11,167 research outputs found
Dissipative quantum mechanics beyond Bloch-Redfield: A consistent weak-coupling expansion of the ohmic spin boson model at arbitrary bias
We study the time dynamics of the ohmic spin boson model at arbitrary bias
and small coupling to the bosonic bath. Using perturbation
theory and the real-time renormalization group (RG) method we present a
consistent zero-temperature weak-coupling expansion for the time evolution of
the reduced density matrix one order beyond the Bloch-Redfield solution. We
develop a renormalized perturbation theory and present an analytical solution
covering the whole range from small to large times, including further results
for exponentially small or large times. Resumming all secular terms in all
orders of perturbation theory we find exponential decay for all terms of the
time evolution. We determine the preexponential functions and find slowly
varying logarithmic terms with the renormalized Rabi frequency as
energy scale together with strongly varying parts falling off asymptocially as
in leading order, in contrast to the unbiased case. Resumming all
logarithmic terms in all orders of perturbation theory via real-time RG we find
the correct renormalized tunneling and a power-law behaviour for the
oscillating modes with exponent crossing over from for exponentially
small times to a bias-dependent value for
exponentially large times. Furthermore, we present a degenerate perturbation
theory to calculate consistently the purely decaying mode one order beyond
Bloch-Redfield.Comment: 27 pages, 2 figure
Hubs and Spokes, and Free Trade in the Americas
This paper compares two possible formats for free trade in the Americas: a system of spokes surrounding a U.S. hub, and a free trade area. The paper identifies the sources of welfare change, and it argues that a country's attitude towards a system depends on whether the arrangement is a complement or a substitute trading club. The paper argues also that rent-seeking activities, and costs of administration and transportation, are likely to be higher in a hub-and-spoke system than in a free trade area.
Isotope effects in the harmonic response from hydrogenlike muonic atoms in strong laser fields
High-harmonic generation from hydrogenlike muonic atoms exposed to
ultraintense high-frequency laser fields is calculated. Systems of low nuclear
charge number Z are considered where a nonrelativistic description applies. By
comparing the radiative response for different isotopes we demonstrate
characteristic signatures of the finite nuclear mass and size in the harmonic
spectra. In particular, for Z>1, an effective muon charge appears in the
Schr\"odinger equation for the relative particle motion, which influences the
position of the harmonic cutoff. Cutoff energies in the MeV domain can be
achieved, offering prospects for the generation of ultrashort coherent
gamma-ray pulses.Comment: 11 pages, 5 figure
Airline Crew Scheduling with Potts Neurons
A Potts feedback neural network approach for finding good solutions to
resource allocation problems with a non-fixed topology is presented. As a
target application the airline crew scheduling problem is chosen. The
topological complication is handled by means of a propagator defined in terms
of Potts neurons. The approach is tested on artificial random problems tuned to
resemble real-world conditions. Very good results are obtained for a variety of
problem sizes. The computer time demand for the approach only grows like
\mbox{(number of flights)}^3. A realistic problem typically is solved within
minutes, partly due to a prior reduction of the problem size, based on an
analysis of the local arrival/departure structure at the single airportsComment: 9 pages LaTeX, 3 postscript figures, uufiles forma
Fractal Threshold Behavior in Vacuum Gravitational Collapse
We present the numerical evidence for fractal threshold behavior in the five
dimensional vacuum Einstein equations satisfying the cohomogeneity-two triaxial
Bianchi type-IX ansatz. In other words, we show that a flip of the wings of a
butterfly may influence the process of the black hole formation.Comment: 4 pages, 6 figures, minor change
Transport through anisotropic magnetic molecules with partially ferromagnetic leads: Spin-charge conversion and negative differential conductance
We theoretically investigate inelastic transport through anisotropic magnetic
molecules weakly coupled to one ferromagnetic and one nonmagnetic lead. We find
that the current is suppressed over wide voltage ranges due to spin blockade.
In this system, spin blockade is associated with successive spin flips of the
molecular spins and depends on the anisotropy energy barrier. This leads to the
appearance of a window of bias voltages between the Coulomb blockade and spin
blockade regimes where the current is large and to negative differential
conductance at low temperatures. Remarkably, negative differential conductance
is also present close to room temperature. Spin-blockade behavior is
accompanied by super-Poissonian shot noise, like in nonmagnetic quantum dots.
Finally, we show that the charge transmitted through the molecule between
initial preparation in a certain spin state and infinite time very strongly
depends on the initial spin state in certain parameter ranges. Thus the
molecule can act as a spin-charge converter, an effect potentially useful as a
read-out mechanism for molecular spintronics.Comment: 8 pages with 5 figures, version as publishe
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