The anomalous velocity in its ultrafast regime
Theoretical Condensed Matter Physics
Final Report Abstract
Carriers in solids have fundamentally altered properties than in free-space. One of the most intriguing differences is the anomalous velocity constituting carriers moving perpendicular to an electrical bias. This velocity is at the origin of fascinating physical phenomena with direct impact on spintronics, topological insulators, and novel quantum computers. Our previous experimental and theoretical work on the anomalous velocity and related phenomena has led to a better understanding of light-matter interaction and produced fascinating results. Yet, it also gave rise to additional fundamental questions on anomalous velocity effects, which we aimed to address in this project. The theory part of this project obtained a number of important novel results. Firstly, we extended the SBE and applied them to compute normal photocurrents in GaAs including excitonic effects. We demonstrated that exciting at the excitonic resonance generates shiftcurrent transients of significant strength. Due to the electron-hole attraction the shift distance is, however, somewhat reduced compared to above-band-gap excitation. We then analyzed GaAs quantum wells, included electron-LO-phonon scattering, and analyzed shift and ballistic, i.e., injection, currents. Interestingly, for a particular excitation geometry, we found a ballistic current which is absent if excitonic effects, i.e., the electron-hole attraction, are neglected. Thus, in this case excitonic effects do not just quantitatively modify photocurrents which originate from single-particle properties, but result in a completely new signal. We achieved to implement the parallel-transport gauge in the SBE and computed photocurrents induced by intense THz radiation, i.e., high-harmonic radiation. Comparing results within the velocity and the length gauges we demonstrated that the length gauge is advantageous since (i) it requires a smaller number of bands to obtain converged results and (ii) it allows one to distinguish unambiguously between inter- and intraband currents. With this approach we demonstrated the existence of perpendicularly-polarized high-harmonic emission which predominantly originates from the Berry curvature. In a combined theoretical and experimental work, we could prove the existence of new anomalous velocity components occurring the GaAs quantum wells under optical and THz excitations. In this situation the anomalous velocity comprises an additional velocity component resulting from quantum beats which are caused by the simultaneous excitation of 1s and 2s excitons. Finally, in the experimental part of this proposal we have mainly studied the anomalous velocity in GaAs by above-bandgap optical femtosecond excitation at various temperatures, excitation energies and constant magnetic fields. While a linear behavior is obtained at room temperature and low magnetic fields, we enter a highly nonlinear regime at higher magnetic fields and both room- and low-temperatures. Moreover, the amplitude of the anomalous velocity shows maxima at excitation photon energies, which agree well with interband transitions between valence and conduction band Landau levels. This behavior is observed for both cryogenic and room temperature. We believe that the detection of Landau levels at elevated temperatures was possible due to the special probing in our experiments. We are sensitive to anomalous velocity effects in the ultrafast regime, i.e., before scattering restores equilibrium. A better understanding of the anomalous velocity and its link to Landau levels, which are at the origin of the Quantum-Hall Effect, will also have impact in metrology. Our studies yielded fundamentally novel insights into the generation and the dynamics of the anomalous velocity in its ultrafast regime.
Publications
- “Anisotropic Excitons and their Contributions to Shift Current Transients in Bulk GaAs”, Phys. Rev. B 96, 205201 (2017)
R. Podzimski, H. T. Duc, and T. Meier
(See online at https://doi.org/10.1103/PhysRevB.96.205201) - "Generation of ultrafast anomalous Hall currents in GaAs in the highly nonlinear regime", French-German-THz-Conference FGTC (2019), Kaiserslautern, Germany
C. Dresler and M. Bieler
- “Ballistic photocurrents in semiconductor quantum wells caused by the excitation of asymmetric excitons”, Phys. Rev. B 100, 045308 (2019)
H. T. Duc, C. Ngo, and T. Meier
(See online at https://doi.org/10.1103/PhysRevB.100.045308) - “Nonlinearity of ultrafast anomalous Hall currents in GaAs”, 44th IRMMW-THz conference (2019), Paris, France
C. Dresler and M. Bieler
(See online at https://doi.org/10.1109/IRMMW-THz.2019.8873895) - “Microscopic analysis of high harmonic generation in semiconductors with degenerate bands”, Phys. Rev. B 103, 085201 (2021)
L. H. Thong, C. Ngo, H. T. Duc, X. Song, and T. Meier
(See online at https://doi.org/10.1103/PhysRevB.103.085201) - “Landau-Level Signatures in Ultrafast Anomalous Hall Currents at Room Temperature”, 47th IRMMW-THz conference (2022), Delft, The Netherlands
C. Dresler, S. Priyadarshi, and M. Bieler
(See online at https://doi.org/10.1109/IRMMW-THz50927.2022.9895973) - “Microscopic simulations of high harmonic generation from semiconductors”, Proceedings of the SPIE 11999, Ultrafast Phenomena and Nanophotonics XXVI, 1199909 (2022)
A. Trautmann, R. Zuo, G. Wang, W.-R. Hannes, S. Yang, L. H. Thong, C. Ngo, J. T. Steiner, M. Ciappina, M. Reichelt, H. T. Duc, X. Song, W. Yang, and T. Meier
(See online at https://doi.org/10.1117/12.2607447)
