in the Little-Parks effect*

Finding unambiguous evidence of non-trivial pairing states is one of the greatest experimental challenges in the field of unconventional superconductivity. Such evidence requires phase-sensitive probes susceptible to the internal structure of the order parameter.

We measure the Little-Parks effect to provide clear evidence of an unconventional superconducting order parameter in 4Hb-TaS₂. Namely, we find a π-shift in the transition-temperature oscillations of rings made of a single crystal. We argue that such an effect can only occur if the underlying order parameter belongs to a two-dimensional representation, in other words there are two degenerate order parameters right at the transition.

Additionally, we show that T_c is enhanced as a function of the out-of-plane field when a constant in-plane field is applied. Such an increase is consistent with a chiral state, which again, in general only emerges from a two-component order parameter. In combination with previous experiments, our results strongly indicate that 4Hb-TaS₂ indeed realizes a chiral superconductor.