22 research outputs found
Crossover from Orthogonal to Unitary Symmetry for Ballistic Electron Transport in Chaotic Microstructures
We study the ensemble-averaged conductance as a function of applied magnetic
field for ballistic electron transport across few-channel microstructures
constructed in the shape of classically chaotic billiards. We analyse the
results of recent experiments, which show suppression of weak localization due
to magnetic field, in the framework of random-matrix theory. By analysing a
random-matrix Hamiltonian for the billiard-lead system with the aid of
Landauer's formula and Efetov's supersymmetry technique, we derive a universal
expression for the weak-localization contribution to the mean conductance that
depends only on the number of channels and the magnetic flux. We consequently
gain a theoretical understanding of the continuous crossover from orthogonal
symmetry to unitary symmetry arising from the violation of time-reversal
invariance for generic chaotic systems.Comment: 49 pages, latex, 9 figures as tar-compressed uuencoded fil
Molecular dynamics approach: from chaotic to statistical properties of compound nuclei
Statistical aspects of the dynamics of chaotic scattering in the classical
model of -cluster nuclei are studied. It is found that the dynamics
governed by hyperbolic instabilities which results in an exponential decay of
the survival probability evolves to a limiting energy distribution whose
density develops the Boltzmann form. The angular distribution of the
corresponding decay products shows symmetry with respect to angle. Time
estimated for the compound nucleus formation ranges within the order of
s.Comment: 11 pages, LaTeX, non
Fine Structure Discussion of Parity-Nonconserving Neutron Scattering at Epithermal Energies
The large magnitude and the sign correlation effect in the parity
non-conserving resonant scattering of epithermal neutrons from Th is
discussed in terms of a non-collective local doorway model. General
conclusions are drawn as to the probability of finding large parity violation
effects in other regions of the periodic table.Comment: 6 pages, Tex. CTP# 2296, to appear in Z. Phys.
Relaxation process in a regime of quantum chaos
We show that the quantum relaxation process in a classically chaotic open
dynamical system is characterized by a quantum relaxation time scale t_q. This
scale is much shorter than the Heisenberg time and much larger than the
Ehrenfest time: t_q ~ g^alpha where g is the conductance of the system and the
exponent alpha is close to 1/2. As a result, quantum and classical decay
probabilities remain close up to values P ~ exp(-sqrt(g)) similarly to the case
of open disordered systems.Comment: revtex, 5 pages, 4 figures discussion of the relations between time
scale t_q and weak localization correction and between dynamical and
disordered systems is adde
Classical and quantum decay of one dimensional finite wells with oscillating walls
To study the time decay laws (tdl) of quasibounded hamiltonian systems we
have considered two finite potential wells with oscillating walls filled by non
interacting particles. We show that the tdl can be qualitatively different for
different movement of the oscillating wall at classical level according to the
characteristic of trapped periodic orbits. However, the quantum dynamics do not
show such differences.Comment: RevTeX, 15 pages, 14 PostScript figures, submitted to Phys. Rev.
Distribution of reflection eigenvalues in many-channel chaotic cavities with absorption
The reflection matrix R=S^{\dagger}S, with S being the scattering matrix,
differs from the unit one, when absorption is finite. Using the random matrix
approach, we calculate analytically the distribution function of its
eigenvalues in the limit of a large number of propagating modes in the leads
attached to a chaotic cavity. The obtained result is independent on the
presence of time-reversal symmetry in the system, being valid at finite
absorption and arbitrary openness of the system. The particular cases of
perfectly and weakly open cavities are considered in detail. An application of
our results to the problem of thermal emission from random media is briefly
discussed.Comment: 4 pages, 2 figures; (Ref.[5b] added, appropriate modification in
text
Effective Coupling for Open Billiards
We derive an explicit expression for the coupling constants of individual
eigenstates of a closed billiard which is opened by attaching a waveguide. The
Wigner time delay and the resonance positions resulting from the coupling
constants are compared to an exact numerical calculation. Deviations can be
attributed to evanescent modes in the waveguide and to the finite number of
eigenstates taken into account. The influence of the shape of the billiard and
of the boundary conditions at the mouth of the waveguide are also discussed.
Finally we show that the mean value of the dimensionless coupling constants
tends to the critical value when the eigenstates of the billiard follow
random-matrix theory
Theory of parity violation in compound nuclear states; one particle aspects
In this work we formulate the reaction theory of parity violation in compound
nuclear states using Feshbach's projection operator formalism. We derive in
this framework a complete set of terms that contribute to the longitudinal
asymmetry measured in experiments with polarized epithermal neutrons. We also
discuss the parity violating spreading width resulting from this formalism. We
then use the above formalism to derive expressions which hold in the case when
the doorway state approximation is introduced. In applying the theory we limit
ourselves in this work to the case when the parity violating potential and the
strong interaction are one-body. In this approximation, using as the doorway
the giant spin-dipole resonance and employing well known optical potentials and
a time-reversal even, parity odd one-body interaction we calculate or estimate
the terms we derived. In our calculations we explicitly orthogonalize the
continuum and bound wave functions. We find the effects of orthogonalization to
be very important. Our conclusion is that the present one-body theory cannot
explain the average longitudinal asymmetry found in the recent polarized
neutron experiments. We also confirm the discrepancy, first pointed out by
Auerbach and Bowman, that emerges, between the calculated average asymmetry and
the parity violating spreading width, when distant doorways are used in the
theory.Comment: 37 pages, REVTEX, 5 figures not included (Postscript, available from
the authors
Weak-Localization in Chaotic Versus Non-Chaotic Cavities: A Striking Difference in the Line Shape
We report experimental evidence that chaotic and non-chaotic scattering
through ballistic cavities display distinct signatures in quantum transport. In
the case of non-chaotic cavities, we observe a linear decrease in the average
resistance with magnetic field which contrasts markedly with a Lorentzian
behavior for a chaotic cavity. This difference in line-shape of the
weak-localization peak is related to the differing distribution of areas
enclosed by electron trajectories. In addition, periodic oscillations are
observed which are probably associated with the Aharonov-Bohm effect through a
periodic orbit within the cavities.Comment: 4 pages revtex + 4 figures on request; amc.hub.94.
Exact eigenstate analysis of finite-frequency conductivity in graphene
We employ the exact eigenstate basis formalism to study electrical
conductivity in graphene, in the presence of short-range diagonal disorder and
inter-valley scattering. We find that for disorder strength, 5, the
density of states is flat. We, then, make connection, using the MRG approach,
with the work of Abrahams \textit{et al.} and find a very good agreement for
disorder strength, = 5. For low disorder strength, = 2, we plot the
energy-resolved current matrix elements squared for different locations of the
Fermi energy from the band centre. We find that the states close to the band
centre are more extended and falls of nearly as as we move away
from the band centre. Further studies of current matrix elements versus
disorder strength suggests a cross-over from weakly localized to a very weakly
localized system. We calculate conductivity using Kubo Greenwood formula and
show that, for low disorder strength, conductivity is in a good qualitative
agreement with the experiments, even for the on-site disorder. The intensity
plots of the eigenstates also reveal clear signatures of puddle formation for
very small carrier concentration. We also make comparison with square lattice
and find that graphene is more easily localized when subject to disorder.Comment: 11 pages,15 figure