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Negative superfluid density and spatial instabilities in driven superconductors

Andrey Grankin1, Victor Galitski1, and Vadim Oganesyan2,3

  • 1Joint Quantum Institute, Department of Physics, University of Maryland, College Park, Maryland 20742, USA
  • 2Physics Program and Initiative for the Theoretical Sciences, The Graduate Center, CUNY, New York, New York 10016, USA
  • 3Department of Physics and Astronomy, College of Staten Island, CUNY, Staten Island, New York 10314, USA

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Phys. Rev. B 113, L140502 – Published 24 April, 2026

DOI: https://doi.org/10.1103/3klc-8873

Abstract

We consider the excitation of Higgs modes via the modulation of the Bardeen-Cooper-Schrieffer coupling within the Migdal-Eliashberg-Keldysh theory of time-dependent superconductivity. Despite the presence of phonons, which break integrability, we observe Higgs amplitude oscillations reminiscent of the integrable case. The dynamics of quasiparticles follows from the effective Bogolyubov–de Gennes (BdG) equations, which represent a Floquet problem for the Bogolyubov quasiparticles. We find that when the Floquet-Bogolyubov bands overlap, the homogeneous solution formally leads to a negative superfluid density, which is no longer proportional to the amplitude of the order parameter. This result indicates an instability, which we explore using spatially resolved BdG equations. Spontaneous appearance of spatial inhomogeneities in the order parameter is observed and they first occur when the superfluid density becomes unphysical. We conclude that the homogeneous solution to time-dependent superconductivity is generally unstable and breaks up into a complicated spatial landscape via an avalanche of topological excitations.

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Phys. Rev. B 113, L140502– Published 24 April, 2026
  • Received 18 August 2025
  • Revised 21 February 2026
  • Accepted 25 February 2026
Article part of CHORUS
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