6,243 research outputs found
Sympathetic cooling of He ions in a radiofrequency trap
We have generated Coulomb crystals of ultracold He ions in a linear
radiofrequency trap, by sympathetic cooling via laser--cooled Be.
Stable crystals containing up to 150 localized He ions at 20 mK were
obtained. Ensembles or single ultracold He ions open up interesting
perspectives for performing precision tests of QED and measurements of nuclear
radii. The present work also indicates the feasibility of cooling and
crystallizing highly charged atomic ions using Be as coolant.Comment: 4 pages, 2 figure
Improvement for Quenched Wilson Fermions
We briefly describe some of our recent results for the mass spectrum and
matrix elements using improved fermions for quenched QCD. Where possible
a comparison is made between improved and Wilson fermions.Comment: 6 pages, Latex, 11 figures, epsf.sty and buckow1.sty needed
(buckow1.sty included). Talk presented at the 31st Ahrenshoop Symposium on
the Theory of Elementary Particles, September 1997, Buckow, German
Coherent control of correlated nanodevices: A hybrid time-dependent numerical renormalization-group approach to periodic switching
The time-dependent numerical renormalization-group approach (TD-NRG),
originally devised for tracking the real-time dynamics of quantum-impurity
systems following a single quantum quench, is extended to multiple switching
events. This generalization of the TD-NRG encompasses the possibility of
periodic switching, allowing for coherent control of strongly correlated
systems by an external time-dependent field. To this end, we have embedded the
TD-NRG in a hybrid framework that combines the outstanding capabilities of the
numerical renormalization group to systematically construct the effective
low-energy Hamiltonian of the system with the prowess of complementary
approaches for calculating the real-time dynamics derived from this
Hamiltonian. We demonstrate the power of our approach by hybridizing the TD-NRG
with the Chebyshev expansion technique in order to investigate periodic
switching in the interacting resonant-level model. Although the interacting
model shares the same low-energy fixed point as its noninteracting counterpart,
we surprisingly find the gradual emergence of damped oscillations as the
interaction strength is increased. Focusing on a single quantum quench and
using a strong-coupling analysis, we reveal the origin of these
interaction-induced oscillations and provide an analytical estimate for their
frequency. The latter agrees well with the numerical results.Comment: 20 pager, Revtex, 10 figures, submitted to Physical Review
The temperature dependent bandstructure of a ferromagnetic semiconductor film
The electronic quasiparticle spectrum of a ferromagnetic film is investigated
within the framework of the s-f model. Starting from the exact solvable case of
a single electron in an otherwise empty conduction band being exchange coupled
to a ferromagnetically saturated localized spin system we extend the theory to
finite temperatures. Our approach is a moment-conserving decoupling procedure
for suitable defined Green functions. The theory for finite temperatures
evolves continuously from the exact limiting case. The restriction to zero
conduction band occupation may be regarded as a proper model description for
ferromagnetic semiconductors like EuO and EuS. Evaluating the theory for a
simple cubic film cut parallel to the (100) crystal plane, we find some marked
correlation effects which depend on the spin of the test electron, on the
exchange coupling, and on the temperature of the local-moment system.Comment: 11 pages, 9 figure
Kondo-lattice model: Application to the temperature-dependent electronic structure of EuO(100) films
We present calculations for the temperature-dependent electronic structure
and magnetic properties of thin ferromagnetic EuO films. The treatment is based
on a combination of a multiband-Kondo lattice model with first-principles
TB-LMTO band structure calculations. The method avoids the problem of
double-counting of relevant interactions and takes into account the correct
symmetry of the atomic orbitals. We discuss the temperature-dependent
electronic structures of EuO(100) films in terms of quasiparticle densities of
states and quasiparticle band structures. The Curie temperature T_C of the EuO
films turns out to be strongly thickness-dependent, starting from a very low
value = 15K for the monolayer and reaching the bulk value at about 25 layers
Novel High-Frequency Air Transducers
Ultrasonic transducers operating in air in the frequency range of 1–10MHz have major applications in robotics and nondestructive evaluation. For robotics applications, high-frequency air transducers make possible range measurements with a resolution in the 30–100 μm range. For nondestructive evaluation, it is possible to make transmission C-scan systems operating in air for the inspection of composites, green ceramics, and even metals at elevated temperatures. In this work, we report on the use of ligneous materials as a matching layer for PZT-based transducers
Feedback Control of Flight Speed to Reduce Unmanned Aerial System Noise
The aim of this initial study is to incorporate an acoustic metric into the flight control system of an unmanned aerial vehicle. This could be used to mitigate the noise impact of unmanned aerial systems operating near residential communities. To incorporate an acoustic metric into a pre-existing flight control system, two things are required: a source noise model, and an acoustic controller. An acoustic model was developed based on Gutin's work to estimate propeller noise. The flight control system was augmented with a controller to reduce propeller noise using feedback control of the commanded flight speed until an acoustic target was met. This control approach focuses on modifying flight speed only, with no perturbation to the trajectory. Multiple flight simulations were performed and the results showed that integrating an acoustic metric into the flight control system of an unmanned aerial system is possible
Nucleon structure in terms of OPE with non-perturbative Wilson coefficients
Lattice calculations could boost our understanding of Deep Inelastic
Scattering by evaluating moments of the Nucleon Structure Functions. To this
end we study the product of electromagnetic currents between quark states. The
Operator Product Expansion (OPE) decomposes it into matrix elements of local
operators (depending on the quark momenta) and Wilson coefficients (as
functions of the larger photon momenta). For consistency with the matrix
elements, we evaluate a set of Wilson coefficients non-perturbatively, based on
propagators for numerous momentum sources, on a 24^3 x 48 lattice. The use of
overlap quarks suppresses unwanted operator mixing and lattice artifacts.
Results for the leading Wilson coefficients are extracted by means of Singular
Value Decomposition.Comment: 7 pages, 3 figures, contribution to the XXVI International Symposium
on Lattice Field Theory, July 14-19 Williamsburg, Virginia, US
Multichannel Kondo Screening in a One-Dimensional Correlated Electron System
We present the exact Bethe Ansatz solution of a multichannel model of one-
dimensional correlated electrons coupled antiferromagnetically to a magnetic
impurity of arbitrary spin S. The solution reveals that interactions in the
bulk make the magnetic impurity drive both spin and charge fluctuations,
producing a mixed valence at the impurity site, with an associated effective
spin S_eff > S in the presence of a magnetic field. The screening of the
impurity spin is controlled by its size independently of the number of
channels, in contrast to the multichannel Kondo effect for free electrons.Comment: 5 pages Revtex. Final revised version to appear in Europhys. Let
Observation of twin beam correlations and quadrature entanglement by frequency doubling in a two-port resonator
We demonstrate production of quantum correlated and entangled beams by second
harmonic generation in a nonlinear resonator with two output ports. The output
beams at wavelength 428.5 nm exhibit 0.9 dB of nonclassical intensity
correlations and 0.3 dB of entanglement.Comment: 5 pages, 7 figure
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