Terahertz Dynamics of a Topologically Protected State: Quantum Hall Effect Plateaus near the Cyclotron Resonance of a Two-Dimensional Electron Gas.

We measure the Hall conductivity of a two-dimensional electron gas formed at a GaAs/AlGaAs heterojunction in the terahertz regime close to the cyclotron resonance frequency using highly sensitive Faraday rotation measurements. The sample is electrically gated, allowing the electron density to be changed continuously by more than a factor of 3. We observe clear plateaulike and steplike features in the Faraday rotation angle vs electron density and magnetic field (Landau-level filling factor) even at fields or frequencies very close to cyclotron resonance absorption.

Aharonov-Bohm excitons at elevated temperatures in type-II ZnTe/ZnSe quantum dots.

Optical emission from type-II ZnTe/ZnSe quantum dots demonstrates large and persistent oscillations in both the peak energy and intensity indicating the formation of coherently rotating states. Furthermore, these Aharonov-Bohm oscillations are shown to be remarkably robust and persist until 180 K. This is at least one order of magnitude greater than the typical temperatures in lithographically defined rings.

Internal transitions of negatively charged magnetoexcitons and many body effects in a two-dimensional electron gas.

Internal transitions of quasi-two-dimensional, negatively charged magnetoexcitons ( X-) and their evolution with excess electron density have been studied in GaAs/AlGaAs quantum wells. In the dilute electron limit, due to magnetic translational invariance, the optically detected resonance spectra are dominated by bound-to-continuum bands in contrast to the negatively charged donor system D-, which exhibits strictly bound-to-bound transitions. With increasing excess electron density Landau-level filling factors nu<2 the X--like transitions are blueshifted; they are absent for nu>2.

Blocking of the polaron effect and spin-split cyclotron resonance in a two-dimensional electron Gas.

Cyclotron resonance (CR) of high density GaAs quantum wells exhibits well-resolved spin splitting above the LO-phonon frequency. The spin-up and spin-down CR frequencies are reversed relative to the order expected from simple band nonparabolicity. We demonstrate that this is a consequence of the blocking of the polaron interaction which is a sensitive function of the filling of the Landau levels.