667 research outputs found
In-Plane Magnetic Field Induced Anisotropy of 2D Fermi Contours and the Field Dependent Cyclotron Mass
The electronic structure of a 2D gas subjected to a tilted magnetic field,
with a strong component parallel to the GaAs/AlGaAs interface and a weak
component oriented perpendicularly, is studied theoretically. It is shown that
the parallel field component modifies the originally circular shape of a Fermi
contour while the perpendicular component drive an electron by the Lorentz
force along a Fermi line with a cyclotron frequency given by its shape. The
corresponding cyclotron effective mass is calculated self-consistently for
several concentrations of 2D carriers as a function of the in-plane magnetic
field. The possibility to detect its field-induced deviations from the zero
field value experimentally is discussed.Comment: written in LaTeX, 9 pages, 4 figures (6 pages) in 1 PS file
(compressed and uuencoded) available on request from [email protected],
SM-JU-93-
Ferromagnetism in Diluted Magnetic Semiconductor Heterojunction Systems
Diluted magnetic semiconductors (DMSs), in which magnetic elements are
substituted for a small fraction of host elements in a semiconductor lattice,
can become ferromagnetic when doped. In this article we discuss the physics of
DMS ferromagnetism in systems with semiconductor heterojunctions. We focus on
the mechanism that cause magnetic and magnetoresistive properties to depend on
doping profiles, defect distributions, gate voltage, and other system
parameters that can in principle be engineered to yield desired results.Comment: 12 pages, 7 figures, review, special issue of Semicon. Sci. Technol.
on semiconductor spintronic
A single-molecule approach to ZnO defect studies: single photons and single defects
Investigations that probe defects one at a time offer a unique opportunity to
observe properties and dynamics that are washed out of ensemble measurements.
Here we present confocal fluorescence measurements of individual defects in
Al-doped ZnO nanoparticles and undoped ZnO sputtered films that are excited
with sub-bandgap energy light. Photon correlation measurements yield both
antibunching and bunching, indicative of single-photon emission from isolated
defects that possess a metastable shelving state. The single-photon emission is
in the range 560 - 720 nm and typically exhibits two broad spectral peaks
separated by approximately 150 meV. The excited state lifetimes range from 1 -
13 ns, consistent with the finite-size and surface effects of nanoparticles and
small grains. We also observe discrete jumps in the fluorescence intensity
between a bright state and a dark state. The dwell times in each state are
exponentially distributed and the average dwell time in the bright (dark) state
does (may) depend on the power of the exciting laser. Taken together, our
measurements demonstrate the utility of a single-molecule approach to
semiconductor defect studies and highlight ZnO as a potential host material for
single-defect based applications.Comment: 33 pages, 7 figure
Anisotropy of Magnetoresistance Hysteresis around the Quantum Hall State in Tilted Magnetic Field
We present an anisotropy of the hysteretic transport around the spin
transition point at Landau level filling factor in tilted magnetic
field. When the direction of the in-plane component of the magnetic field
is normal to the probe current , a strong hysteretic
transport due to the current-induced nuclear spin polarization occurs. When
is parallel to , on the other hand, the hysteresis almost
disappears. We also demonstrate that the nuclear spin-lattice relaxation rate
at the transition point increases with decreasing angle between
the directions of and . These results suggest that the
morphology of electron spin domains around is affected by the
current direction.Comment: 4 pages, 4 figure
Orientation of the Stripe Formed by the Two-Dimensional Electrons in Higher Landau Levels
Effect of periodic potential on the stripe phase realized in the higher
Landau levels is investigated by the Hartree-Fock approximation. The period of
the potential is chosen to be two to six times of the fundamental period of the
stripe phase. It is found that the stripe aligns perpendicularly to the
external potential in contrast to a naive expectation and hydrodynamic theory.
Charge modulation towards the Wigner crystallization along the stripe is
essential for the present unexpected new result.Comment: 5 pages, RevTex, two figures included in the tex
Modulation Induced Phase Transition from Fractional Quantum Hall to Stripe State at nu=5/3
We have investigated the effect of unidirectional periodic potential
modulation on the fractional quantum Hall (FQH) state at filling factors nu=5/3
and 4/3. For large enough modulation amplitude, we find that the resistivity
minimum at nu=5/3 gives way to a peak that grows with decreasing temperature.
Density matrix renormalization group calculation reveals that phase transition
from FQH state to unidirectional striped state having a period sim 4 l (with l
the magnetic length) takes place at nu=1/3 (equivalent to nu=5/3 by the
particle-hole symmetry) with the increase of the modulation amplitude,
suggesting that the observed peak is the manifestation of the stripe phase.Comment: 4 pages, 6 figures; minor revisio
Reorientation of Anisotropy in a Square Well Quantum Hall Sample
We have measured magnetotransport at half-filled high Landau levels in a
quantum well with two occupied electric subbands. We find resistivities that
are {\em isotropic} in perpendicular magnetic field but become strongly {\em
anisotropic} at = 9/2 and 11/2 on tilting the field. The anisotropy
appears at an in-plane field, 2.5T, with the easy-current
direction {\em parallel} to but rotates by 90 at 10T and points now in the same direction as in single-subband samples.
This complex behavior is in quantitative agreement with theoretical
calculations based on a unidirectional charge density wave state model.Comment: 4 pages, 4 figure
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