Speaker
Description
Ultrathin superconducting films have become the material of choice for compact, high-impedance microwave elements such as superinductors and kinetic-inductance detectors [1,2,4], because reduced dimensionality and disorder strongly enhance the kinetic inductance. Yet the same ingredients that boost it reshape the condensate itself: this property is set by the superfluid stiffness Θ, so enhancing one suppresses the other. This raises a question of both fundamental and technological significance—when disorder is used to engineer a large response, does it merely tune a circuit parameter, or fundamentally alter the condensate the device relies on? We address this directly using ultrathin NbN microwave resonators with thicknesses down to 2.8 nm [3], where the electrodynamic response grants simultaneous access to both the device-relevant inductance and the underlying superfluid stiffness.
Our thickness-controlled platform (25 nm → 2.8 nm) reaches a high sheet kinetic inductance near 300 pH/□, tuning disorder and dimensionality continuously within one technologically relevant material. We uncover an anomalous low-temperature power-law suppression of superfluid stiffness, incompatible with Mattis–Bardeen theory, that crosses over continuously into BCS-like electrodynamics as thickness increases [5]. We identify the governing parameter, Θ(0)/Tc, and show the characteristic energy scale obeys a two-scale relation, T0 ≈ 0.18 Θ(0) + 1.1 Tc, revealing that both phase stiffness and pairing gap set the electrodynamics, unlike the pure phase-stiffness limit reported near the SIT [5, 6]. NbN thus bridges the two regimes and serves as a practical quantum-circuit material: the kinetic inductance is intrinsic, while a self-terminating native oxide confines TLS loss to the extreme ultrathin limit.
References:
[1] S. Frasca, I. N. Arabadzhiev, S. Y. Bros de Puechredon, F. Oppliger, V. Jouanny, R. Musio, M. Scigliuzzo, F. Minganti, P. Scarlino, and E. Charbon, Phys. Rev. Applied 20, 044021 (2023).
[2] V. Jouanny, S. Frasca, V. J. Weibel, L. Peyruchat, M. Scigliuzzo, F. Oppliger, F. De Palma, D. Sbroggiò, G. Beaulieu, O. Zilberberg, et al., Nat. Commun. 16, 3396 (2025).
[3] M. Khorramshahi, M. Spiecker, P. Paluch, S. Geisert, N. Gosling, N. Zapata, L. Brauch, C. Kübel, S. Dehm, R. Krupke, et al., Phys. Rev. Applied 24, 024066 (2025).
[4] X. Wei, J. Jiang, W. Xu, T. Guo, K. Zhang, Z. Li, T. Zhou, Y. Sheng, C. Cao, G. Sun, and P. Wu, Appl. Phys. Lett. 123, 154005 (2023).
[5] A. Weitzel, L. Pfaffinger, I. Maccari, K. Kronfeldner, T. Huber, L. Fuchs, J. Mallord, S. Linzen, E. Il’ichev, N. Paradiso, et al., Phys. Rev. Lett. 131, 186002 (2023).
[6] A. V. Khvalyuk, T. Charpentier, N. Roch, B. Sacépé, and M. V. Feigel’man, Phys. Rev. B 109, 144501 (2024).