Anton soutiendra sa thèse intitulée « Low-energy theory of strongly disordered superconductors » le mardi 4 novembre 2025 à 14 heures, dans la salle G-421.
Résumé :
Macroscopic electromagnetic response of a superconductor is set by its finite superfluid stiffness, which underlies hallmark phenomena such as dissipationless current flow and the Meissner effect. In conventional superconductors a hard excitation gap lets these properties persist at finite temperature and frequency, enabling advanced superconducting technologies.
The standard Mattis‑Bardeen framework predicts that increasing disorder reduces the superfluid stiffness, thus raising the kinetic inductance—a desirable trait for microwave‑device applications. Consequently, deliberate introduction of disorder should yield super‑inductive devices with dissipationless transport.
However, heavily disordered samples deviate from the conventional BCS-like theory. Both the phase diagram and microwave response reveal a hard “pseudogap” persisting above the transition temperature and unusually large sub‑gap dissipation. Recent experiments have uncovered further anomalies: the suppression of the superfluid stiffness with temperature follows an unexpected power law over a decade of temperature, and the microwave dissipation shows a non‑monotonic temperature trend that cannot be explained by conventional means.
In this thesis I develop a theoretical framework that bridges these measurements to intrinsic material parameters and exposes the limits of existing models. Based on the recently proposed pseudospin model, I formulate a microscopic description of the electromagnetic response in strongly disordered superconductors. A combination of numerical simulations and theoretical analysis links this response to disorder‑induced spatial inhomogeneity of the superconducting state. By analytically characterizing the associated statistical distribution, I derive expressions for both the superfluid stiffness and low‑frequency dissipation that align with experimental data.
The analysis identifies the low‑energy excitations responsible for the anomalous behavior as localized collective two‑level systems emerging from intrinsic inhomogeneity of the superconducting state. This insight also supports recent proposals explaining the non‑monotonic shape of the superconducting transition line in the temperature–disorder plane.
The standard Mattis‑Bardeen framework predicts that increasing disorder reduces the superfluid stiffness, thus raising the kinetic inductance—a desirable trait for microwave‑device applications. Consequently, deliberate introduction of disorder should yield super‑inductive devices with dissipationless transport.
However, heavily disordered samples deviate from the conventional BCS-like theory. Both the phase diagram and microwave response reveal a hard “pseudogap” persisting above the transition temperature and unusually large sub‑gap dissipation. Recent experiments have uncovered further anomalies: the suppression of the superfluid stiffness with temperature follows an unexpected power law over a decade of temperature, and the microwave dissipation shows a non‑monotonic temperature trend that cannot be explained by conventional means.
In this thesis I develop a theoretical framework that bridges these measurements to intrinsic material parameters and exposes the limits of existing models. Based on the recently proposed pseudospin model, I formulate a microscopic description of the electromagnetic response in strongly disordered superconductors. A combination of numerical simulations and theoretical analysis links this response to disorder‑induced spatial inhomogeneity of the superconducting state. By analytically characterizing the associated statistical distribution, I derive expressions for both the superfluid stiffness and low‑frequency dissipation that align with experimental data.
The analysis identifies the low‑energy excitations responsible for the anomalous behavior as localized collective two‑level systems emerging from intrinsic inhomogeneity of the superconducting state. This insight also supports recent proposals explaining the non‑monotonic shape of the superconducting transition line in the temperature–disorder plane.
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