Optical birefringence and dichroism of
cuprate superconductors in the THz regime
Peter Armitage
The presence of optical polarization
anisotropies, such as Faraday/Kerr effects, linear birefringence, and
magnetoelectric birefringence are evidence for broken symmetry states of
matter. The recent discovery of a Kerr effect using near-IR light in the
pseudogap phase of the cuprates can be regarded as a strong evidence for a
spontaneous symmetry breaking and the existence of an anomalous long-range
ordered state. In this work we present a high precision study of the polarimetry
properties of the cuprates in the THz regime. While no Faraday effect was found
in this frequency range to the limits of our experimental uncertainty (1.3
milli-radian or 0.07∘), a small but significant polarization
rotation was detected that derives from an anomalous linear dichroism. In YBa2Cu3Oy the effect has a
temperature onset that mirrors the pseudogap temperature T∗ and is enhanced in magnitude in underdoped samples. In x=1/8 La2−xBaxCuO4, the effect onsets above room temperature, but shows a dramatic
enhancement near a temperature scale known to be associated with spin and
charge ordered states. These features are consistent with a loss of both C4 rotation and mirror symmetry in the electronic structure of the CuO2 planes in the pseudogap state.
Host: Prof. G. Kotliar