunity to investigate odd-parity superconductivity further. It also allows for testing mechanisms for a transition between two superconducting states, and especially their relation to spin-orbit coupling, multiband physics, and additional ordered states occurring in this material.