185 research outputs found
Transport properties of clean and disordered Josephson junction arrays
We investigate the influence of quantum fluctuations and weak disorder on the
vortex dynamics in a two-dimensional superconducting
Berezinskii-Kosterlitz-Thouless system. The temperature below which quantum
fluctuations dominate the vortex creep is determined, and the transport in this
quantum regime is described. The crossover from quantum to classical regime is
discussed and the quantum correction to the classical current-voltage relation
is determined. It is found that weak disorder can effectively reduce the
critical current as compared to that in the clean system.Comment: 4 pages, 2 figure
Non-exponential relaxation and hierarchically constrained dynamics in a protein
A scaling analysis within a model of hierarchically constrained dynamics is
shown to reproduce the main features of non-exponential relaxation observed in
kinetic studies of carbonmonoxymyoglobin.Comment: 4 pages, 3 figures in text. Reference errors have been correcte
Nuclear-resonant electron scattering
We investigate nuclear-resonant electron scattering as occurring in the
two-step process of nuclear excitation by electron capture (NEEC) followed by
internal conversion. The nuclear excitation and decay are treated by a
phenomenological collective model in which nuclear states and transition
probabilities are described by experimental parameters. We present capture
rates and resonant strengths for a number of heavy ion collision systems
considering various scenarios for the resonant electron scattering process. The
results show that for certain cases resonant electron scattering can have
significantly larger resonance strengths than NEEC followed by the radiative
decay of the nucleus. We discuss the impact of our findings on the possible
experimental observation of NEEC.Comment: 24 pages, 2 plots, 5 table
The Order of Phase Transitions in Barrier Crossing
A spatially extended classical system with metastable states subject to weak
spatiotemporal noise can exhibit a transition in its activation behavior when
one or more external parameters are varied. Depending on the potential, the
transition can be first or second-order, but there exists no systematic theory
of the relation between the order of the transition and the shape of the
potential barrier. In this paper, we address that question in detail for a
general class of systems whose order parameter is describable by a classical
field that can vary both in space and time, and whose zero-noise dynamics are
governed by a smooth polynomial potential. We show that a quartic potential
barrier can only have second-order transitions, confirming an earlier
conjecture [1]. We then derive, through a combination of analytical and
numerical arguments, both necessary conditions and sufficient conditions to
have a first-order vs. a second-order transition in noise-induced activation
behavior, for a large class of systems with smooth polynomial potentials of
arbitrary order. We find in particular that the order of the transition is
especially sensitive to the potential behavior near the top of the barrier.Comment: 8 pages, 6 figures with extended introduction and discussion; version
accepted for publication by Phys. Rev.
Stochastic Approach to Enantiomeric Excess Amplification and Chiral Symmetry Breaking
Stochastic aspects of chemical reaction models related to the Soai reactions
as well as to the homochirality in life are studied analytically and
numerically by the use of the master equation and random walk model. For
systems with a recycling process, a unique final probability distribution is
obtained by means of detailed balance conditions. With a nonlinear
autocatalysis the distribution has a double-peak structure, indicating the
chiral symmetry breaking. This problem is further analyzed by examining
eigenvalues and eigenfunctions of the master equation. In the case without
recycling process, final probability distributions depend on the initial
conditions. In the nonlinear autocatalytic case, time-evolution starting from a
complete achiral state leads to a final distribution which differs from that
deduced from the nonzero recycling result. This is due to the absence of the
detailed balance, and a directed random walk model is shown to give the correct
final profile. When the nonlinear autocatalysis is sufficiently strong and the
initial state is achiral, the final probability distribution has a double-peak
structure, related to the enantiomeric excess amplification. It is argued that
with autocatalyses and a very small but nonzero spontaneous production, a
single mother scenario could be a main mechanism to produce the homochirality.Comment: 25 pages, 6 figure
Dissociation in a polymerization model of homochirality
A fully self-contained model of homochirality is presented that contains the
effects of both polymerization and dissociation. The dissociation fragments are
assumed to replenish the substrate from which new monomers can grow and undergo
new polymerization. The mean length of isotactic polymers is found to grow
slowly with the normalized total number of corresponding building blocks.
Alternatively, if one assumes that the dissociation fragments themselves can
polymerize further, then this corresponds to a strong source of short polymers,
and an unrealistically short average length of only 3. By contrast, without
dissociation, isotactic polymers becomes infinitely long.Comment: 16 pages, 6 figures, submitted to Orig. Life Evol. Biosp
Is the tetraneutron a bound dineutron-dineutron molecule?
In light of a new experiment which claims a positive identification, we
discuss the possible existence of the tetraneutron. We explore a novel model
based on a dineutron-dineutron molecule. We show that this model is not able to
explain the tetraneutron as a bound state, in agreement with other theoretical
models already discussed in the literature.Comment: 9 pages, 3 figures, J. Phys. G, in pres
First- and Second-Order Transitions between Quantum and Classical Regimes for the Escape Rate of a Spin System
We have found a novel feature of the bistable large-spin model described by
the Hamiltonian H = -DS_z^2 - H_xS_x.The crossover from thermal to quantum
regime for the escape rate can be either first (H_x<SD/2) or second
(SD/2<H_x<2SD) order, that is, sharp or smooth, depending on the strength of
the transverse field. This prediction can be tested experimentally in molecular
magnets like Mn_12Ac.Comment: 4 pages, 4 figure
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