Ultrafast dynamics and light-induced superconductivity from first principles

TL;DR

This paper develops a first-principles model to simulate the ultrafast light-induced superconductivity in materials, accurately reproducing experimental optical responses and predicting new photo-induced superconducting states in various compounds.

Contribution

The authors introduce a novel ab-initio approach solving Migdal-Eliashberg equations on the real-frequency axis, enabling quantitative predictions of nonequilibrium superconducting phenomena.

Findings

Successfully reproduces pump-probe experimental data for Pb and LaH$_{10}$

Identifies photo-induced superconductivity in K$_3$C$_{60}$ after mid-infrared pulse irradiation

Predicts similar photo-induced superconducting gap in CaC$_6$

Abstract

Experiments on superconducting materials have unveiled unique emergent properties when they are driven far from equilibrium. However, a quantitative first-principles treatment that describes experimental observations is lacking. In this work, we develop an ab-initio model for the nonequilibrium response of optically irradiated superconducting films within the framework of conventional electron-phonon-mediated superconductivity, leveraging new numerical techniques to solve the Migdal-Eliashberg equations directly on the real-frequency axis. This enables us to quantitatively reproduce the optical response of superconducting films in pump-probe experiments and validate our approach on measurements of the differential reflectance of Pb and LaH$_{10}$ in response to a pump excitation. Similar calculations performed on the alkali-doped fulleride K$_3$C$_{60}$ reveal that a photo-induced superconducting state is generated after irradiation by an ultrafast mid-infrared pulse of sufficient intensity, as reported in prior experimental work. The enhancement in this framework is attributed to the excitation of quasiparticles to energies resonant with the strongest electron-phonon coupling in K$_3$C$_{60}$, in close analogy to the mechanism for enhancement of superconductivity under microwave irradiation, explaining the nature of the photo-induced superconducting state and elucidating the subsequent quasiparticle and phonon dynamics. Our results suggest that photo-induced superconductivity is accessible in more materials than previously recognized. We demonstrate this by performing calculations on calcium-intercalated graphite, CaC$_6$, and predict a similar photo-induced superconducting gap.