Probing Inhomogeneous Cuprate Superconductivity by Terahertz Josephson Echo Spectroscopy

TL;DR

This study uses terahertz spectroscopy to investigate inhomogeneities in cuprate superconductors, revealing that interlayer Josephson plasma resonances are only weakly affected by disorder, contrasting with in-plane gap variations.

Contribution

It introduces a novel terahertz Josephson echo spectroscopy method to quantify inhomogeneous broadening in cuprates, showing reduced impact of disorder on interlayer charge fluctuations.

Findings

Josephson plasmons are weakly inhomogeneously broadened

Extrinsic broadening remains constant up to 0.7Tc

Disorder effects are much smaller than in-plane gap variations

Abstract

Inhomogeneities play a crucial role in determining the properties of quantum materials. Yet methods that can measure these inhomogeneities are few, and apply to only a fraction of the relevant microscopic phenomena. For example, the electronic properties of cuprate materials are known to be inhomogeneous over nanometer length scales, although questions remain about how such disorder influences supercurrents and their dynamics. Here, two-dimensional terahertz spectroscopy is used to study interlayer superconducting tunneling in near-optimally-doped La1.83Sr0.17CuO4. We isolate a 2 THz Josephson echo signal with which we disentangle intrinsic lifetime broadening from extrinsic inhomogeneous broadening. We find that the Josephson plasmons are only weakly inhomogeneously broadened, with an inhomogeneous linewidth that is three times smaller than their intrinsic lifetime broadening. This extrinsic broadening remains constant up to 0.7Tc, above which it is overcome by the thermally-increased lifetime broadening. Crucially, the effects of disorder on the Josephson plasma resonance are nearly two orders of magnitude smaller than the in-plane variations in the superconducting gap in this compound, which have been previously documented using Scanning Tunnelling Microscopy (STM) measurements. Hence, even in the presence of significant disorder in the superfluid density, the finite frequency interlayer charge fluctuations exhibit dramatically reduced inhomogeneous broadening. We present a model that relates disorder in the superfluid density to the observed lifetimes.