Twisting Anderson pseudospins with light: Quench dynamics in THz-pumped BCS superconductors

TL;DR

This paper investigates how intense THz pulses can induce and control nonequilibrium dynamics in BCS superconductors, revealing the ability to reach a gapless phase more efficiently than traditional interaction quenches.

Contribution

It demonstrates that THz pump pulses can induce far-from-equilibrium superconducting phases and introduces the use of Lax reduction as a quantitative tool for analyzing coherent BCS dynamics.

Findings

THz pulses twist Anderson pseudospins, inducing coherent Higgs mode oscillations.

Intense THz pumping can induce a gapless superconducting phase at lower energy densities.

Lax reduction effectively computes nonequilibrium BCS dynamics.

Abstract

We study the preparation (pump) and the detection (probe) of far-from-equilibrium BCS superconductor dynamics in THz pump-probe experiments. In a recent experiment [R. Matsunaga, Y. I. Hamada, K. Makise, Y. Uzawa, H. Terai, Z. Wang, and R. Shimano, Phys. Rev. Lett. {\bf 111}, 057002 (2013)], an intense monocycle THz pulse with center frequency $\omega \simeq \Delta$ was injected into a superconductor with BCS gap $\Delta$; the subsequent post-pump evolution was detected via the optical conductivity. It was argued that nonlinear coupling of the pump to the Anderson pseudospins of the superconductor induces coherent dynamics of the Higgs (amplitude) mode $\Delta(t)$. We validate this picture in a two-dimensional BCS model with a combination of exact numerics and the Lax reduction method, and we compute the nonequilibrium phase diagram as a function of the pump intensity. The main effect of the pump is to scramble the orientations of Anderson pseudospins along the Fermi surface by twisting them in the $xy$-plane. We show that more intense pump pulses can induce a far-from-equilibrium phase of gapless superconductivity ("phase I"), originally predicted in the context of interaction quenches in ultracold atoms. We show that the THz pump method can reach phase I at much lower energy densities than an interaction quench, and we demonstrate that Lax reduction (tied to the integrability of the BCS Hamiltonian) provides a general quantitative tool for computing coherent BCS dynamics. We also calculate the Mattis-Bardeen optical conductivity for the nonequilibrium states discussed here.