44,955 research outputs found
Edge excitations and Topological orders in rotating Bose gases
The edge excitations and related topological orders of correlated states of a
fast rotating Bose gas are studied. Using exact diagonalization of small
systems, we compute the energies and number of edge excitations, as well as the
boson occupancy near the edge for various states. The chiral Luttinger-liquid
theory of Wen is found to be a good description of the edges of the bosonic
Laughlin and other states identified as members of the principal Jain sequence
for bosons. However, we find that in a harmonic trap the edge of the state
identified as the Moore-Read (Pfaffian) state shows a number of anomalies. An
experimental way of detecting these correlated states is also discussed.Comment: Results extended to larger systems. Improved presentatio
Robustness of Fractional Quantum Hall States with Dipolar Atoms in Artificial Gauge Fields
The robustness of fractional quantum Hall states is measured as the energy
gap separating the Laughlin ground-state from excitations. Using thermodynamic
approximations for the correlation functions of the Laughlin state and the
quasihole state, we evaluate the gap in a two-dimensional system of dipolar
atoms exposed to an artificial gauge field. For Abelian fields, our results
agree well with the results of exact diagonalization for small systems, but
indicate that the large value of the gap predicted in [Phys. Rev. Lett. 94,
070404 (2005)] was overestimated. However, we are able to show that the small
gap found in the Abelian scenario is dramatically increased if we turn to
non-Abelian fields squeezing the Landau levels
Formulas for Continued Fractions. An Automated Guess and Prove Approach
We describe a simple method that produces automatically closed forms for the
coefficients of continued fractions expansions of a large number of special
functions. The function is specified by a non-linear differential equation and
initial conditions. This is used to generate the first few coefficients and
from there a conjectured formula. This formula is then proved automatically
thanks to a linear recurrence satisfied by some remainder terms. Extensive
experiments show that this simple approach and its straightforward
generalization to difference and -difference equations capture a large part
of the formulas in the literature on continued fractions.Comment: Maple worksheet attache
Dynamic nuclear polarisation in biased quantum wires with spin-orbit interaction
We propose a new method for dynamic nuclear polarisation in a quasi
one-dimensional quantum wire utilising the spin-orbit interaction, the
hyperfine interaction, and a finite source-drain potential difference. In
contrast with current methods, our scheme does not rely on external magnetic or
optical sources which makes independent control of closely placed devices much
more feasible. Using this method, a significant polarisation of a few per cent
is possible in currently available InAs wires which may be detected by
conductance measurements. This may prove useful for nuclear-magnetic-resonance
studies in nanoscale systems as well as in spin-based devices where external
magnetic and optical sources will not be suitable.Comment: 6 pages, published versio
Solitary Waves of Planar Ferromagnets and the Breakdown of the Spin-Polarized Quantum Hall Effect
A branch of uniformly-propagating solitary waves of planar ferromagnets is
identified. The energy dispersion and structures of the solitary waves are
determined for an isotropic ferromagnet as functions of a conserved momentum.
With increasing momentum, their structure undergoes a transition from a form
ressembling a droplet of spin-waves to a Skyrmion/anti-Skyrmion pair. An
instability to the formation of these solitary waves is shown to provide a
mechanism for the electric field-induced breakdown of the spin-polarized
quantum Hall effect.Comment: 5 pages, 3 eps-figures, revtex with epsf.tex and multicol.st
Exact and approximate dynamics of the quantum mechanical O(N) model
We study a quantum dynamical system of N, O(N) symmetric, nonlinear
oscillators as a toy model to investigate the systematics of a 1/N expansion.
The closed time path (CTP) formalism melded with an expansion in 1/N is used to
derive time evolution equations valid to order 1/N (next-to-leading order). The
effective potential is also obtained to this order and its properties
areelucidated. In order to compare theoretical predictions against numerical
solutions of the time-dependent Schrodinger equation, we consider two initial
conditions consistent with O(N) symmetry, one of them a quantum roll, the other
a wave packet initially to one side of the potential minimum, whose center has
all coordinates equal. For the case of the quantum roll we map out the domain
of validity of the large-N expansion. We discuss unitarity violation in the 1/N
expansion; a well-known problem faced by moment truncation techniques. The 1/N
results, both static and dynamic, are also compared to those given by the
Hartree variational ansatz at given values of N. We conclude that late-time
behavior, where nonlinear effects are significant, is not well-described by
either approximation.Comment: 16 pages, 12 figrures, revte
Inorganic Surface Passivation of PbS Nanocrystals resulting in Strong Photoluminescent Emission
Strong photoluminescent emission has been obtained from 3 nm PbS nanocrystals
in aqueous colloidal solution, following treatment with CdS precursors. The
observed emission can extend across the entire visible spectrum and usually
includes a peak near 1.95 eV. We show that much of the visible emission results
from absorption by higher-lying excited states above 3.0 eV with subsequent
relaxation to and emission from states lying above the observed band-edge of
the PbS nanocrystals. The fluorescent lifetimes for this emission are in the
nanosecond regime, characteristic of exciton recombination.Comment: Preprint, 23 pages, 6 figure
Oceanic stochastic parametrizations in a seasonal forecast system
We study the impact of three stochastic parametrizations in the ocean
component of a coupled model, on forecast reliability over seasonal timescales.
The relative impacts of these schemes upon the ocean mean state and ensemble
spread are analyzed. The oceanic variability induced by the atmospheric forcing
of the coupled system is, in most regions, the major source of ensemble spread.
The largest impact on spread and bias came from the Stochastically Perturbed
Parametrization Tendency (SPPT) scheme - which has proven particularly
effective in the atmosphere. The key regions affected are eddy-active regions,
namely the western boundary currents and the Southern Ocean. However, unlike
its impact in the atmosphere, SPPT in the ocean did not result in a significant
decrease in forecast error. Whilst there are good grounds for implementing
stochastic schemes in ocean models, our results suggest that they will have to
be more sophisticated. Some suggestions for next-generation stochastic schemes
are made.Comment: 24 pages, 3 figure
Oceanic stochastic parametrizations in a seasonal forecast system
We study the impact of three stochastic parametrizations in the ocean
component of a coupled model, on forecast reliability over seasonal timescales.
The relative impacts of these schemes upon the ocean mean state and ensemble
spread are analyzed. The oceanic variability induced by the atmospheric forcing
of the coupled system is, in most regions, the major source of ensemble spread.
The largest impact on spread and bias came from the Stochastically Perturbed
Parametrization Tendency (SPPT) scheme - which has proven particularly
effective in the atmosphere. The key regions affected are eddy-active regions,
namely the western boundary currents and the Southern Ocean. However, unlike
its impact in the atmosphere, SPPT in the ocean did not result in a significant
decrease in forecast error. Whilst there are good grounds for implementing
stochastic schemes in ocean models, our results suggest that they will have to
be more sophisticated. Some suggestions for next-generation stochastic schemes
are made.Comment: 24 pages, 3 figure
Effects of Disorder and Momentum Relaxation on the Intertube Transport of Incommensurate Carbon Nanotube Ropes and Multiwall Nanotubes
We study theoretically the electrical transport between aligned carbon
nanotubes in nanotube ropes, and between shells in multiwall carbon nanotubes.
We focus on transport between two metallic nanotubes (or shells) of different
chiralities with mismatched Fermi momenta and incommensurate periodicities. We
perform numerical calculations of the transport properties of such systems
within a tight-binding formalism. For clean (disorder-free) nanotubes the
intertube transport is strongly suppressed as a result of momentum
conservation. For clean nanotubes, the intertube transport is typically
dominated by the loss of momentum conservation at the contacts. We discuss in
detail the effects of disorder, which also breaks momentum conservation, and
calculate the effects of localised scatterers of various types. We show that
physically relevant disorder potentials lead to very dramatic enhancements of
the intertube conductance. We show that recent experimental measurements of the
intershell transport in multiwall nanotubes are consistent with our theoretical
results for a model of short-ranged correlated disorder.Comment: References adde
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