47 research outputs found
Quantum Hall Ferromagnetism in a Two-Dimensional Electron System
Experiments on a nearly spin degenerate two-dimensional electron system
reveals unusual hysteretic and relaxational transport in the fractional quantum
Hall effect regime. The transition between the spin-polarized (with fill
fraction ) and spin-unpolarized () states is accompanied
by a complicated series of hysteresis loops reminiscent of a classical
ferromagnet. In correlation with the hysteresis, magnetoresistance can either
grow or decay logarithmically in time with remarkable persistence and does not
saturate. In contrast to the established models of relaxation, the relaxation
rate exhibits an anomalous divergence as temperature is reduced. These results
indicate the presence of novel two-dimensional ferromagnetism with a
complicated magnetic domain dynamic.Comment: 15 pages, 5 figure
Excitation Energy Dependence of the Exciton Inner Ring
We report on the excitation energy dependence of the inner ring in the
exciton emission pattern. The contrast of the inner ring is found to decrease
with lowering excitation energy. Excitation by light tuned to the direct
exciton resonance is found to effectively suppress excitation-induced heating
of indirect excitons and facilitate the realization of a cold and dense exciton
gas. The excitation energy dependence of the inner ring is explained in terms
of exciton transport and cooling.Comment: 5 pages, 4 figure
Pancharatnam-Berry phase in condensate of indirect excitons
We report on the observation of the Pancharatnam-Berry phase in a condensate
of indirect excitons (IXs) in a GaAs coupled quantum well structure. The
Pancharatnam-Berry phase leads to phase shifts of interference fringes in IX
interference patterns. Correlations are found between the phase shifts,
polarization pattern of IX emission, and onset of IX spontaneous coherence. The
Pancharatnam-Berry phase is acquired due to coherent spin precession in IX
condensate. The effect of the Pancharatnam-Berry phase on the IX phase pattern
is described in terms of an associated momentum.Comment: 6 pages, 5 figures + 2 pages supplemental material, 3 supplemental
figure
Hybridization of electron subbands in a double quantum well at quantizing magnetic field
We employ magnetocapacitance and far-infrared spectroscopy techniques to
study the spectrum of the double-layer electron system in a parabolic quantum
well with a narrow tunnel barrier in the centre. For gate-bias-controlled
asymmetric electron density distributions in this soft two-subband system we
observe both individual subband gaps and double layer gaps at integer filling
factor . The bilayer gaps are shown to be either trivial common for two
subbands or caused by hybridization of electron subbands in magnetic field. We
describe the observed hybrid gaps at and within a simple model
for the modified bilayer spectrum.Comment: REVTeX, 24 pages, 9 figures included. Submitted to Phys. Rev.
Canted antiferromagnetic phase in a double quantum well in a tilted quantizing magnetic field
We investigate the double-layer electron system in a parabolic quantum well
at filling factor in a tilted magnetic field using capacitance
spectroscopy. The competition between two ground states is found at the Zeeman
splitting appreciably smaller than the symmetric-antisymmetric splitting.
Although at the transition point the system breaks up into domains of the two
competing states, the activation energy turns out to be finite, signaling the
occurrence of a new insulator-insulator quantum phase transition. We interpret
the obtained results in terms of a predicted canted antiferromagnetic phase.Comment: 4 pages, 3 figures included, accepted to PR
Spin-Orbit Coupling, Antilocalization, and Parallel Magnetic Fields in Quantum Dots
We investigate antilocalization due to spin-orbit coupling in ballistic GaAs
quantum dots. Antilocalization that is prominent in large dots is suppressed in
small dots, as anticipated theoretically. Parallel magnetic fields suppress
both antilocalization and also, at larger fields, weak localization, consistent
with random matrix theory results once orbital coupling of the parallel field
is included. In situ control of spin-orbit coupling in dots is demonstrated as
a gate-controlled crossover from weak localization to antilocalization.Comment: related papers at http://marcuslab.harvard.ed