Optical spectroscopy of plasmons and excitons in cuprate superconductors

TL;DR

This paper explores collective excitations like plasmons and excitons in high-temperature cuprate superconductors, demonstrating how photons acquire mass inside the superconductor and analyzing various Josephson plasma modes through optical spectroscopy.

Contribution

It provides experimental evidence for Anderson's mechanism of photon mass generation and details the observation of Josephson plasmons in layered cuprates, linking them to collective phase oscillations.

Findings

Photon mass relates to condensate spectral weight

Transverse optical plasma modes observed in bi-layer cuprates

Collective modes consistent with Leggett's phase oscillation predictions

Abstract

An introduction is given to collective modes in layered, high Tc superconductors. An experimental demonstration is treated of the mechanism proposed by Anderson whereby photons travelling inside the superconductor become massive, when the U(1) gauge symmetry is broken in the superconductor to which the photons are coupled. Using the Ferrell-Tinkham sumrule the photon mass is shown to have a simple relation to the spectral weight of the condensate. Various forms of Josephson plasmons can exist in single-layer, and bi-layecuprates. In the bi-layer cuprates a transverse optical plasma mode can be observed as a peak in the c-axis optical conductivity. This mode appears as a consequence of the existence of two different intrinsic Josephson couplings between the CuO2 layers. It is strongly related to a collective oscillation corresponding to small fluctuations of the relative phases of the two condensates, which has been predicted in 1966 by A.J. Leggett for superconductors with two bands of charge carriers. A description is given of optical data of the high Tc cuprates demonstrating the presence of these and similar collective modes.