1,213 research outputs found
Fish schooling as a basis for vertical axis wind turbine farm design
Most wind farms consist of horizontal axis wind turbines (HAWTs) due to the
high power coefficient (mechanical power output divided by the power of the
free-stream air through the turbine cross-sectional area) of an isolated
turbine. However when in close proximity to neighbouring turbines, HAWTs suffer
from a reduced power coefficient. In contrast, previous research on vertical
axis wind turbines (VAWTs) suggests that closely-spaced VAWTs may experience
only small decreases (or even increases) in an individual turbine's power
coefficient when placed in close proximity to neighbours, thus yielding much
higher power outputs for a given area of land. A potential flow model of
inter-VAWT interactions is developed to investigate the effect of changes in
VAWT spatial arrangement on the array performance coefficient, which compares
the expected average power coefficient of turbines in an array to a
spatially-isolated turbine. A geometric arrangement based on the configuration
of shed vortices in the wake of schooling fish is shown to significantly
increase the array performance coefficient based upon an array of 16x16 wind
turbines. Results suggest increases in power output of over one order of
magnitude for a given area of land as compared to HAWTs.Comment: Submitted for publication in BioInspiration and Biomimetics. Note:
The technology described in this paper is protected under both US and
international pending patents filed by the California Institute of Technolog
Impurity state in Haldane gap for S=1 Heisenberg antiferromagnetic chain with bond doping
Using a new impurity density matrix renormalization group scheme, we
establish a reliable picture of how the low lying energy levels of a
Heisenberg antiferromagnetic chain change {\it quantitatively} upon bond
doping. A new impurity state gradually occurs in the Haldane gap as ,
while it appears only if with as . The
system is non-perturbative as . This explains the
appearance of a new state in the Haldane gap in a recent experiment on
YCaBaNiO [J.F. DiTusa, et al., Phys. Rev. Lett. 73 1857(1994)].Comment: 4 pages of uuencoded gzip'd postscrip
Sub-Riemannian Geometry and Time Optimal Control of Three Spin Systems: Quantum Gates and Coherence Transfer
Many coherence transfer experiments in Nuclear Magnetic Resonance
Spectroscopy, involving network of coupled spins, use temporary spin-decoupling
to produce desired effective Hamiltonians. In this paper, we show that
significant time can be saved in producing an effective Hamiltonian, if
spin-decoupling is avoided. We provide time optimal pulse sequences for
producing an important class of effective Hamiltonians in three spin networks.
These effective Hamiltonians are useful for coherence transfer experiments and
implementation of quantum logic gates in NMR quantum computing. It is
demonstrated that computing these time optimal pulse sequences can be reduced
to geometric problems that involve computing sub-Riemannian geodesics on
Homogeneous spaces
Nonlocal description of X waves in quadratic nonlinear materials
We study localized light bullets and X waves in quadratic media and show how the notion of nonlocality can provide an alternative simple physical picture of both types of multidimensional nonlinear waves. For X waves we show that a local cascading limit in terms of a nonlinear Schrödinger equation does not exist—one needs
to use the nonlocal description, because the nonlocal response function does not converge toward a function.
Also, we use the nonlocal theory to show that the coupling to the second harmonic is able to generate an X shape in the fundamental field despite having anomalous dispersion, in contrast to the predictions of the cascading limit
Implementation of NMR quantum computation with para-hydrogen derived high purity quantum states
We demonstrate the first implementation of a quantum algorithm on a liquid
state nuclear magnetic resonance (NMR) quantum computer using almost pure
states. This was achieved using a two qubit device where the initial state is
an almost pure singlet nuclear spin state of a pair of 1H nuclei arising from a
chemical reaction involving para-hydrogen. We have implemented Deutsch's
algorithm for distinguishing between constant and balanced functions with a
single query.Comment: 7 pages RevTex including 6 figures. Figures 4-6 are low quality to
save space. Submitted to Phys Rev
Practical Implementations of Twirl Operations
Twirl operations, which convert impure singlet states into Werner states,
play an important role in many schemes for entanglement purification. In this
paper we describe strategies for implementing twirl operations, with an
emphasis on methods suitable for ensemble quantum information processors such
as nuclear magnetic resonance (NMR) quantum computers. We implement our twirl
operation on a general two-spin mixed state using liquid state NMR techniques,
demonstrating that we can obtain the singlet Werner state with high fidelity.Comment: 6 pages RevTex4 including 2 figures (fig 1 low quality to save space
Magnetization profiles and NMR spectra of doped Haldane chains at finite temperatures
Open segments of S=1 antiferromagnetic spin chains are studied at finite
temperatures and fields using continuous time Quantum Monte Carlo techniques.
By calculating the resulting magnetization profiles for a large range of chain
lengths with fixed field and temperature we reconstruct the experimentally
measured NMR spectrum of impurity doped YBaNiMgO. For
temperatures above the gap the calculated NMR spectra are in excellent
agreement with the experimental results, confirming the existence of
excitations at the end of open S=1 chain segments. At temperatures below the
gap, neglecting inter chain couplings, we still find well defined peaks in the
calculated NMR spectra corresponding to the chain end excitations. At
low temperatures, inter chain couplings could be important, resulting in a more
complicated phase.Comment: 7 pages, 5 figures, minor correction
Spin-Orbit-Induced Magnetic Anisotropy for Impurities in Metallic Samples I. Surface Anisotropy
Motivated by the recent measurements of Kondo resistivity in thin films and
wires, where the Kondo amplitude is suppressed for thinner samples, the surface
anisotropy for magnetic impurities is studied. That anisotropy is developed in
those cases where in addition to the exchange interaction with the impurity
there is strong spin-orbit interaction for conduction electrons around the
impurity in the ballistic region. The asymmetry in the neighborhood of the
magnetic impurity exhibits the anisotropy axis which, in the case of a
plane surface, is perpendicular to the surface. The anisotropy energy is
for spin , and the anisotropy constant is
inversionally proportional to distance measured from the surface and
. Thus at low temperature the spin is frozen in a singlet or doublet of
lowest energy. The influence of that anisotropy on the electrical resistivity
is the subject of the following paper (part II).Comment: 28 pages, RevTeX (using epsfig), 8 eps figures included, submitted to
PR
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