We report a study of superconductivity in the heavy fermion metal YbRh₂Si₂ using SQUID-based measurements of complex electrical impedance down to 0.4 mK [1]. The superconductivity manifests below 10 mK on the background of two magnetic phases [2]: electronic antiferromagnet (AFM) below 70 mK and electro-nuclear spin-density wave (SDW) below 1.5 mK. Our study reveals the presence of distinct superconducting states, that respond differently to the magnetic field, providing the experimental measure of the Pauli limiting field and demonstrating superconductivity beyond this limit. This constitutes direct evidence for odd-parity spin-triplet superconductivity in YbRh₂Si₂ and leads to the identification of the helical superconducting state, the analogue of the topological planar phase of the superfluid ³He.

A striking observation is the boost of the helical state and simultaneous suppression of other superconducting states at the onset of the electro-nuclear SDW, that we attribute to the formation of a spin-triplet pair density wave (PDW). The observed interplay between the superconductivity and magnetism and correlations with the field dependence of the Sommerfeld-Wilson ratio point towards pairing mediated by spin fluctuations.

The superconductivity in YbRh₂Si₂ is heterogeneous, as indicated by the nature of transport signatures and sample-to-sample variation. We discuss the possible origins of this heterogeneity and strategies for obtaining homogeneous superconducting states.

The low transition temperatures confer advantages through modest energy scales: the magnetism and superconductivity are effectively tuneable by modest magnetic field, uniaxial strain and Yb isotopic substitution. This flexibility makes YbRh₂Si₂ a promising material for topological superconducting technology.

[1] https://www.nature.com/articles/s41567-026-03247-x

[2] https://journals.aps.org/prresearch/pdf/10.1103/yl3t-1gj5