4,480 research outputs found
Conductance of Disordered Wires with Symplectic Symmetry: Comparison between Odd- and Even-Channel Cases
The conductance of disordered wires with symplectic symmetry is studied by
numerical simulations on the basis of a tight-binding model on a square lattice
consisting of M lattice sites in the transverse direction. If the potential
range of scatterers is much larger than the lattice constant, the number N of
conducting channels becomes odd (even) when M is odd (even). The average
dimensionless conductance g is calculated as a function of system length L. It
is shown that when N is odd, the conductance behaves as g --> 1 with increasing
L. This indicates the absence of Anderson localization. In the even-channel
case, the ordinary localization behavior arises and g decays exponentially with
increasing L. It is also shown that the decay of g is much faster in the
odd-channel case than in the even-channel case. These numerical results are in
qualitative agreement with existing analytic theories.Comment: 4 page
Wave Scattering through Classically Chaotic Cavities in the Presence of Absorption: An Information-Theoretic Model
We propose an information-theoretic model for the transport of waves through
a chaotic cavity in the presence of absorption. The entropy of the S-matrix
statistical distribution is maximized, with the constraint : n is the dimensionality of S, and meaning complete (no) absorption. For strong absorption our result
agrees with a number of analytical calculations already given in the
literature. In that limit, the distribution of the individual (angular)
transmission and reflection coefficients becomes exponential -Rayleigh
statistics- even for n=1. For Rayleigh statistics is attained even
with no absorption; here we extend the study to . The model is
compared with random-matrix-theory numerical simulations: it describes the
problem very well for strong absorption, but fails for moderate and weak
absorptions. Thus, in the latter regime, some important physical constraint is
missing in the construction of the model.Comment: 4 pages, latex, 3 ps figure
Exact Solution for the Distribution of Transmission Eigenvalues in a Disordered Wire and Comparison with Random-Matrix Theory
An exact solution is presented of the Fokker-Planck equation which governs
the evolution of an ensemble of disordered metal wires of increasing length, in
a magnetic field. By a mapping onto a free-fermion problem, the complete
probability distribution function of the transmission eigenvalues is obtained.
The logarithmic eigenvalue repulsion of random-matrix theory is shown to break
down for transmission eigenvalues which are not close to unity. ***Submitted to
Physical Review B.****Comment: 20 pages, REVTeX-3.0, INLO-PUB-931028
Vacuum polarization by topological defects in de Sitter spacetime
In this paper we investigate the vacuum polarization effects associated with
a massive quantum scalar field in de Sitter spacetime in the presence of
gravitational topological defects. Specifically we calculate the vacuum
expectation value of the field square, . Because this investigation
has been developed in a pure de Sitter space, here we are mainly interested on
the effects induced by the presence of the defects.Comment: Talk presented at the 1st. Mediterranean Conference on Classical and
Quantum Gravity (MCCQG
Path Integral Approach to the Scattering Theory of Quantum Transport
The scattering theory of quantum transport relates transport properties of
disordered mesoscopic conductors to their transfer matrix \bbox{T}. We
introduce a novel approach to the statistics of transport quantities which
expresses the probability distribution of \bbox{T} as a path integral. The
path integal is derived for a model of conductors with broken time reversal
invariance in arbitrary dimensions. It is applied to the
Dorokhov-Mello-Pereyra-Kumar (DMPK) equation which describes
quasi-one-dimensional wires. We use the equivalent channel model whose
probability distribution for the eigenvalues of \bbox{TT}^{\dagger} is
equivalent to the DMPK equation independent of the values of the forward
scattering mean free paths. We find that infinitely strong forward scattering
corresponds to diffusion on the coset space of the transfer matrix group. It is
shown that the saddle point of the path integral corresponds to ballistic
conductors with large conductances. We solve the saddle point equation and
recover random matrix theory from the saddle point approximation to the path
integral.Comment: REVTEX, 9 pages, no figure
Reflectance Fluctuations in an Absorbing Random Waveguide
We study the statistics of the reflectance (the ratio of reflected and
incident intensities) of an -mode disordered waveguide with weak absorption
per mean free path. Two distinct regimes are identified. The regime
shows universal fluctuations.
With increasing length of the waveguide, the variance of the reflectance
changes from the value , characteristic for universal conductance
fluctuations in disordered wires, to another value , characteristic
for chaotic cavities. The weak-localization correction to the average
reflectance performs a similar crossover from the value to . In
the regime , the large- distribution of the reflectance
becomes very wide and asymmetric, for .Comment: 7 pages, RevTeX, 2 postscript figure
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