Yuli Lyanda-Geller

Gate-controlled spin-orbit quantum interference effects in lateral transport.

In situ control of spin-orbit coupling in coherent transport using a clean GaAs/AlGaAs two-dimensional electron gas is realized, leading to a gate-tunable crossover from weak localization to antilocalization. The necessary theory of 2D magnetotransport in the presence of spin-orbit coupling beyond the diffusive approximation is developed and used to analyze experimental data. With this theory the Rashba contribution and linear and cubic Dresselhaus contributions to spin-orbit coupling are separately estimated, allowing the angular dependence of spin-orbit precession to be extracted at various gate voltages.

Polarized fine structure in the photoluminescence excitation spectrum of a negatively charged quantum dot.

We report polarized photoluminescence excitation spectroscopy of the negative trion in single charge-tunable quantum dots. The spectrum exhibits a p-shell resonance with polarized fine structure arising from the direct excitation of the electron spin triplet states. The energy splitting arises from the axially symmetric electron-hole exchange interaction. The magnitude and sign of the polarization are understood from the spin character of the triplet states and a small amount of quantum dot asymmetry, which mixes the wave functions through asymmetric e-e and e-h exchange interactions.

DOMAIN WALLS AND CONDUCTIVITY OF MESOSCOPIC FERROMAGNETS

Quantum interference phenomena in conductivity of mesoscopic ferromagnets are considered, particularly with regard to geometric phases acquired by electrons propagating through regions of spatially varying magnetization such as those caused by, for example, magnetic domain walls. Weak localization and electron-electron interaction quantum corrections to the conductivity, and universal conductance fluctuations are discussed. Experiments are proposed for multiply connected geometries that should reveal conductance oscillations with variations in the profile of the magnetization. {copyright} {ital 1998} {ital The American Physical Society}

Magnetotransport in Manganites and the Role of Quantal Phases: Theory and Experiment

While low-temperature Hall resisitivity rhoxy of La2/3(Ca,Pb)1/3MnO3 single crystals can be separated into ordinary (OHE) and anomalous (AHE) contributions, no such decomposition is possible near the Curie temperature Tc. Rather, the rhoxy data collapse to a single function of the reduced magnetization m=M/Msat, with an extremum at approximately 0.4 m. A new mechanism for the AHE in the inelastic hopping regime is identified that reproduces the scaling curve. An extension of Holsteins model for the hopping OHE, the mechanism arises from the combined effects of the double-exchange-induced quantal phase in triads of Mn ions and spin-orbit interactions.

Charge transport in manganites: Hopping conduction, the anomalous Hall effect, and universal scaling

The low-temperature Hall resistivity r xy of La2/3A1/3MnO3 single crystals ~where A stands for Ca, Pb, and Ca, or Sr! can be separated into ordinary and anomalous contributions, giving rise to ordinary and anomalous Hall effects, respectively. However, no such decomposition is possible near the Curie temperature which, in these systems, is close to metal-to-insulator transition. Rather, for all of these compounds and to a good approximation, the r xy data at various temperatures and magnetic fields collapse~up to an overall scale! ,o n to a single function of the reduced magnetization m[M/ M sat , the extremum of this function lying at m’0.4. A mechanism for the anomalous Hall effect in the inelastic hopping regime, which reproduces these scaling curves, is identified. This mechanism, which is an extension of Holstein’s model for the ordinary Hall effect in the hopping regime, arises from the combined effects of the double-exchange-induced quantal phase in triads of Mn ions and spin-orbit interactions. We identify processes that lead to the anomalous Hall effect for localized carriers and, along the way, analyze issues of quantum interference in the presence of phononassisted hopping. Our results suggest that, near the ferromagnet-to-paramagnet transition, it is appropriate to describe transport in manganites in terms of carrier hopping between states that are localized due to the combined effect of magnetic and nonmagnetic disorder. We attribute the qualitative variations in resistivity characteristics across manganite compounds to the differing strengths of their carrier self-trapping, and conclude that both disorder-induced localization and self-trapping effects are important for transport.

Weak localization in Ga1-xMnxAs: Evidence of impurity band transport

We report the observation of negative magnetoresistance in the ferromagnetic semiconductor GaMnAs at low temperatures (

QUANTUM INTERFERENCE AND ELECTRON-ELECTRON INTERACTIONS AT STRONG SPIN-ORBIT COUPLING IN DISORDERED SYSTEMS

T<3

Andreev reflection and pair-breaking effects at the superconductor/magnetic semiconductor interface

K) and low magnetic fields (

Coulomb blockade of nuclear spin relaxation in quantum dots

0< B <20

Quantum steering of electron wave function in an InAs Y-branch switch

mT). We attribute this effect to weak localization. Observation of weak localization provides a strong evidence of impurity band transport in these materials, since for valence band transport one expects either weak anti-localization due to strong spin-orbit interactions or total suppression of interference by intrinsic magnetization. In addition to the weak localization, we observe Altshuler-Aronov electron-electron interactions effect in this material.