13,230 research outputs found
Quantum Hall Effect on the Hofstadter Butterfly
Motivated by recent experimental attempts to detect the Hofstadter butterfly,
we numerically calculate the Hall conductivity in a modulated two-dimensional
electron system with disorder in the quantum Hall regime. We identify the
critical energies where the states are extended for each of butterfly subbands,
and obtain the trajectory as a function of the disorder. Remarkably, we find
that when the modulation becomes anisotropic, the critical energy branches
accompanying a change of the Hall conductivity.Comment: 4 pages, 6 figure
Absence of correlation between built-in electric dipole moment and quantum Stark effect in InAs/GaAs self-assembled quantum dots
We report significant deviations from the usual quadratic dependence of the
ground state interband transition energy on applied electric fields in
InAs/GaAs self-assembled quantum dots. In particular, we show that conventional
second-order perturbation theory fails to correctly describe the Stark shift
for electric field below kV/cm in high dots. Eight-band calculations demonstrate this effect is predominantly due to
the three-dimensional strain field distribution which for various dot shapes
and stoichiometric compositions drastically affects the hole ground state. Our
conclusions are supported by two independent experiments.Comment: 4 pages, 4 figure
A Novel Cable-Driven Robotic Training Improves Locomotor Function in Individuals Post-Stroke
A novel cable-driven robotic gait training system has been tested to improve the locomotor function in individuals post stroke. Seven subjects with chronic stroke were recruited to participate in this 6 weeks robot-assisted treadmill training paradigm. A controlled assistance force was applied to the paretic leg at the ankle through a cable-driven robotic system. The force was applied from late stance to mid-swing during treadmill training. Body weight support was provided as necessary to prevent knee buckling or toe drag. Subjects were trained 3 times a week for 6 weeks. Overground gait speed, 6 minute walking distance, and balance were evaluated at pre, post 6 weeks robotic training, and at 8 weeks follow up. Significant improvements in gait speed and 6 minute walking distance were obtained following robotic treadmill training through a cable-driven robotic system. Results from this study indicate that it is feasible to improve the locomotor function in individuals post stroke through a flexible cable-driven robot
Device-spectroscopy of magnetic field effects in a polyfluorene organic light-emitting diode
We perform charge-induced absorption and electroluminescence spectroscopy in
a polyfluorene organic magnetoresistive device. Our experiments allow us to
measure the singlet exciton, triplet exciton and polaron densities in a live
device under an applied magnetic field, and to distinguish between three
different models that were proposed to explain organic magnetoresistance. These
models are based on different spin-dependent interactions, namely exciton
formation, triplet exciton-polaron quenching and bipolaron formation. We show
that the singlet exciton, triplet exciton and polaron densities and
conductivity all increase with increasing magnetic field. Our data are
inconsistent with the exciton formation and triplet-exciton polaron quenching
models.Comment: 4 pages, two figure
Anomalous quantum confined Stark effects in stacked InAs/GaAs self-assembled quantum dots
Vertically stacked and coupled InAs/GaAs self-assembled quantum dots (SADs)
are predicted to exhibit a strong non-parabolic dependence of the interband
transition energy on the electric field, which is not encountered in single SAD
structures nor in other types of quantum structures. Our study based on an
eight-band strain-dependent Hamiltonian indicates that
this anomalous quantum confined Stark effect is caused by the three-dimensional
strain field distribution which influences drastically the hole states in the
stacked SAD structures.Comment: 4 pages, 4 figure
Phase Diagram of Integer Quantum Hall Effect
The phase diagram of integer quantum Hall effect is numerically determined in
the tight-binding model, which can account for overall features of recently
obtained experimental phase diagram. In particular, the quantum Hall plateaus
are terminated by two distinct insulating phases, characterized by the Hall
resistance with classic and quantized values, respectively, which is also in
good agreement with experiments.Comment: 4 pages, RevTex, 4 PostScript figures; one new figure is added; minor
modifications in the tex
Thermal expansion in carbon nanotubes and graphene: nonequilibrium Green's function approach
The nonequilibrium Green's function method is applied to investigate the
coefficient of thermal expansion (CTE) in single-walled carbon nanotubes
(SWCNT) and graphene. It is found that atoms deviate about 1% from equilibrium
positions at T=0 K, resulting from the interplay between quantum zero-point
motion and nonlinear interaction. The CTE in SWCNT of different sizes is
studied and analyzed in terms of the competition between various vibration
modes. As a result of this competition, the axial CTE is positive in the whole
temperature range, while the radial CTE is negative at low temperatures. In
graphene, the CTE is very sensitive to the substrate. Without substrate, CTE
has large negative region at low temperature and very small value at high
temperature limit, and the value of CTE at T=300 K is
K which is very close to recent experimental result,
K (Nat. Nanotechnol. \textbf{10}, 1038 (2009)). A very weak substrate
interaction (about 0.06% of the in-plane interaction) can largely reduce the
negative CTE region and greatly enhance the value of CTE. If the substrate
interaction is strong enough, the CTE will be positive in whole temperature
range and the saturate value at high temperature reaches
K.Comment: final version, to appear in PR
Mesoscopic Effects in the Quantum Hall Regime
We report results of a study of (integer) quantum Hall transitions in a
single or multiple Landau levels for non-interacting electrons in disordered
two-dimensional systems, obtained by projecting a tight-binding Hamiltonian to
corresponding magnetic subbands. In finite-size systems, we find that
mesoscopic effects often dominate, leading to apparent non-universal scaling
behaviour in higher Landau levels. This is because localization length, which
grows exponentially with Landau level index, exceeds the system sizes amenable
to numerical study at present. When band mixing between multiple Landau levels
is present, mesoscopic effects cause a crossover from a sequence of quantum
Hall transitions for weak disorder to classical behaviour for strong disorder.
This behaviour may be of relevance to experimentally observed transitions
between quantum Hall states and the insulating phase at low magnetic fields.Comment: 13 pages, 6 figures, Proceedings of the International Meeting on
Mesoscopic and Disordered Systems, Bangalore December 2000, to appear in
Pramana, February 200
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