184 research outputs found
Observation of Quantum Hall Valley Skyrmions
We report measurements of the interaction-induced quantum Hall effect in a
spin-polarized AlAs two-dimensional electron system where the electrons occupy
two in-plane conduction band valleys. Via the application of in-plane strain,
we tune the energies of these valleys and measure the energy gap of the quantum
Hall state at filling factor = 1. The gap has a finite value even at zero
strain and, with strain, rises much faster than expected from a single-particle
picture, suggesting that the lowest energy charged excitations at are
"valley Skyrmions".Comment: 4 pages, 3 figure
Energy lowering of current-carrying single-particle states in open-shell atoms due to an exchange-correlation vector potential
Current-density-functional theory is used to perturbatively calculate
single-particle energies of open-shell atoms prepared in a current-carrying
state. We focus on the highest occupied such energy, because its negative is,
in principle, the exact ionization energy. A variety of different density
functionals and calculational schemes are compared with each other and
experiment. When the atom is prepared in a current-carrying state, a
current-dependent exchange-correlation functional is found to slightly lower
the single-particle energy of the current-carrying orbital, as compared to a
calculation using standard (current independent) density functionals for the
same system. The current-dependent terms in the exchange-correlation functional
thus provide additional stabilization of the current-carrying state.Comment: 13 pages, accepted by Int. J. Quantum Che
Condensate fluctuations of a trapped, ideal Bose gas
For a non-self-interacting Bose gas with a fixed, large number of particles
confined to a trap, as the ground state occupation becomes macroscopic, the
condensate number fluctuations remain micrscopic. However, this is the only
significant aspect in which the grand canonical description differs from
canonical or microcanonical in the thermodynamic limit. General arguments and
estimates including some vanishingly small quantities are compared to explicit,
fixed-number calculations for 10^2 to 10^6 particles.Comment: 16 pages (REVTeX) plus 4 figures (ps), revision includes brief
comparison of repulsive-interaction vs. fixed-N fluctuation damping. To be
published in Phys. Rev.
SU(N) Quantum Hall Skyrmions
We have investigated skyrmions in N-component quantum Hall systems. We find
that SU(N) skyrmions are the lowest energy charged excitations for filling
factors \nu = 1,2,...,N-1 for small enough symmetry breaking terms. N>2
skyrmions can be realized in Si QH systems based on the (110) or (111)
interfaces of Si, or perhaps in Si (100) systems, where the spin and valley
isospin together provide an SU(4)-symmetry, or in multilayer QH systems. We
also present Hartree-Fock results for a phenomenological easy-axis
SU(2)-breaking model appropriate to valley degeneracy.Comment: 5 pages, 2 figure
Superconductivity of Quasi-One-Dimensional Electrons in Strong Magnetic Field
The superconductivity of quasi-one-dimensional electrons in the magnetic
field is studied. The system is described as the one-dimensional electrons with
no frustration due to the magnetic field. The interaction is assumed to be
attractive between electrons in the nearest chains, which corresponds to the
lines of nodes of the energy gap in the absence of the magnetic field. The
effective interaction depends on the magnetic field and the transverse
momentum. As the magnetic field becomes strong, the transition temperature of
the spin-triplet superconductivity oscillates, while that of the spin-singlet
increases monotonically.Comment: 15 pages, RevTeX, 3 PostScript figures in uuencoded compressed tar
file are appende
Two-dimensional limit of exchange-correlation energy functional approximations in density functional theory
We investigate the behavior of three-dimensional (3D) exchange-correlation
energy functional approximations of density functional theory in anisotropic
systems with two-dimensional (2D) character. Using two simple models, quasi-2D
electron gas and two-electron quantum dot, we show a {\it fundamental
limitation} of the local density approximation (LDA), and its semi-local
extensions, generalized gradient approximation (GGA) and meta-GGA (MGGA), the
most widely used forms of which are worse than the LDA in the strong 2D limit.
The origin of these shortcomings is in the inability of the local (LDA) and
semi-local (GGA/MGGA) approximations to describe systems with 2D character in
which the nature of the exchange-correlation hole is very nonlocal. Nonlocal
functionals provide an alternative approach, and explicitly the average density
approximation (ADA) is shown to be remarkably accurate for the quasi-2D
electron gas system. Our study is not only relevant for understanding of the
functionals but also practical applications to semiconductor quantum structures
and materials such as graphite and metal surfaces. We also comment on the
implication of our findings to the practical device simulations based on the
(semi-)local density functional method.Comment: 21 pages including 9 figures, to be published in Phys. Rev.
Anomalous Behavior of the Upper Critical Field in Extreme Type-II Superconductors at Low Temperatures
We present a detailed numerical calculation of the upper critical field
for a bulk extreme type-II superconductor. Particular emphasis is
placed on the high-field, low-temperature regime of the HT-phase diagram. In
this regime it is necessary to go beyond the standard semi-classical theory and
include the effects of Landau quantization of the electronic motion on the
superconducting state. The presence of Landau level quantization induces an
upward curvature in at of for those
superconducting systems in which the slope of at is 0.2 Tesla/Kelvin. We construct a simple analytical model that can account for
this behavior based on the renormalization of the BCS coupling constant by the
off-diagonal pairing of electrons on Landau levels.Comment: 13 pages, Revtex, 3 ps figures, to appear in Physica
Upper Critical Field in a Spin-Charge Separated Superconductor
It is demonstrated that the spatial decay of the pair propagator in a
Luttinger liquid with spin charge separation contains a logarithmic correction
relative to the free fermi gas result in a finite interval between the spin and
charge thermal lengths. It is argued that similar effects can be expected in
higher dimensional systems with spin charge separation and that the temperature
dependence of the upper critical field curve is a probe of this
effect.Comment: 3 pages, postscript file (compressed and uuencoded
Valley degeneracy in biaxially strained aluminum arsenide quantum wells
This paper details a complete formalism for calculating electron subband
energy and degeneracy in strained multi-valley quantum wells grown along any
orientation with explicit results for the AlAs quantum well case. A
standardized rotation matrix is defined to transform from the conventional-
cubic-cell basis to the quantum-well-transport basis whereby effective mass
tensors, valley vectors, strain matrices, anisotropic strain ratios, and
scattering vectors are all defined in their respective bases. The specific
cases of (001)-, (110)-, and (111)-oriented aluminum arsenide (AlAs) quantum
wells are examined, as is the unconventional (411) facet, which is of
particular importance in AlAs literature. Calculations of electron confinement
and strain in the (001), (110), and (411) facets determine the critical well
width for crossover from double- to single-valley degeneracy in each system.
The notation is generalized to include miscut angles, and can be adapted to
other multi-valley systems. To help classify anisotropic inter-valley
scattering events, a new primitive unit cell is defined in momentum space which
allows one to distinguish purely in-plane inter-valley scattering events from
those that requires an out-of-plane momentum scattering component.Comment: 17 pages, 4 figures, 2 table
Mixed-State Quasiparticle Spectrum for d-wave Superconductors
Controversy concerning the pairing symmetry of high- materials has
motivated an interest in those measurable properties of superconductors for
which qualitative differences exist between the s-wave and d-wave cases. We
report on a comparison between the microscopic electronic properties of d-wave
and s-wave superconductors in the mixed state. Our study is based on
self-consistent numerical solutions of the mean-field Bogoliubov-de Gennes
equations for phenomenological BCS models which have s-wave and d-wave
condensates in the absence of a magnetic field. We discuss differences between
the s-wave and the d-wave local density-of-states, both near and away from
vortex cores. Experimental implications for both scanning-tunneling-microscopy
measurements and specific heat measurements are discussed.Comment: 10 pages, REVTEX3.0, 3 figures available upon reques
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