Understanding resistance oscillation in CsV$_3$Sb$_5$ superconductor

Abstract

A recent demonstration of the periodic oscillation of resistance in the thin film of CsV$_3$Sb$_5$ superconductor with a hole in the film suggests that charge-$4e$ and charge-$6e$ Cooper pairs may have condensed in this compound. While exciting, such interpretation calls for a precise determination of the effective area for the passage of Cooper pairs from one end of the lead to the other. Unlike the traditional Little-Parks effect where the rim around the hole is thin, the effective hole area is not obviously defined for the "thick-rim geometry" adopted in [arxiv:2201.10352]. Here, we note that the experiment was conducted in a regime where the superconctivity is strongly fluctuating, which motivates an analysis based on the spacetime formulation of the time-dependent Ginzburg-Landau theory. We argue that under appropriate conditions, the optimal semi-classical path is not the geometrically shortest one, but the one that moves along the edge of the hole and takes advantage of the reduced fluctuations at the boundary of the hole. The condition for the cross-over from the geometrically shortest path to the path that sticks to the wall is clarified. In such a scenario, the geometric area of the hole indeed emerges as the effective area for the flux, providing a theoretical justification to the interpretation given in [arxiv:2201.10352]. The conclusion of our analysis may have implication for similar experiments in other superconductors where the geometry of the device is not obviously that of the Little-Parks experiment employing the thin wall, but of the thick-rim type such as used in [arxiv:2201.10352].