Linear dichroism infrared resonance in over-, under-, and optimally-doped cuprate superconductors

TL;DR

This study investigates symmetry-breaking in cuprate high-temperature superconductors through polarization measurements across a broad energy spectrum, revealing a resonant linear dichroism linked to the pseudogap state and nematic order.

Contribution

It provides the first detailed polarization analysis over a wide energy range, establishing a connection between linear dichroism and symmetry-breaking phenomena in various doping levels.

Findings

Resonant LD signal peaks in the few hundred meV range.

Strongest LD observed in under-doped samples.

LD linked to pseudogap formation and nematic order.

Abstract

By measuring the polarization changes in THz, infrared, and visible radiation over an extended energy range (3-2330 meV), we observe symmetry-breaking in cuprate high temperature superconductors over wide energy, doping, and temperature ranges. We measure the polarization rotation and ellipticity of transmitted radiation though thin films as the sample is rotated. We observe a two-fold rotational symmetry in the Faraday angle, which is associated with linear dichroism (LD) and occurs when electromagnetic radiation polarized along one direction is absorbed more strongly than radiation polarized in the perpendicular direction. Such polarization anisotropies can be generally associated with symmetry breakings. We measure the amplitude of the LD signal and study its temperature, energy, and doping dependence. The LD signal shows a resonant behavior with a peak in the few hundred meV range, which is coincident with the mid-infrared optical feature that has been associated with the formation of the pseudogap state. The strongest LD signal is found in under-doped films, although it is also observed in optimally- and over-doped samples. The LD signal is consistent with nematic charge ordering as well as novel magnetoelectric effects.