896 research outputs found
Electronic States of Graphene Nanoribbons
We study the electronic states of narrow graphene ribbons (``nanoribbons'')
with zigzag and armchair edges. The finite width of these systems breaks the
spectrum into an infinite set of bands, which we demonstrate can be
quantitatively understood using the Dirac equation with appropriate boundary
conditions. For the zigzag nanoribbon we demonstrate that the boundary
condition allows a particle- and a hole-like band with evanescent wavefunctions
confined to the surfaces, which continuously turn into the well-known zero
energy surface states as the width gets large. For armchair edges, we show that
the boundary condition leads to admixing of valley states, and the band
structure is metallic when the width of the sample in lattice constant units is
divisible by 3, and insulating otherwise. A comparison of the wavefunctions and
energies from tight-binding calculations and solutions of the Dirac equations
yields quantitative agreement for all but the narrowest ribbons.Comment: 5 pages, 6 figure
Canted phase in double quantum dots
We perform a Hartree-Fock calculation in order to describe the ground state
of a vertical double quantum dot in the absence of magnetic fields parallel to
the growth direction. Intra- and interdot exchange interactions determine the
singlet or triplet character of the system as the tunneling is tuned. At finite
Zeeman splittings due to in-plane magnetic fields, we observe the continuous
quantum phase transition from ferromagnetic to symmetric phase through a canted
antiferromagnetic state. The latter is obtained even at zero Zeeman energy for
an odd electron number.Comment: 5 pages, 3 figure
Piezoelectric mechanism of orientation of stripe structures in two-dimensional electron systems
A piezoelectric mechanism of orientation of stripes in two-dimensional
quantum Hall systems in GaAs heterostructures is considered. The anisotropy of
the elastic moduli and the boundary of the sample are taken into account. It is
found that in the average the stripes line up with the [110] axis. In double
layer systems the wave vector of the stripe structure rotates from the [110] to
[100] axis if the period of density modulation becomes large than the
interlayer distance. From the experimental point of view it means that in
double layer systems anisotropic part of resistivity changes its sign under
variation of the external magnetic field.Comment: 8 page
Spin depolarization in the transport of holes across GaMnAs/GaAlAs/p-GaAs
We study the spin polarization of tunneling holes injected from ferromagnetic
GaMnAs into a p-doped semiconductor through a tunneling barrier. We obtain an
upper limit to the spin injection rate. We find that spin-orbit interaction
interaction in the barrier and in the drain limits severely spin injection.
Spin depolarization is stronger when the magnetization is parallel to the
current than when is perpendicular to it.Comment: Accepted in Phys. Rev. B. 4 pages, 4 figure
Instability of the symmetric Couette-flow in a granular gas: hydrodynamic field profiles and transport
We investigate the inelastic hard disk gas sheared by two parallel bumpy
walls (Couette-flow). In our molecular dynamic simulations we found a
sensitivity to the asymmetries of the initial condition of the particle places
and velocities and an asymmetric stationary state, where the deviation from
(anti)symmetric hydrodynamic fields is stronger as the normal restitution
coefficient decreases. For the better understanding of this sensitivity we
carried out a linear stability analysis of the former kinetic theoretical
solution [Jenkins and Richman: J. Fluid. Mech. {\bf 171} (1986)] and found it
to be unstable. The effect of this asymmetry on the self-diffusion coefficient
is also discussed.Comment: 9 pages RevTeX, 14 postscript figures, sent to Phys. Rev.
Transport coefficients for dense hard-disk systems
A study of the transport coefficients of a system of elastic hard disks,
based on the use of Helfand-Einstein expressions is reported. The
self-diffusion, the viscosity, and the heat conductivity are examined with
averaging techniques especially appropriate for the use in event-driven
molecular dynamics algorithms with periodic boundary conditions. The density
and size dependence of the results is analyzed, and comparison with the
predictions from Enskog's theory is carried out. In particular, the behavior of
the transport coefficients in the vicinity of the fluid-solid transition is
investigated and a striking power law divergence of the viscosity in this
region is obtained, while all other examined transport coefficients show a drop
in that density range.Comment: submitted to PR
Stripes in Quantum Hall Double Layer Systems
We present results of a study of double layer quantum Hall systems in which
each layer has a high-index Landau level that is half-filled. Hartree-Fock
calculations indicate that, above a critical layer separation, the system
becomes unstable to the formation of a unidirectional coherent charge density
wave (UCCDW), which is related to stripe states in single layer systems. The
UCCDW state supports a quantized Hall effect when there is tunneling between
layers, and is {\it always} stable against formation of an isotropic Wigner
crystal for Landau indices . The state does become unstable to the
formation of modulations within the stripes at large enough layer separation.
The UCCDW state supports low-energy modes associated with interlayer coherence.
The coherence allows the formation of charged soliton excitations, which become
gapless in the limit of vanishing tunneling. We argue that this may result in a
novel {\it ``critical Hall state''}, characterized by a power law in
tunneling experiments.Comment: 10 pages, 8 figures include
Canted ground state in artificial molecules at high magnetic fields
We analyze the transitions that a magnetic field provokes in the ground state
of an artificial homonuclear diatomic molecule. For that purpose, we have
performed numerical diagonalizations for a double quantum dot around the regime
of filling factor 2. We present phase diagrams in terms of tunneling and Zeeman
couplings, and confinement strength. We identify a series of transitions from
ferromagnetic to symmetric states through a set of canted states with
antiferromagnetic couping between the two quantum dots
Fluctuating Navier-Stokes equations for inelastic hard spheres or disks
Starting from the fluctuating Boltzmann equation for smooth inelastic hard
spheres or disks, closed equations for the fluctuating hydrodynamic fields to
Navier-Stokes order are derived. This requires to derive constitutive relations
for both the fluctuating fluxes and the correlations of the random forces. The
former are identified as having the same form as the macroscopic average fluxes
and involving the same transport coefficients. On the other hand, the random
force terms exhibit two peculiarities as compared with their elastic limit for
molecular systems. Firstly, they are not white, but have some finite relaxation
time. Secondly, their amplitude is not determined by the macroscopic transport
coefficients, but involves new coefficients
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