Infrared symmetry breaking in YBa 2 Cu 3 O 7-x as a function of energy, doping, and temperature

We explore the symmetry-breaking in cuprate high temperature superconductors (HTS) as a function of energy (117–2330 meV), doping, and temperature (30–300 K). We measure the Faraday rotation angle $\theta_{\mathrm{F}}$ of transmitted radiation through thin YBa 2 Cu 3 O 7-x (YBCO) films as the sample is rotated. We observe twofold rotational symmetry in $\theta_\mathrm{{F}}$ , which is associated with linear dichroism (LD) that can be generally associated with symmetry breaking. The LD signal shows a peak in the few hundred meV range and is strongest in under-doped films.