1,243 research outputs found
Critical Exponents of the Statistical Multifragmentation Model
For the statistical multifragmentation model the critical indices
are calculated as functions of
the Fisher parameter . It is found that these indices have different
values than in Fisher's droplet model. Some peculiarities of the scaling
relations are discussed. The basic model predicts for the index a narrow
range of values, , which is consistent with two
experiments on nuclear multifragmentation.Comment: minor changes of the text, four references adde
Statistical Multifragmentation in Thermodynamic Limit
An exact analytical solution of the statistical multifragmentation model is
found in thermodynamic limit. The model exhibits a 1-st order phase transition
of the liquid-gas type. The mixed phase region of the phase diagram, where the
gas of nuclear fragments coexists with the infinite liquid condensate, is
unambiguously identified. The peculiar thermodynamic properties of the model
near the boundary between the mixed phase and the pure gaseous phase are
studied. The results for the caloric curve and specific heat are presented and
a physical picture of the nuclear liquid-gas phase transition is clarified.Comment: 4 figure
Depinning with dynamic stress overshoots: A hybrid of critical and pseudohysteretic behavior
A model of an elastic manifold driven through a random medium by an applied
force F is studied focussing on the effects of inertia and elastic waves, in
particular {\it stress overshoots} in which motion of one segment of the
manifold causes a temporary stress on its neighboring segments in addition to
the static stress. Such stress overshoots decrease the critical force for
depinning and make the depinning transition hysteretic. We find that the steady
state velocity of the moving phase is nevertheless history independent and the
critical behavior as the force is decreased is in the same universality class
as in the absence of stress overshoots: the dissipative limit which has been
studied analytically. To reach this conclusion, finite-size scaling analyses of
a variety of quantities have been supplemented by heuristic arguments.
If the force is increased slowly from zero, the spectrum of avalanche sizes
that occurs appears to be quite different from the dissipative limit. After
stopping from the moving phase, the restarting involves both fractal and
bubble-like nucleation. Hysteresis loops can be understood in terms of a
depletion layer caused by the stress overshoots, but surprisingly, in the limit
of very large samples the hysteresis loops vanish. We argue that, although
there can be striking differences over a wide range of length scales, the
universality class governing this pseudohysteresis is again that of the
dissipative limit. Consequences of this picture for the statistics and dynamics
of earthquakes on geological faults are briefly discussed.Comment: 43 pages, 57 figures (yes, that's a five followed by a seven), revte
Mechanisms for electron transport in atomic-scale one-dimensional wires: soliton and polaron effects
We study one-electron tunneling through atomic-scale one-dimensional wires in
the presence of coherent electron-phonon (e-ph) coupling. We use a full quantum
model for the e-ph interaction within the wire with open boundary conditions.
We illustrate the mechanisms of transport in the context of molecular wires
subject to boundary conditions imposing the presence of a soliton defect in the
molecule. Competition between polarons and solitons in the coherent transport
is examined. The transport mechanisms proposed are generally applicable to
other one-dimensional nanoscale systems with strong e-ph coupling.Comment: 7 pages, 4 figures, accepted for publication in Europhys. Let
Applicability of the Fisher Equation to Bacterial Population Dynamics
The applicability of the Fisher equation, which combines diffusion with
logistic nonlinearity, to population dynamics of bacterial colonies is studied
with the help of explicit analytic solutions for the spatial distribution of a
stationary bacterial population under a static mask. The mask protects the
bacteria from ultraviolet light. The solution, which is in terms of Jacobian
elliptic functions, is used to provide a practical prescription to extract
Fisher equation parameters from observations and to decide on the validity of
the Fisher equation.Comment: 5 pages, 3 figs. include
On the stability of scalar-vacuum space-times
We study the stability of static, spherically symmetric solutions to the
Einstein equations with a scalar field as the source. We describe a general
methodology of studying small radial perturbations of scalar-vacuum
configurations with arbitrary potentials V(\phi), and in particular space-times
with throats (including wormholes), which are possible if the scalar is
phantom. At such a throat, the effective potential for perturbations V_eff has
a positive pole (a potential wall) that prevents a complete perturbation
analysis. We show that, generically, (i) V_eff has precisely the form required
for regularization by the known S-deformation method, and (ii) a solution with
the regularized potential leads to regular scalar field and metric
perturbations of the initial configuration. The well-known conformal mappings
make these results also applicable to scalar-tensor and f(R) theories of
gravity. As a particular example, we prove the instability of all static
solutions with both normal and phantom scalars and V(\phi) = 0 under spherical
perturbations. We thus confirm the previous results on the unstable nature of
anti-Fisher wormholes and Fisher's singular solution and prove the instability
of other branches of these solutions including the anti-Fisher "cold black
holes".Comment: 18 pages, 5 figures. A few comments and references added. Final
version accepted at EPJ
Resonant tunneling and the multichannel Kondo problem: the quantum Brownian motion description
We study mesoscopic resonant tunneling as well as multichannel Kondo problems
by mapping them to a first-quantized quantum mechanical model of a particle
moving in a multi-dimensional periodic potential with Ohmic dissipation. From a
renormalization group analysis, we obtain phase diagrams of the quantum
Brownian motion model with various lattice symmetries. For a symmorphic
lattice, there are two phases at T=0: a localized phase in which the particle
is trapped in a potential minimum, and a free phase in which the particle is
unaffected by the periodic potential. For a non-symmorphic lattice, however,
there may be an additional intermediate phase in which the particle is neither
localized nor completely free. The fixed point governing the intermediate phase
is shown to be identical to the well-known multichannel Kondo fixed point in
the Toulouse limit as well as the resonance fixed point of a quantum dot model
and a double-barrier Luttinger liquid model. The mapping allows us to compute
the fixed-poing mobility of the quantum Brownian motion model exactly,
using known conformal-field-theory results of the Kondo problem. From the
mobility, we find that the peak value of the conductance resonance of a
spin-1/2 quantum dot problem is given by . The scaling form of the
resonance line shape is predicted
Low-temperature nonequilibrium transport in a Luttinger liquid
The temperature-dependent nonlinear conductance for transport of a Luttinger
liquid through a barrier is calculated in the nonperturbative regime for
, where is the dimensionless interaction constant. To
describe the low-energy behavior, we perform a leading-log summation of all
diagrams contributing to the conductance which is valid for .
With increasing external voltage, the asymptotic low-temperature behavior
displays a turnover from the to a universal law.Comment: 13 pages RevTeX 3.0, accepted by Physical Review
Disordered Hubbard Model with Attraction: Coupling Energy of Cooper Pairs in Small Clusters
We generalize the Cooper problem to the case of many interacting particles in
the vicinity of the Fermi level in the presence of disorder. On the basis of
this approach we study numerically the variation of the pair coupling energy in
small clusters as a function of disorder. We show that the Cooper pair energy
is strongly enhanced by disorder, which at the same time leads to the
localization of pairs.Comment: revtex, 5 pages, 6 figure
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