Orateur
Description
Macroscopic electromagnetic response of a superconductor is described by a finite superfluid stiffness $\theta$, which underlies hallmark phenomena such as dissipationless current flow. In conventional superconductors, these properties persist at finite temperature $T$ and frequency $\omega$ due to a hard superconducting gap. The conventional Mattis‑Bardeen framework further predicts that increasing disorder reduces $\theta$—a desirable trait for microwave-device applications.
However, strongly disordered superconductors (amorphous indium oxide, niobium nitride, titanium nitride) run counter to the conventional theory of superconductivity. Tunneling spectroscopy reveals a hard “pseudogap” persisting above the transition temperature. Furthermore, the suppression of $\theta$ with $T$ follows an unexpected power law spanning more than a decade of $T$ [1], and microwave dissipation shows a non‑monotonic $T$ trend that cannot be explained by conventional means [2, 3].
This talk presents a theoretical framework that consistently addresses these observations. A combination of custom numerical simulations and theoretical analysis links macroscopic electromagnetic response to disorder‑induced spatial inhomogeneity of the superconducting state—a key feature of strong disorder. By analytically characterizing the statistical order parameter distribution, we derive expressions for both $\theta$ and low-$\omega$ dissipation that agree with experimental data [1, 3]. The analysis further identifies the relevant low‑energy collective excitations that are phenomenologically similar to two-level systems [3]. The steep profile of the spectral density of these excitations suggests a strong $\omega$ dependence of dissipation. Finally, these insights help explain the non‑monotonic shape of the superconducting transition line in the temperature–disorder plane [2].
References:
[1] AVK et al., Phys. Rev. B 109, 144501 (2024)
[2] T. Charpentier et al., Nature Physics 21, 104-109 (2025)
[3] AVK and M. V. Feigel'man, Phys. Rev. Lett. 136, 256001 (2026)