Frequency dependence of the anomalous Hall effect: possible transition from extrinsic to intrinsic behavior John Cerne, University at Buffalo, SUNY, DMR 0449899 In metals, magnetic fields deflect moving charges to produce a voltage perpendicular to the flow of the charges. This phenomenon is known as the Hall effect and is critical to characterizing materials as well as fundamental science. Two Nobel prizes in the last few decades have been awarded for the integer and fractional quantum Hall effect in semiconductors. In some magnetized metals, the moving charges are deflected even when no magnetic field is present. This is known as the anomalous Hall effect (AHE). Two competing models are used to explain the AHE. The intrinsic model demonstrates that AHE can arise from the fundamental electronic structure of the material. The extrinsic model shows that impurity scattering (which depends on the purity of a particular sample) can produce the AHE. In many materials such as SrRuO 3 , it is not clear which model is more appropriate. The AHE effect remains a rich and challenging problem in condensed matter physics. The AHE conductivity xy as a function of probe energy. The thin lines are from a calculation of the intrinsic AHE by Fang Science, 2003) From Kim PRB, 2010. Copyright (2010) by the American Physical Society. Unlike conventional Hall measurements, which measure the static Hall voltage produced by a dc current at zero frequency, we have extended Hall measurements to higher frequencies by looking at the oscillating Hall voltage responding to an alternating current, which is produced by infrared light (wavelengths from 11 to 0.6 m). Instead of looking at just one point, our infrared Hall measurements map out the Hall response over a broad energy range. The frequency, temperature and magnetic field dependence of the infrared AHE conductivity xy in SrRuO 3 (see figure) suggest that in fact both mechanisms are present, with the extrinsic dominating at low energy and the intrinsic accounting for the high energy behavior. These results have been published by Kim et al. in Physical Review B in June 2010.