Coherent Emission from Surface Josephson Plasmons in Striped Cuprates

TL;DR

This study demonstrates that stripe-induced symmetry breaking in cuprates enables surface Josephson plasmons to emit coherent terahertz radiation, revealing a link between charge order and electromagnetic emission.

Contribution

It shows that in striped cuprates, breaking of c-axis inversion symmetry activates surface Josephson plasmons to emit terahertz radiation, a phenomenon previously considered forbidden.

Findings

Emission occurs only when stripe symmetry is broken.

Terahertz radiation is emitted by surface Josephson plasmons.

Emission is suppressed in quasi-static stripe phases.

Abstract

The interplay between charge order and superconductivity remains one of the central themes of research in quantum materials. In the case of cuprates, the coupling between striped charge fluctuations and local electromagnetic fields is especially important, as it affects transport properties, coherence and dimensionality of superconducting correlations. Here, we study the emission of coherent terahertz radiation in single-layer cuprates of the La$_{2-x}$Ba$_x$CuO$_4$ family, for which this effect is expected to be forbidden by symmetry. We find that emission vanishes for compounds in which the stripes are quasi-static, but is activated when $c$-axis inversion symmetry is broken by incommensurate or fluctuating charge stripes, such as in La$_{1.905}$Ba$_{0.095}$CuO$_4$ and in La$_{1.845}$Ba$_{0.155}$CuO$_4$. In this case, terahertz radiation is emitted by surface Josephson plasmons, which are generally dark modes, but couple to free space electromagnetic radiation because of the stripe modulation.