Population inversion and dynamical phase transitions in a driven superconductor

TL;DR

This paper explores how periodic driving of a superconductor induces resonant quasiparticle excitations, leading to Rabi oscillations, population inversion, and various dynamical phases of the order parameter.

Contribution

It introduces the concept of a Rabi-Higgs mode resulting from resonant quasiparticle excitations under high-frequency drive in superconductors.

Findings

Resonant quasiparticle excitations cause Rabi oscillations.

A new Rabi-Higgs mode emerges from the driven superconductor.

Different dynamical phases of the order parameter are accessible by modulating the drive.

Abstract

We consider a superconductor in which the density of states at the Fermi level or the pairing interaction is driven periodically with a frequency larger than the superconducting gap in the collisionless regime. We show by numerical and analytical computations that a subset of quasiparticle excitations enter into resonance and perform synchronous Rabi oscillations leading to cyclic population inversion with a frequency that depends on the amplitude of the drive. As a consequence a new "Rabi-Higgs" mode emerges. Turning off the drive at different times and modulating the strength allows access to all known dynamical phases of the order parameter: persistent oscillations, oscillations with damping and overdamped dynamics. We discuss physical realizations of the drive and methods to detect the dynamics.