Gyrotropic birefringence in the underdoped cuprates

The optical effects due to the loop-current order parameter in underdoped cuprates are studied in order to understand the recent observation of unusual birefringence in electromagnetic propagation in underdoped cuprates. It is shown why birefringence occurs even in samples with multiple domains of size much smaller than the wavelength and in twinned samples. Not only is there a rotation of polarization of incident light but also a rotation of the principal optical axis from the crystalline axes. Both are calculated in relative agreement with experiments in terms of the same parameters. The magnitude of the effect is orders of magnitude larger than the unusual Kerr effect observed in underdoped cuprates earlier. The new observations, including their comparison with the Kerr effect, test the symmetry of the proposed order decisively and confirm the conclusions from polarized neutron scattering