Theory for Anomalous Terahertz Emission in Striped Cuprate Superconductors

TL;DR

This paper presents a theoretical model explaining anomalous terahertz emission in striped cuprate superconductors as a result of nonlinear electrodynamics and photocurrents driven by stripe-induced symmetry breaking, matching experimental observations.

Contribution

It introduces a new theoretical framework linking stripe-induced symmetry breaking and nonlinear photocurrents to terahertz emission in cuprates, highlighting the role of Umklapp processes and surface Josephson plasmons.

Findings

Photocurrents are generated via second-order nonlinearity activated by stripe symmetry breaking.

Umklapp photocurrents modulate at stripe periodicity, exciting surface Josephson plasmons.

The model explains the resonant terahertz radiation observed experimentally.

Abstract

Recent experiments in the doped cuprates La$_{2-x}$Ba$_x$CuO$_4$ have revealed the emission of anomalous terahertz radiation after impulsive optical excitation. Here, we theoretically investigate the nonlinear electrodynamics of such striped superconductors and explore the origin of the observed radiation. We argue that photoexcitation is converted into a photocurrent by a second-order optical nonlinearity, which is activated by the breaking of inversion symmetry in certain stripe configurations. We point out the importance of including Umklapp photocurrents modulated at the stripe periodicity itself, which impulsively drive surface Josephson plasmons and lead to a resonant structure of outgoing radiation, consistent with the experiments. We speculate on the utility of the proposed mechanism in the context of generating tunable terahertz radiation.