A mechanism for $\pi$ phase shifts in Little-Parks experiments: application to 4Hb-TaS$_2$ and to 2H-TaS$_2$ intercalated with chiral molecules

Abstract

Recently, unusual $\pi$ phase shifts in Little-Parks experiments performed on two systems derived from the layered superconductor 2H-TaS$_2$ were reported. These systems share the common feature that additional layers have been inserted between the 1H-TaS$_2$ layers. In both cases, the $\pi$ phase shift has been interpreted as evidence for the emergence of exotic superconductivity in the 1H layers. Here, we propose an alternative explanation assuming that superconductivity in the individual 1H layers is of conventional $s$-wave nature derived from the parent 2H-TaS$_2$. We show that a negative Josephson coupling between otherwise decoupled neighboring 1H layers can explain the observations. Furthermore, we find that the negative coupling can arise naturally assuming a tunneling barrier containing paramagnetic impurities. An important ingredient is the suppression of non-spin-flip tunneling due to spin-momentum locking of Ising type in a single 1H layer together with the inversion symmetry of the double layer. In the exotic superconductivity scenario, it is challenging to explain why the critical temperature is almost the same as in the parent material and, in the 4Hb case, the superconductivity's robustness to disorder. Both are non-issues in our picture, which also exposes the common features that are special in these two systems.