42,941 research outputs found
Energy transfer in finite-size exciton-phonon systems : confinement-enhanced quantum decoherence
Based on the operatorial formulation of the perturbation theory, the
exciton-phonon problem is revisited for investigating exciton-mediated energy
flow in a finite-size lattice. Within this method, the exciton-phonon
entanglement is taken into account through a dual dressing mechanism so that
exciton and phonons are treated on an equal footing. In a marked contrast with
what happens in an infinite lattice, it is shown that the dynamics of the
exciton density is governed by several time scales. The density evolves
coherently in the short-time limit whereas a relaxation mechanism occurs over
intermediated time scales. Consequently, in the long-time limit, the density
converges toward a nearly uniform distributed equilibrium distribution. Such a
behavior results from quantum decoherence that originates in the fact that the
phonons evolve differently depending on the path followed by the exciton to
tunnel along the lattice. Although the relaxation rate increases with the
temperature and with the coupling, it decreases with the lattice size,
suggesting that the decoherence is inherent to the confinement
Interface Width and Bulk Stability: requirements for the simulation of Deeply Quenched Liquid-Gas Systems
Simulations of liquid-gas systems with extended interfaces are observed to
fail to give accurate results for two reasons: the interface can get ``stuck''
on the lattice or a density overshoot develops around the interface. In the
first case the bulk densities can take a range of values, dependent on the
initial conditions. In the second case inaccurate bulk densities are found. In
this communication we derive the minimum interface width required for the
accurate simulation of liquid gas systems with a diffuse interface. We
demonstrate this criterion for lattice Boltzmann simulations of a van der Waals
gas. When combining this criterion with predictions for the bulk stability we
can predict the parameter range that leads to stable and accurate simulation
results. This allows us to identify parameter ranges leading to high density
ratios of over 1000. This is despite the fact that lattice Boltzmann
simulations of liquid-gas systems were believed to be restricted to modest
density ratios of less than 20.Comment: 5 pages, 3 figure
Symmetric Rotating Wave Approximation for the Generalized Single-Mode Spin-Boson System
The single-mode spin-boson model exhibits behavior not included in the
rotating wave approximation (RWA) in the ultra and deep-strong coupling
regimes, where counter-rotating contributions become important. We introduce a
symmetric rotating wave approximation that treats rotating and counter-rotating
terms equally, preserves the invariances of the Hamiltonian with respect to its
parameters, and reproduces several qualitative features of the spin-boson
spectrum not present in the original rotating wave approximation both
off-resonance and at deep strong coupling. The symmetric rotating wave
approximation allows for the treatment of certain ultra and deep-strong
coupling regimes with similar accuracy and mathematical simplicity as does the
RWA in the weak coupling regime. Additionally, we symmetrize the generalized
form of the rotating wave approximation to obtain the same qualitative
correspondence with the addition of improved quantitative agreement with the
exact numerical results. The method is readily extended to higher accuracy if
needed. Finally, we introduce the two-photon parity operator for the two-photon
Rabi Hamiltonian and obtain its generalized symmetric rotating wave
approximation. The existence of this operator reveals a parity symmetry similar
to that in the Rabi Hamiltonian as well as another symmetry that is unique to
the two-photon case, providing insight into the mathematical structure of the
two-photon spectrum, significantly simplifying the numerics, and revealing some
interesting dynamical properties.Comment: 11 pages, 5 figure
The evidence of quasi-free positronium state in GiPS-AMOC spectra of glycerol
We present the results of processing of Age-Momentum Correlation (AMOC)
spectra that were measured for glycerol by the Gamma-induced positron
spectroscopy (GiPS) facility. Our research has shown that the shape of
experimental s(t) curve cannot be explained without introduction of the
intermediate state of positronium (Ps), called quasi-free Ps. This state yields
the wide Doppler line near zero lifetimes. We discuss the possible properties
of this intermediate Ps state from the viewpoint of developed model. The amount
of annihilation events produced by quasi-free Ps is estimated to be less than
5% of total annihilations. In the proposed model, quasi-free Ps serves as a
precursor for trapped Ps of para- and ortho-states
A study of local and non-local spatial densities in quantum field theory
We use a one-dimensional model system to compare the predictions of two
different 'yardsticks' to compute the position of a particle from its quantum
field theoretical state. Based on the first yardstick (defined by the
Newton-Wigner position operator), the spatial density can be arbitrarily narrow
and its time-evolution is superluminal for short time intervals. Furthermore,
two spatially distant particles might be able to interact with each other
outside the light cone, which is manifested by an asymmetric spreading of the
spatial density. The second yardstick (defined by the quantum field operator)
does not permit localized states and the time evolution is subluminal.Comment: 29 pages, 3 figure
Hybrid Superconducting Neutron Detectors
A new neutron detection concept is presented that is based on superconductive
niobium (Nb) strips coated by a boron (B) layer. The working principle of the
detector relies on the nuclear reaction 10B+n + 7Li ,
with and Li ions generating a hot spot on the current-biased Nb strip
which in turn induces a superconducting-normal state transition. The latter is
recognized as a voltage signal which is the evidence of the incident neutron.
The above described detection principle has been experimentally assessed and
verified by irradiating the samples with a pulsed neutron beam at the ISIS
spallation neutron source (UK). It is found that the boron coated
superconducting strips, kept at a temperature T = 8 K and current-biased below
the critical current Ic, are driven into the normal state upon thermal neutron
irradiation. As a result of the transition, voltage pulses in excess of 40 mV
are measured while the bias current can be properly modulated to bring the
strip back to the superconducting state, thus resetting the detector.
Measurements on the counting rate of the device are presented and the future
perspectives leading to neutron detectors with unprecedented spatial
resolutions and efficiency are highlighted.Comment: 8 pages 6 figure
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