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Résumé |
It is well accepted that, in two dimensions, the zero resistance state should be destroyed by the proliferation of phase fluctuations at a critical temperature T_BKT < T_c0, where T_c0 is the mean-field transition temperature [1]. Experiments so far showed a strong broadening of the expected universal jump of the superfluid stiffness J_S(T) at the Berezinski-Kosterlitz-Thouless (BKT) transition [2,3]. Here, we report AC and DC transport measurements of meso-scale NbN meanders, revealing a sharp BKT transition that is consistent in all experimental observables [4]. Reducing meander width from 20 microns to 200 nm leads to the development of a foot in the resistive transition that clearly scales with the sample width. Our data can be understood in terms of established theory, without resorting to dominant sample inhomogeneity [1,5].
When increasing the normal state sheet resistance R_N up to 15 kOhm, T_c0, T_BKT and J_S(0) decrease by nearly two orders of magnitude down to 0.2 K, while the BKT transition remains sharp. For the higher levels of disorder, the phase fluctuation regime T_BKT < T < T_c0 covers up to 85% of T_c0.
For strong disorder, numerical implementations of mean-field theory predict striking deviations from Anderson’s theorem: the spectral gap Eg, the pair potential Delta, and the mean-field critical temperature T_c0 all decrease and start to significantly deviate from each other [6]. The evolution of the measured J_S(T)-curves with disorder reveals that another energy scale and not Delta determines J_S(T) in the limit of strong disorder.
[1] B. I. Halperin, D. R. Nelson, J. Low Temp. Phys. 36, 599 (1979).
[2] M. Mondal et al., Phys. Rev. Lett. 106, 047001 (2011).
[3] Y. Jong et al., Phys. Rev. B 87, 184505 (2013).
[4] A. Weitzel, L. Pfaffinger, et al., Phys. Rev. Lett. 131, 186002 (2023).
[5] L. Benfatto, C. Castellani, T. Giamarchi, Phys. Rev. B 80, 214506, (2009).
[6] A. Ghosal, M. Randeria, and N. Trivedi, Phys. Rev. B 65, 014501 (2001); M. Stosiek, B. Lang,
F. Evers, Phys. Rev. B 101, 144503 (2020); M. Stosiek, et al., Phys. Rev. B 105, L140504 (2022). |
