Search

The drift of electron spin helices in an external in-plane electric field in GaAs quantum wells is studied by means of time-resolved magneto-optical Kerr microscopy. The evolution of the spin distribution measured for different excitation powers reveals that, for short delay times and higher excitation powers, the spin helix drift slows down while its envelope becomes anisotropic. The effect is understood as a local decrease of the electron gas mobility due to electron collisions with nonequilibrium holes within the excitation spot and is reproduced well in the kinetic theory framework. For larger delay times, when the electrons constituting the spin helix and nonequilibrium holes are separated by an electric field, the spin helix drift accelerates and the mobility reaches its unperturbed value again.

We conduct photoluminescence microscopy that is sensitive to both electron and nuclear spin polarization to investigate the changes that occur in the magnetic ordering in the vicinity of the first integer quantum Hall state in a GaAs two-dimensional electron system (2DES). We observe a discontinuity in the electron spin polarization and nuclear spin longitudinal relaxation time which heralds a spontaneous transition in the magnetic ordering. We image in real space the spin phase domains that coexist at this transition, and observe hysteresis in their formation as a function of the 2DES's chemical potential. Based on measurements in a tilted magnetic field orientation, we found that the transition is protected by an energy gap containing the Zeeman energy. We explain that these observations are consistent with a phase of skyrmions forming at the transition.

Using photoluminescence microscopy enhanced by magnetic resonance, we visualize in real space both electron and nuclear polarization occurring in nonequilibrium fraction quantum Hall (FQH) liquids. We observe stripelike domain regions comprising FQH excited states which discretely form when the FQH liquid is excited by a source-drain current. These regions are deformable and give rise to bidirectionally polarized nuclear spins as spin-resolved electrons flow across their boundaries.

We study the photoluminescence (PL) spectrum of a two-dimensional electron system at the high magnetic field limit, where all electrons reside at the lowest Landau level (ν

The scattering of carriers in a two-dimensional electron gas (2DEG) in a series of novel GaAs/ n-AlGaAs heterojunctions is studied for the case where InAlAs anti-dots are embedded near the channel. Mobilities μ are measured as functions of the electron concentrations N e and are compared with theoretical models which take into account the shape and distribution of anti-dots. In samples with low dot densities, mobilities are well explained in terms of the scattering by anti-dots with a random distribution. In contrast, in samples with dense and overlapping dots and a thick wetting layer, a roughness-like potential with Gaussian-type correlation is shown to account for the data. Influences of growth conditions and the alloy composition on the diameter and the density of InAlAs dots are also discussed.

By a computer simulation we study the real space and energy distributions of 0D electrons bound in a planar array of quantum dots, including both intra-dot charging and inter-dot Coulomb interaction effects, size fluctuations, as well as the screening by a parallel gas of 2D electrons. It is demonstrated that the mutual Coulomb shifts between different dots cause pronounced many-body correlation effects and in-plane potential fluctuations, which can be significant for experiments such as capacitance and tunneling spectroscopy. In addition we investigate the influence of charged dot scattering on the mobility of a conducting channel parallel to the dot layer.

The trapping of electrons and holes in self-assembled InAs quantum dots (QDs) has been studied at low temperatures in GaAs/n-AlGaAs heterostructures. It has been found that the concentration Ns of two-dimensional electrons at a given gate voltage Vg is persistently enhanced by laser illumination, because of the trapping of holes by QDs. We also studied how the number of trapped holes depends on the energy of excitation photons from a Ti-sapphire laser. Applications of this device for a memory and near-infrared detector are discussed.

The trapping of photogenerated carriers by embedded InAs quantum dots (QDs) has been studied at 77 K in novel GaAs/n-AlGaAs structures. It is found that the concentration Ns of two dimensional electrons at a given gate voltage Vg is persistently increased by light illumination, because of the trapping of holes by QDs. By the interplay of the gate voltage and photocarrier generation, a distinct hysteresis is observed in the Ns-Vg characteristics. A drastic change of electron mobility by a factor of 19 is achieved by light illumination. The applications of this device for a novel light-controllable floating dot memory is suggested.

The resonant tunneling of electrons through a 20 nm scale InAs quantum dot bound by a pair of very thin AlAs barriers is studied. A well-resolved composite peak resulting from the ground 1s states was observed at 4.2 K in current–voltage characteristics. By investigating the effects of inplane magnetic fields, the shape of the wave function and the spatial extent of the first two electronic states are clarified.

We study electron‐nuclear spin coupling implemented in mesoscopic fractional quantum Hall (FQH) devices. We find that longitudinal resistance in such systems oscillates with a period of several hundreds of seconds driven by a constant voltage instead of a constant current. The anomalous behavior suggests that an average nuclear spin polarization self‐sustainingly oscillates between randomized and polarized states, which reveal nonlinear nature of the mesoscopic electron‐nuclear spin coupled systems.

We study electron-nuclear spin coupled systems implemented in mesoscopic fractional quantum Hall devices. We find that longitudinal resistance in such systems, oscillates with a period of several hundreds of seconds driven by a constant voltage instead of a constant current. The anomalous behavior suggests that an average nuclear spin polarization self-sustainingly oscillates between randomized and polarized states, which reveal the nonlinear nature of the mesoscopic electron-nuclear spin coupled systems.

We study electron-nuclear spin coupled systems implemented in microscopic fractional quantum Hall (FQH) devices. We find that the longtitudinal resistance in such systems oscillates with a period of about ∼200 s and is driven by a constant voltage instead of a constant current. This anomalous behavior suggests that an average nuclear spin polarization self-sustainingly oscillates between randomized and polarized states which reveal the nonlinear nature of the mesoscopic electron-nuclear spin coupled systems.

We study the photoluminescence spectrum of a low density ($��, 10 pages, 5 figures

Novel GaAs/n-AlGaAs FETs have been developed by placing InAs quantum dots near the channel. The authors show that the electron concentration increases linearly with gate voltage but its threshold voltage can be programmable by trapping electrons in these dots. Analysis has shown that one electron is trapped by each InAs dot.