743 research outputs found
Vortex States of a Superconducting Film from a Magnetic Dot Array
Using Ginzburg-Landau theory, we find novel configurations of vortices in
superconducting thin films subject to the magnetic field of a magnetic dot
array, with dipole moments oriented perpendicular to the film. Sufficiently
strong magnets cause the formation of vortex-antivortex pairs. In most cases,
the vortices are confined to dot regions, while the antivortices can form a
rich variety of lattice states. We propose an experiment in which the
perpendicular component of the dot dipole moments can be tuned using an
in-plane magnetic field. We show that in such an experiment the
vortex-antivortex pair density shows broad plateaus as a function of the dipole
strength. Many of the plateaus correspond to vortex configurations which break
dot lattice symmetries. In some of these states, the vortex cores are strongly
distorted. Possible experimental consequences are mentioned.Comment: 4 pages, 4 figure
Theory of Phonon Shakeup Effects on Photoluminescence from the Wigner Crystal in a Strong Magnetic Field
We develop a method to compute shakeup effects on photoluminescence from a
strong magnetic field induced two-dimensional Wigner crystal. Only localized
holes are considered. Our method treats the lattice electrons and the tunneling
electron on an equal footing, and uses a quantum-mechanical calculation of the
collective modes that does not depend in any way on a harmonic approximation.
We find that shakeup produces a series of sidebands that may be identified with
maxima in the collective mode density of states, and definitively distinguishes
the crystal state from a liquid state in the absence of electron-hole
interaction. In the presence of electron-hole interaction, sidebands also
appear in the liquid state coming from short-range density fluctuations around
the hole. However, the sidebands in the liquid state and the crystal state have
different qualitative behaviors. We also find a shift in the main luminescence
peak, that is associated with lattice relaxation in the vicinity of a vacancy.
The relationship of the shakeup spectrum with previous mean-field calculations
is discussed.Comment: 14 pages, uuencoded postscript file for entire paper, also available
at (click phd14) http://rainbow.uchicago.edu/~ldz/paper/paper.htm
Orbital maneuvering engine feed system coupled stability investigation
A digital computer model used to analyze and predict engine feed system coupled instabilities over a frequency range of 10 to 1000 Hz was developed and verified. The analytical approach to modeling the feed system hydrodynamics, combustion dynamics, chamber dynamics, and overall engineering model structure is described and the governing equations in each of the technical areas are presented. This is followed by a description of the generalized computer model, including formulation of the discrete subprograms and their integration into an overall engineering model structure. The operation and capabilities of the engineering model were verified by comparing the model's theoretical predictions with experimental data from an OMS-type engine with a known feed system/engine chugging history
Orbital Maneuvering Engine Feed System Coupled Stability Investigation, Computer User's Manual
An operating manual for the feed system coupled stability model was given, in partial fulfillment of a program designed to develop, verify, and document a digital computer model that can be used to analyze and predict engine/feed system coupled instabilities in pressure-fed storable propellant propulsion systems over a frequency range of 10 to 1,000 Hz. The first section describes the analytical approach to modelling the feed system hydrodynamics, combustion dynamics, chamber dynamics, and overall engineering model structure, and presents the governing equations in each of the technical areas. This is followed by the program user's guide, which is a complete description of the structure and operation of the computerized model. Last, appendices provide an alphabetized FORTRAN symbol table, detailed program logic diagrams, computer code listings, and sample case input and output data listings
Diluted Graphene Antiferromagnet
We study RKKY interactions between local magnetic moments for both doped and
undoped graphene. We find in both cases that the interactions are primarily
ferromagnetic for moments on the same sublattice, and antiferromagnetic for
moments on opposite sublattices. This suggests that at sufficiently low
temperatures dilute magnetic moments embedded in graphene can order into a
state analogous to that of a dilute antiferromagnet. We find that in the
undoped case one expects no net magnetic moment, and demonstrate numerically
that this effect generalizes to ribbons where the magnetic response is
strongest at the edge, suggesting the possibility of an unusual spin-transfer
device. For doped graphene we find that moments at definite lattice sites
interact over longer distances than those placed in interstitial sites of the
lattice ( vs. ) because the former support a Kohn anomaly that is
suppressed in the latter due to the absence of backscattering.Comment: 5 pages, two figures include
Strongly inhibited transport of a 1D Bose gas in a lattice
We report the observation of strongly damped dipole oscillations of a quantum
degenerate 1D atomic Bose gas in a combined harmonic and optical lattice
potential. Damping is significant for very shallow axial lattices (0.25 photon
recoil energies), and increases dramatically with increasing lattice depth,
such that the gas becomes nearly immobile for times an order of magnitude
longer than the single-particle tunneling time. Surprisingly, we see no
broadening of the atomic quasimomentum distribution after damped motion. Recent
theoretical work suggests that quantum fluctuations can strongly damp dipole
oscillations of 1D atomic Bose gas, providing a possible explanation for our
observations.Comment: 5 pages, 4 figure
Optimierung von p GaP Halbleitermaterialien zur photoelektrochemischen Wasserspaltung Optimization of p GaP semiconductor materials for photoelectrochemical water splitting
Local density of states of electron-crystal phases in graphene in the quantum Hall regime
We calculate, within a self-consistent Hartree-Fock approximation, the local
density of states for different electron crystals in graphene subject to a
strong magnetic field. We investigate both the Wigner crystal and bubble
crystals with M_e electrons per lattice site. The total density of states
consists of several pronounced peaks, the number of which in the negative
energy range coincides with the number of electrons M_e per lattice site, as
for the case of electron-solid phases in the conventional two-dimensional
electron gas. Analyzing the local density of states at the peak energies, we
find particular scaling properties of the density patterns if one fixes the
ratio nu_N/M_e between the filling factor nu_N of the last partially filled
Landau level and the number of electrons per bubble. Although the total density
profile depends explicitly on M_e, the local density of states of the lowest
peaks turns out to be identical regardless the number of electrons M_e. Whereas
these electron-solid phases are reminiscent to those expected in the
conventional two-dimensional electron gas in GaAs heterostructures in the
quantum Hall regime, the local density of states and the scaling relations we
highlight in this paper may be, in graphene, directly measured by spectroscopic
means, such as e.g. scanning tunneling microscopy.Comment: 8 pages, 7 figures; minor correction
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