Optical tests for broken time-reversal symmetry in the cuprate superconductors

Optical measurements have been made on YBa2Cu3O7 and Bi2Sr2CaCu2O8 to search for both spontaneous Faraday rotation (in transmission) and spontaneous Kerr rotation (reflection). The observation of these magneto-optical effects near Tc would be a clear indication that the superconductivity in the cuprate superconductors can be explained by a theory which predicts broken time-reversal (T) symmetry. In particular, the open-quotes any onclose quotes model of these materials predicts that T symmetry will be broken spontaneously as the material is cooled. The magneto-optical effects are expected to be quite small compared to the birefringence of the material. A method has been developed that uses a fiber-optic Sagnac interferometer to search for the magneto-optic signal. Incident upon the samples are two counter-propagating circularly polarized beams, whose electric fields have opposite time dependence. The interferometer detects Faraday or Kerr rotation by measuring the difference in phase between these beams after they have interacted with the sample. Because it is immune to birefringence in the sample, this method is superior to simply analyzing the polarization state of a single beam. Transmission experiments at 1060 nm wavelength were done with a modified fiber-optic gyroscope (a type of Sagnac interferometer). The nonreciprocal phase shift observed in the cuprates was less than the sensitivity (2 μrad). A 670 nm interferometer was constructed to look for these effects in the visible portion of the spectrum, where other researchers have witnessed unusual polarization effects.Both reflection and transmission measurements at this wavelength indicate there are no magneto-optic effects to a sensitivity of 5 μrad. This technique has been applied successfully to study ferromagnetic phase transitions in films of EuO and Gd