TITLE: Infrared Hall Effect in High T_c Superconductors

SPEAKER: Dr. John Cerne

TIME: 3 PM, Tuesday, April 20, 1999

PLACE: George P. Williams, Jr. Lecture Hall, (Olin 101)

Since the discovery of cuprate high temperature superconductors (HTSC) in 1987, intense experimental and theoretical efforts have failed to fully resolve the unconventional behavior of HTSC in the "normal" (non-superconducting) state. One of the most puzzling anomalies in the normal state of HTSC is the temperature dependence of the DC Hall angle. Furthermore, the scattering rate associated with the Hall angle shows striking qualitative and quantitative differences from the rate associated with the longitudinal conductivity. Although the normal state is a good metal, this behavior has been cited as evidence for non-Drude and even non-Fermi liquid (FL) physics . A number of FL models (with low lying electron-like excitations, but an anisotropic scattering rate [1,2]) and non-FL models (with more exotic excitations such as spinons and holons [3]) have been used to explain the DC Hall angle measurements. By extending Hall angle measurements into the infrared (IR), we can test the frequency dependence of these models while decreasing the effects of impurity scattering which can dominate DC measurements. This is accomplished with a sensitive IR (900-1100 cm-1, 112-136 meV) photolelastic polarization modulation technique which can measure simultaneously both Faraday rotation and circular dichroism. These two quantities are used to determine the complex IR Hall angle, which provides insight into a number of fundamental properties such as scattering rates and effective masses. Measurements on Au and Cu thin films show anisotropic scattering rates that are related to the anisotropy of their Fermi surfaces, in addition to demonstrating the accuracy of this technique. These results also may be relevant to HTSC where two scattering rates are observed. Measurements on YBCO thin films are used to test several prevailing theories on the normal state of HTSC. [1] A.T. Zheleznyak et al., Phys. Rev. B 57, 3089 (1998). [2] L.B. Ioffe and A.J. Millis, cond-mat/9801092. [3] P.W. Anderson, Phys. Rev. Lett. 67, 2092 (1991).