Multi-Dimensional Coherent Spectroscopy of Light-Driven States and their Collective Modes in Multi-Band Superconductors

TL;DR

This paper develops a comprehensive theory for light-driven multi-band superconductivity, predicting unique signatures in terahertz multi-dimensional coherent spectroscopy that reveal long-lived non-equilibrium states and collective modes.

Contribution

It introduces gauge-invariant Maxwell-Bloch equations for multi-band superconductors, including quantum transport effects, and predicts distinctive spectroscopic signatures of non-equilibrium states.

Findings

Identification of long-lived finite-momentum Cooper-pairing states

Prediction of spectral shape changes due to parametric driving

Dependence of spectral features on interband-to-intraband interactions

Abstract

We present a comprehensive theory of light-controlled multi-band superconductivity and apply it to predict distinctive signatures of light-driven superconducting (SC) states in terahertz multi-dimensional coherent spectroscopy (THz-MDCS) experiments. We first derive gauge-invariant Maxwell-Bloch equations for multi-band BCS superconductors. For this, we go beyond previously considered Anderson pseudo-spin precession models to include quantum transport effects. By calculating the THz-MDCS spectra measured experimentally, we then identify unique signatures of finite-momentum Cooper-pairing states that live longer than the laser pulse. These non-equilibrium SC states are characterized by long-lived canting of Anderson pseudo-spins. The pseudo-spin oscillators that describe the properties of these SC states are parametrically driven by both finite-momentum Cooper pairing and by time oscillations of the order parameter relative phase. We show that such strong parametric driving leads to drastic changes in the THz-MDCS spectral shape from the predictions of third-order nonlinear susceptibility calculations. These spectral changes strongly depend on the interband-to-intraband interaction ratio and on the collective modes of the light-driven state.