The fate of superconductivity in the vicinity of the superconductor - insulator transition probed by kinetic inductance

In the 2D limit, the resistive transition towards superconductivity is
governed by amplitude and phase fluctuations of the superconducting
order parameter. Moreover, the transition can be broadened by
inhomogeneities of the materials [1,2]. Investigating ultra-thin
disordered NbN-films, we observe an intrinsically sharp
Berezinskii-Kosterlitz-Thouless (BKT) transition in both DC- and AC
transport properties. Four independent observables allow for an
independent and consistent determination of the mean-field and BKT
transition temperatures, as well as the superfluid stiffness [3].

In NbN strips with a width of less than 3 mm, a foot appears in the
resistive transition, consistent with expectations from finite size
scaling.

When the sheet resistance of the films is gradually increased towards
e2/h, a paradigmatic quantum phase transition is approached: the
superconductor-insulator transition. We follow the evolution of the
superfluid stiffness down to BKT-transition temperatures of 250mK. The
transition remains sharp and still agrees well for the resistive and
the inductive transition. In our samples, we find no evidence for a
discontinuous drop of the stiffness at the transition in the low-T
limit [4].

[1] M. Mondal, et al., Phys. Rev. Lett. 106, 047001 (2011); Phys. Rev.
Lett. 107, 217003 (2011).
[2] J. Yong, et al., Phys. Rev. B 87, 184505 (2013).
[3] A. Weitzel et al., Phys. Rev. Lett. 131, 186002 (2023).
[4] T. Charpentier et al., arXiv:2404.09855.