Magnetic- and electric-field-induced anomalous Hall currents from optical excitation of Landau transitions in bulk GaAs

We study a certain type of anomalous Hall current in magnetically and electrically biased bulk GaAs samples under the formation of Landau levels. The currents are generated by ultrafast optical excitation of spin-polarized carriers and detected by time-resolved measurements of the simultaneously emitted terahertz (THz) radiation. Due to the requirements of simultaneous magnetic and electric driving fields we refer to these currents as magnetic- and electric-field-induced anomalous Hall currents (BE-AHC). We find that the BE-AHC peaks for optical transitions between the band extrema of Landau levels of valence and conduction bands. These discrete features are attributed to the energy dependence of the geometric phases being responsible for anomalous transport effects in a semiconductor band structure. Surprisingly, we even detect the discrete Landau band transitions at room temperature, most likely due to the ultrafast local probing realized in our experiment. An analysis of the THz spectra using a model, based on the Boltzmann transport equation of optically excited carriers, shows that electron and hole contributions lead to complex current dynamics. While the cyclotronic motion of electrons results in a dip in the THz spectral response of BE-AHC, it causes a peak in the spectral response of normal Hall currents. Additionally, our experimental results strongly suggest that the Landau levels of the valence band play a significant role in the generation of BE-AHC. We expect that our results will initiate further studies on Berry-phase effects in Landau bands.