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

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. 111, 057002 (2013)], an intense monocycle THz pulse with center frequency ω ≃ ∆ was injected into a superconductor with BCS gap ∆; 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 ∆(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.

Nonlinear optical response of collective modes in multiband superconductors assisted by nonmagnetic impurities

“…When the considered energy region is far smaller than γ (the dirty limit), the transition matrix element can be approximated by an energy-independent constant. Such an approach has been used to calculate transient optical conductivity of photoexcited superconductors [51,52].…”

Postquench gap dynamics of two-band superconductors

“…7 Finally, we note that the topological classification explains why terahertz induced gap dynamics is qualitatively similar to the case of a parameter quench, as has been observed in various numerical studies. 24,[35][36][37][38][39] In this paper, we extend these previous studies by addressing numerically and analytically the gap dynamics of two-band superconductors following an interaction quench. Our motivation is on the fact that multiband superconductivity is realized in a variety of materials with conventional and unconventional pairing mechanisms.…”

Classification and characterization of nonequilibrium Higgs modes in unconventional superconductors

“…Quantum quenches. While the nonequilibrium probe of collective excitations in conventional s-wave superconductors has been studied intensively [29][30][31]33,34,[47][48][49][50] , the response for unconventional superconductors is still in its infancy. These systems often exhibit very complicated correlations 51,52 and a variety of different mechanisms which can lead to superconductivity.…”

“…Independent of the actual coupling to the external field, the following instructive picture can be drawn to understand the excitation process by an ultrashort THz pulse, where Higgs oscillations are excited by taking the superconductor out of equilibrium 15,[29][30][31][32][33][34][35][36][37][38][39] . Hereby, Cooper pairs are partially broken and the landscape of the free energy changes suddenly, i.e.…”