Mechanism for π phase shifts in Little-Parks experiments: Application to 4Hb−TaS2 and to 2H Physical Review Pub Date : 2023-11-14 DOI:10.1103/physrevb.108.l180505 Mark H. Fischer, Patrick A. Lee, Jonathan Ruhman

Abstract

Recently, unusual $\ensuremath{\pi}$ phase shifts in Little-Parks experiments performed on two systems derived from the layered superconductor $2H\text{\ensuremath{-}}{\mathrm{TaS}}_{2}$ were reported. These systems share the common feature that additional layers have been inserted between the $1H\text{\ensuremath{-}}{\mathrm{TaS}}_{2}$ layers. In both cases, the $\ensuremath{\pi}$ 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 $2H\text{\ensuremath{-}}{\mathrm{TaS}}_{2}$. 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 $4\mathit{Hb}$ case, the superconductivity's robustness to disorder. Both are nonissues in our picture, which also exposes the common features that are special in these two systems.