18,363 research outputs found
Transport into the south polar vortex in early spring
Estimates of the mean circulation and diffusive transport of ozone and other species into the Antarctic polar vortex during the spring of 1987 are made using data from the Airborne Antarctic Ozone Experiment. Measurements of long-lived tracers of tropospheric origin remained relatively constant at the levels of the maximum rate of decline of ozone during September. At lower levels in the stratosphere some evidence exists to support intrusions of tropospheric or low latitude air. Given the distribution in latitude and height of these tracers measured from the ER-2 aircraft, it can be inferred that the Lagrangian or diabatic mean circulation was zero or downward over Antarctica during the period of the ozone decline. The observation of a decline in ozone therefore requires a photochemical sink for ozone. The magnitude of the required photochemical sink must be sufficient to offset the transport of ozone into the polar region and produce the observed decline. Quasi-isentropic mixing and downward motion are coupled and are difficult to estimate from a single tracer. The full suite of measured tracers and auxiliary information are brought together to provide an estimate of the rate at which air is cycled through the polar vortex during spring. Estimates of large scale transport of potential vorticity and ozone from previous years are generally consistent with the data from the airborne experiment in suggesting a relatively slow rate of mass flow through the polar vortex in the lower stratosphere during September
Statistics of lowest excitations in two dimensional Gaussian spin glasses
A detailed investigation of lowest excitations in two-dimensional Gaussian
spin glasses is presented. We show the existence of a new zero-temperature
exponent lambda describing the relative number of finite-volume excitations
with respect to large-scale ones. This exponent yields the standard thermal
exponent of droplet theory theta through the relation, theta=d(lambda-1). Our
work provides a new way to measure the thermal exponent theta without any
assumption about the procedure to generate typical low-lying excitations. We
find clear evidence that theta < theta_{DW} where theta_{DW} is the thermal
exponent obtained in domain-wall theory showing that MacMillan excitations are
not typical.Comment: 4 pages, 3 figures, (v2) revised version, (v3) corrected typo
From ferromagnetism to spin-density wave: Magnetism in the two channel periodic Anderson model
The magnetic properties of the two-channel periodic Anderson model for
uranium ions, comprised of a quadrupolar and a magnetic doublet are
investigated through the crossover from the mixed-valent to the stable moment
regime using dynamical mean field theory. In the mixed-valent regime
ferromagnetism is found for low carrier concentration on a hyper-cubic lattice.
The Kondo regime is governed by band magnetism with small effective moments and
an ordering vector \q close to the perfect nesting vector. In the stable
moment regime nearest neighbour anti-ferromagnetism dominates for less than
half band filling and a spin density wave transition for larger than half
filling. is governed by the renormalized RKKY energy scale \mu_{eff}^2
^2 J^2\rho_0(\mu).Comment: 4 pages, RevTeX, 3 eps figure
Controlling decoherence of a two-level-atom in a lossy cavity
By use of external periodic driving sources, we demonstrate the possibility
of controlling the coherent as well as the decoherent dynamics of a two-level
atom placed in a lossy cavity.
The control of the coherent dynamics is elucidated for the phenomenon of
coherent destruction of tunneling (CDT), i.e., the coherent dynamics of a
driven two-level atom in a quantum superposition state can be brought
practically to a complete standstill. We study this phenomenon for different
initial preparations of the two-level atom. We then proceed to investigate the
decoherence originating from the interaction of the two-level atom with a lossy
cavity mode. The loss mechanism is described in terms of a microscopic model
that couples the cavity mode to a bath of harmonic field modes. A suitably
tuned external cw-laser field applied to the two-level atom slows down
considerably the decoherence of the atom. We demonstrate the suppression of
decoherence for two opposite initial preparations of the atomic state: a
quantum superposition state as well as the ground state. These findings can be
used to the effect of a proficient battling of decoherence in qubit
manipulation processes.Comment: 12 pages including 3 figures, submitted for publicatio
Networks of nonlinear superconducting transmission line resonators
We investigate a network of coupled superconducting transmission line
resonators, each of them made nonlinear with a capacitively shunted Josephson
junction coupling to the odd flux modes of the resonator. The resulting
eigenmode spectrum shows anticrossings between the plasma mode of the shunted
junction and the odd resonator modes. Notably, we find that the combined device
can inherit the complete nonlinearity of the junction, allowing for a
description as a harmonic oscillator with a Kerr nonlinearity. Using a dc SQUID
instead of a single junction, the nonlinearity can be tuned between 10 kHz and
4 MHz while maintaining resonance frequencies of a few gigahertz for realistic
device parameters. An array of such nonlinear resonators can be considered a
scalable superconducting quantum simulator for a Bose-Hubbard Hamiltonian. The
device would be capable of accessing the strongly correlated regime and be
particularly well suited for investigating quantum many-body dynamics of
interacting particles under the influence of drive and dissipation.Comment: 18 pages, 3 figure
Systematic and Causal Corrections to the Coherent Potential Approximation
The Dynamical Cluster Approximation (DCA) is modified to include disorder.
The DCA incorporates non-local corrections to local approximations such as the
Coherent Potential Approximation (CPA) by mapping the lattice problem with
disorder, and in the thermodynamic limit, to a self-consistently embedded
finite-sized cluster problem. It satisfies all of the characteristics of a
successful cluster approximation. It is causal, preserves the point-group and
translational symmetry of the original lattice, recovers the CPA when the
cluster size equals one, and becomes exact as . We use the DCA to
study the Anderson model with binary diagonal disorder. It restores sharp
features and band tailing in the density of states which reflect correlations
in the local environment of each site. While the DCA does not describe the
localization transition, it does describe precursor effects of localization.Comment: 11 pages, LaTeX, and 11 PS figures, to appear in Phys. Rev. B.
Revised version with typos corrected and references adde
A new method for analyzing ground-state landscapes: ballistic search
A ``ballistic-search'' algorithm is presented which allows the identification
of clusters (or funnels) of ground states in Ising spin glasses even for
moderate system sizes. The clusters are defined to be sets of states, which are
connected in state-space by chains of zero-energy flips of spins. The technique
can also be used to estimate the sizes of such clusters. The performance of the
method is tested with respect to different system sizes and choices of
parameters. As an application the ground-state funnel structure of
two-dimensional +or- J spin glasses of systems up to size L=20 is analyzed by
calculating a huge number of ground states per realization. A T=0 entropy per
spin of s_0=0.086(4)k_B is obtained.Comment: 10 pages, 11 figures, 35 references, revte
Arrays of waveguide-coupled optical cavities that interact strongly with atoms
We describe a realistic scheme for coupling atoms or other quantum emitters
with an array of coupled optical cavities. We consider open Fabry-Perot
microcavities coupled to the emitters. Our central innovation is to connect the
microcavities to waveguide resonators, which are in turn evanescently coupled
to each other on a photonic chip to form a coupled cavity chain. In this paper,
we describe the components, their technical limitations and the factors that
need to be determined experimentally. This provides the basis for a detailed
theoretical analysis of two possible experiments to realize quantum squeezing
and controlled quantum dynamics. We close with an outline of more advanced
applications.Comment: 30 pages, 8 figures. Submitted to New Journal of Physic
Berry's phase for large spins in external fields
It is shown that even for large spins the fundamental difference between
integer and half-integer spins persists. In a quasi-classical description this
difference enters via Berry's connection. This general phenomenon is derived
and illustrated for large spins confined to a plane by crystalline electric
fields. Physical realizations are rare-earth Nickel Borocarbides. Magnetic
moments for half-integer spin
(Dy, ) and magnetic susceptibilities for integer spin
(Ho, ) are calculated. Experiments are proposed to furnish evidence
for the predicted fundamental difference.Comment: 4 pages RevTe
Scaling of stiffness energy for 3d +/-J Ising spin glasses
Large numbers of ground states of 3d EA Ising spin glasses are calculated for
sizes up to 10^3 using a combination of a genetic algorithm and Cluster-Exact
Approximation. A detailed analysis shows that true ground states are obtained.
The ground state stiffness (or domain wall) energy D is calculated. A D ~ L^t
behavior with t=0.19(2) is found which strongly indicates that the 3d model has
an equilibrium spin-glass-paramagnet transition for non-zero T_c.Comment: 4 pages, 4 figure
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