4,284 research outputs found
Probing microscopic models for system-bath interactions via parametric driving
We show that strong parametric driving of a quantum harmonic oscillator
coupled to a thermal bath allows one to distinguish between different
microscopic models for the oscillator-bath coupling. We consider a bath with an
Ohmic spectral density and a model where the system-bath interaction can be
tuned continuously between position and momentum coupling via the coupling
angle . We derive a master equation for the reduced density operator of
the oscillator in Born-Markov approximation and investigate its quasi-steady
state as a function of the driving parameters, the temperature of the bath and
the coupling angle . We find that the time-averaged variance of
position and momentum exhibits a strong dependence on these parameters. In
particular, we identify parameter regimes that maximise the -dependence
and provide an intuitive explanation of our results.Comment: 13 pages, 8 figure
Optical control of the current-voltage relation in stacked superconductors
We simulate the current-voltage relation of short layered superconductors,
which we model as stacks of capacitively coupled Josephson junctions. The
system is driven by external laser fields, in order to optically control the
voltage drop across the junction. We identify parameter regimes in which
supercurrents can be stabilised against thermally induced phase slips, thus
reducing the effective voltage across the superconductor. Furthermore, single
driven Josephson junctions are known to exhibit phase-locked states, where the
superconducting phase is locked to the driving field. We numerically observe
their persistence in the presence of thermal fluctuations and capacitive
coupling between adjacent Josephson junctions. Our results indicate how
macroscopic material properties can be manipulated by exploiting the large
optical nonlinearities of Josephson plasmons.Comment: 7 pages, 7 figure
Terahertz field control of interlayer transport modes in cuprate superconductors
We theoretically show that terahertz pulses with controlled amplitude and
frequency can be used to switch between stable transport modes in layered
superconductors, modelled as stacks of Josephson junctions. We find pulse
shapes that deterministically switch the transport mode between
superconducting, resistive and solitonic states. We develop a simple model that
explains the switching mechanism as a destablization of the centre of mass
excitation of the Josephson phase, made possible by the highly non-linear
nature of the light-matter coupling
Lattice Universes in 2+1-dimensional gravity
Lattice universes are spatially closed space-times of spherical topology in
the large, containing masses or black holes arranged in the symmetry of a
regular polygon or polytope. Exact solutions for such spacetimes are found in
2+1 dimensions for Einstein gravity with a non-positive cosmological constant.
By means of a mapping that preserves the essential nature of geodesics we
establish analogies between the flat and the negative curvature cases. This map
also allows treatment of point particles and black holes on a similar footing.Comment: 14 pages 7 figures, to appear in Festschrift for Vince Moncrief (CQG
Quantum Electrodynamics near a Dielectric Half-space
We determine the photon propagator in the presence of a non-dispersive
dielectric half-space and use it to calculate the self-energy of an electron
near a dielectric surface
Acupuncture randomized trials (ART) in patients with chronic low back pain and osteoarthritis of the knee - Design and protocols
Background: We report on the study design and protocols of two randomized controlled trials (Acupuncture Randomized Trials = ART) that investigate the efficacy of acupuncture in the treatment of chronic low back pain and osteoarthritis of the knee, respectively. Objective: To investigate whether acupuncture is more efficacious than (a) no treatment or (b) minimal acupuncture in the treatment of low back pain and osteoarthritis. Design: Two randomized, controlled, multicenter trials with three treatment arms and a total follow-up time of 52 weeks. Setting: 30 practitioners and outpatient units in Germany specialized in acupuncture treatment. Patients: 300 patients will be included in each study. In the low back pain trial, patients will be included according to clinical diagnosis. In the osteoarthritis pain trial, patients will be included according to the American College of Rheumatology criteria. Interventions: Patients are randomly assigned to receive either (1) semi-standardized acupuncture (150 patients), (2) minimal acupuncture at non-acupuncture points (75 patients), or (3) no treatment for two months followed by semi-standardized acupuncture (75 patients, waiting list control). Acupuncture treatment consists of 12 sessions per patient over a period of 8 weeks. Main Outcome Measure: The main outcome measure is the difference between baseline and the end of the 8-week treatment period in the following parameters: pain intensity as measured by a visual analogue scale (VAS; 0-100 mm) in the low back pain trial and by the Western Ontario and McMaster Universities Osteoarthritis Score (WOMAC) in the osteoarthritis trial. Outlook: The results of these two studies (available in 2004) will provide health care providers and policy makers with the information needed to make scientifically sound assessments of acupuncture therapy
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