1,913 research outputs found
Antiferromagnetic noise correlations in optical lattices
We analyze how noise correlations probed by time-of-flight (TOF) experiments
reveal antiferromagnetic (AF) correlations of fermionic atoms in
two-dimensional (2D) and three-dimensional (3D) optical lattices. Combining
analytical and quantum Monte Carlo (QMC) calculations using experimentally
realistic parameters, we show that AF correlations can be detected for
temperatures above and below the critical temperature for AF ordering. It is
demonstrated that spin-resolved noise correlations yield important information
about the spin ordering. Finally, we show how to extract the spin correlation
length and the related critical exponent of the AF transition from the noise.Comment: 4 pages, 4 figure
Error tolerance in an NMR Implementation of Grover's Fixed-Point Quantum Search Algorithm
We describe an implementation of Grover's fixed-point quantum search
algorithm on a nuclear magnetic resonance (NMR) quantum computer, searching for
either one or two matching items in an unsorted database of four items. In this
new algorithm the target state (an equally weighted superposition of the
matching states) is a fixed point of the recursive search operator, and so the
algorithm always moves towards the desired state. The effects of systematic
errors in the implementation are briefly explored.Comment: 5 Pages RevTex4 including three figures. Changes made at request of
referees; now in press at Phys Rev
Structure of boson systems beyond the mean-field
We investigate systems of identical bosons with the focus on two-body
correlations. We use the hyperspherical adiabatic method and a decomposition of
the wave function in two-body amplitudes. An analytic parametrization is used
for the adiabatic effective radial potential. We discuss the structure of a
condensate for arbitrary scattering length. Stability and time scales for
various decay processes are estimated. The previously predicted Efimov-like
states are found to be very narrow. We discuss the validity conditions and
formal connections between the zero- and finite-range mean-field
approximations, Faddeev-Yakubovskii formulation, Jastrow ansatz, and the
present method. We compare numerical results from present work with mean-field
calculations and discuss qualitatively the connection with measurements.Comment: 26 pages, 6 figures, submitted to J. Phys. B. Ver. 2 is 28 pages with
modified figures and discussion
Correlated N-boson systems for arbitrary scattering length
We investigate systems of identical bosons with the focus on two-body
correlations and attractive finite-range potentials. We use a hyperspherical
adiabatic method and apply a Faddeev type of decomposition of the wave
function. We discuss the structure of a condensate as function of particle
number and scattering length. We establish universal scaling relations for the
critical effective radial potentials for distances where the average distance
between particle pairs is larger than the interaction range. The correlations
in the wave function restore the large distance mean-field behaviour with the
correct two-body interaction. We discuss various processes limiting the
stability of condensates. With correlations we confirm that macroscopic
tunneling dominates when the trap length is about half of the particle number
times the scattering length.Comment: 15 pages (RevTeX4), 11 figures (LaTeX), submitted to Phys. Rev. A.
Second version includes an explicit comparison to N=3, a restructured
manuscript, and updated figure
Latest results from the EU project AVATAR: aerodynamic modelling of 10 MW wind turbines
This paper presents the most recent results from the EU project AVATAR in which aerodynamic models are improved and validated for wind turbines on a scale of 10 MW and more. Measurements on a DU 00-W-212 airfoil are presented which have been taken in the pressurized DNW-HDG wind tunnel up to a Reynolds number of 15 Million. These measurements are compared with measurements in the LM wind tunnel for Reynolds numbers of 3 and 6 Million and with calculational results. In the analysis of results special attention is paid to high Reynolds numbers effects. CFD calculations on airfoil performance showed an unexpected large scatter which eventually was reduced by paying even more attention to grid independency and domain size in relation to grid topology. Moreover calculations are presented on flow devices (leading and trailing edge flaps and vortex generators). Finally results are shown between results from 3D rotor models where a comparison is made between results from vortex wake methods and BEM methods at yawed conditions
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
Theory of magnetoresistance in films of dilute magnetic alloys
Earlier a magnetic anisotropy for magnetic impurities nearby the surface of
non-magnetic host was proposed in order to explain the size dependence of the
Kondo effect in dilute magnetic alloys. Recently Giordano has measured the
magnetoresistance of dilute Au(Fe) films for different thicknesses well above
the Kondo temperature . In this way he verified the existence of that
anisotropy even for such a case where the Kondo effect is not dominating. For
detailed comparison of that suggestion with experiments, the magnetic field
dependence of the magnetoresistance is calculated in the lowest approximation,
thus in the second order of the exchange coupling. The strength of the
anisotropy is very close to earlier estimates deduced from the size dependence
of the Kondo resistivity amplitude.Comment: (11 pages, 8 figures, essential changes compared to the old version
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
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