Realization of a large T_BKT–T_BCS separation in two-dimensional superconductors

Determine whether two-dimensional superconductors can realize a regime in which the Berezinskii–Kosterlitz–Thouless transition temperature T_BKT is much less than the pairing temperature T_BCS, corresponding to the London limit where quasiparticle contributions can be neglected.

Background

The theoretical framework used in this paper assumes that superconductivity is effectively two-dimensional and, specifically, that the BKT transition temperature is much lower than the mean-field pairing temperature. This large separation (T_BKT ≪ T_BCS) corresponds to the London limit, under which quasiparticle effects can be neglected and vortex dynamics dominate the magnetic noise.

The authors explicitly note that it is unclear whether this separation can be fully realized in practice, though they argue their predictions should remain relevant whenever there is significant separation between these temperatures. Establishing the realizability of this regime would validate the assumptions underlying the proposed noise magnetometry probe and clarify the applicability across material platforms.

References

To obtain the noise spectrum we assume the superconductivity is truly two-dimensional, i.e., i) the sample thickness is much smaller than the penetration depth and ii) the BKT transition temperature T_{\rm BKT} is much less than the pairing temperature T_{\rm BCS} -- corresponding to the London limit and allowing us to neglect effects due to quasiparticles (for a more precise discussion see Sec.~\ref{sub:qp}). While it is unclear whether assumption (ii) can be truly realized, predictions of our theory should still be relevant in any situation where there is a significant separation between T_{\rm BKT} and the three-dimensional transition temperature, as discussed in Sec.~\ref{sec:expt}.

— Probing the Berezinskii-Kosterlitz-Thouless vortex unbinding transition in two-dimensional superconductors using local noise magnetometry (2404.06147 - Curtis et al., 9 Apr 2024) in Section 2 (Vortex Interactions and magnetic noise), assumptions paragraph near the beginning