457 research outputs found
Some generic aspects of bosonic excitations in disordered systems
We consider non-interacting bosonic excitations in disordered systems,
emphasising generic features of quadratic Hamiltonians in the absence of
Goldstone modes. We discuss relationships between such Hamiltonians and the
symmetry classes established for fermionic systems. We examine the density
\rho(\omega) of excitation frequencies \omega, showing how the universal
behavior \rho(\omega) ~ \omega^4 for small \omega can be obtained both from
general arguments and by detailed calculations for one-dimensional models
Coupled Maps on Trees
We study coupled maps on a Cayley tree, with local (nearest-neighbor)
interactions, and with a variety of boundary conditions. The homogeneous state
(where every lattice site has the same value) and the node-synchronized state
(where sites of a given generation have the same value) are both shown to occur
for particular values of the parameters and coupling constants. We study the
stability of these states and their domains of attraction. As the number of
sites that become synchronized is much higher compared to that on a regular
lattice, control is easier to effect. A general procedure is given to deduce
the eigenvalue spectrum for these states. Perturbations of the synchronized
state lead to different spatio-temporal structures. We find that a mean-field
like treatment is valid on this (effectively infinite dimensional) lattice.Comment: latex file (25 pages), 4 figures included. To be published in Phys.
Rev.
Synchronisation in Coupled Sine Circle Maps
We study the spatially synchronized and temporally periodic solutions of a
1-d lattice of coupled sine circle maps. We carry out an analytic stability
analysis of this spatially synchronized and temporally periodic case and obtain
the stability matrix in a neat block diagonal form. We find spatially
synchronized behaviour over a substantial range of parameter space. We have
also extended the analysis to higher spatial periods with similar results.
Numerical simulations for various temporal periods of the synchronized
solution, reveal that the entire structure of the Arnold tongues and the
devil's staircase seen in the case of the single circle map can also be
observed for the synchronized coupled sine circle map lattice. Our formalism
should be useful in the study of spatially periodic behaviour in other coupled
map lattices.Comment: uuencoded, 1 rextex file 14 pages, 3 postscript figure
Antilocalization in a 2D Electron Gas in a Random Magnetic Field
We construct a supersymmetric field theory for the problem of a
two-dimensional electron gas in a random, static magnetic field. We find a new
term in the free energy, additional to those present in the conventional
unitary sigma-model, whose presence relies on the long-range nature of the
disorder correlations. Under a perturbative renormalization group analysis of
the free energy, the new term contributes to the scaling function at one-loop
order and leads to antilocalization.Comment: 4 pages, RevTe
Conductance scaling at the band center of wide wires with pure non--diagonal disorder
Kubo formula is used to get the scaling behavior of the static conductance
distribution of wide wires showing pure non-diagonal disorder. Following recent
works that point to unusual phenomena in some circumstances, scaling at the
band center of wires of odd widths has been numerically investigated. While the
conductance mean shows a decrease that is only proportional to the inverse
square root of the wire length, the median of the distribution exponentially
decreases as a function of the square root of the length. Actually, the whole
distribution decays as the inverse square root of the length except close to
G=0 where the distribution accumulates the weight lost at larger conductances.
It accurately follows the theoretical prediction once the free parameter is
correctly fitted. Moreover, when the number of channels equals the wire length
but contacts are kept finite, the conductance distribution is still described
by the previous model. It is shown that the common origin of this behavior is a
simple Gaussian statistics followed by the logarithm of the E=0 wavefunction
weight ratio of a system showing chiral symmetry. A finite value of the
two-dimensional conductance mean is obtained in the infinite size limit. Both
conductance and the wavefunction statistics distributions are given in this
limit. This results are consistent with the 'critical' character of the E=0
wavefunction predicted in the literature.Comment: 10 pages, 9 figures, RevTeX macr
Shell structure at N=28 near the dripline: spectroscopy of Si, P and S
Measurements of the N=28 isotones 42Si, 43P and 44S using one- and two-proton
knockout reactions from the radioactive beam nuclei 44S and 46Ar are reported.
The knockout reaction cross sections for populating 42Si and 43P and a 184 keV
gamma-ray observed in 43P establish that the d_{3/2} and s_{1/2} proton orbits
are nearly degenerate in these nuclei and that there is a substantial Z=14
subshell closure separating these two orbits from the d_{5/2} orbit. The
increase in the inclusive two-proton knockout cross section from 42Si to 44S
demonstrates the importance of the availability of valence protons for
determining the cross section. New calculations of the two-proton knockout
reactions that include diffractive effects are presented. In addition, it is
proposed that a search for the d_{5/2} proton strength in 43P via a higher
statistics one-proton knockout experiment could help determine the size of the
Z=14 closure.Comment: Phys. Rev. C, in pres
Singular Density of States of Disordered Dirac Fermions in the Chiral Models
The Dirac fermion in the random chiral models is studied which includes the
random gauge field model and the random hopping model. We focus on a connection
between continuum and lattice models to give a clear perspective for the random
chiral models. Two distinct structures of density of states (DoS) around zero
energy, one is a power-law dependence on energy in the intermediate energy
range and the other is a diverging one at zero energy, are revealed by an
extensive numerical study for large systems up to . For the
random hopping model, our finding of the diverging DoS within very narrow
energy range reconciles previous inconsistencies between the lattice and the
continuum models.Comment: 4 pages, 4 figure
Particle-hole symmetric localization in two dimensions
We revisit two-dimensional particle-hole symmetric sublattice localization
problem, focusing on the origin of the observed singularities in the density of
states at the band center E=0. The most general such system [R. Gade,
Nucl. Phys. B {\bf 398}, 499 (1993)] exhibits critical behavior and has
that diverges stronger than any integrable power-law, while the
special {\it random vector potential model} of Ludwiget al [Phys. Rev. B {\bf
50}, 7526 (1994)] has instead a power-law density of states with a continuously
varying dynamical exponent. We show that the latter model undergoes a dynamical
transition with increasing disorder--this transition is a counterpart of the
static transition known to occur in this system; in the strong-disorder regime,
we identify the low-energy states of this model with the local extrema of the
defining two-dimensional Gaussian random surface. Furthermore, combining this
``surface fluctuation'' mechanism with a renormalization group treatment of a
related vortex glass problem leads us to argue that the asymptotic low
behavior of the density of states in the {\it general} case is , different from earlier prediction of Gade. We also
study the localized phases of such particle-hole symmetric systems and identify
a Griffiths ``string'' mechanism that generates singular power-law
contributions to the low-energy density of states in this case.Comment: 18 pages (two-column PRB format), 10 eps figures include
Variation with mass of \boldmath{B(E3; 0_1^+ \to 3_1^-)} transition rates in even-mass xenon nuclei
transition matrix elements have been measured for
even-mass Xe nuclei using sub-barrier Coulomb excitation in inverse
kinematics. The trends in energy and
excitation strengths are well reproduced using phenomenological models based on
a strong coupling picture with a soft quadrupole mode and an increasing
occupation of the intruder orbital.Comment: 5 pages, 4 figures, PRC in pres
- …