888 research outputs found
Transmission, reflection and localization in a random medium with absorption or gain
We study reflection and transmission of waves in a random tight-binding
system with absorption or gain for weak disorder, using a scattering matrix
formalism. Our aim is to discuss analytically the effects of absorption or gain
on the statistics of wave transport. Treating the effects of absorption or gain
exactly in the limit of no disorder, allows us to identify short- and long
lengths regimes relative to absorption- or gain lengths, where the effects of
absorption/gain on statistical properties are essentially different. In the
long-lengths regime we find that a weak absorption or a weak gain induce
identical statistical corrections in the inverse localization length, but lead
to different corrections in the mean reflection coefficient. In contrast, a
strong absorption or a strong gain strongly suppress the effect of disorder in
identical ways (to leading order), both in the localization length and in the
mean reflection coefficient.Comment: Important revisions and expansion caused by a crucial property of
$\hat Q
Mean Free Path in Disordered Multichannel Tight-Binding Wires
Transport in a disordered tight-binding wire involves a collection of
different mean free paths resulting from the distinct fermi points, which
correspond to the various scattering channels of the wire. The generalization
of Thouless' relation between the mean free path and the localization length
permits to define an average channel mean free path,, such that
in an -channel system. The averaged mean free path
is expressed exactly in terms of the total reflection coefficient of
the wire and compared with the mean free path defined in the maximum entropy
approach
Conductance and localization in disordered wires: role of evanescent states
This paper extends an earlier analytical scattering matrix treatment of
conductance and localization in coupled two- and three Anderson chain systems
for weak disorder when evanescent states are present at the Fermi level. Such
states exist typically when the interchain coupling exceeds the width of
propagating energy bands associated with the various transverse eigenvalues of
the coupled tight-binding systems. We calculate reflection- and transmission
coefficients in cases where, besides propagating states, one or two evanescent
states are available at the Fermi level for elastic scattering of electrons by
the disordered systems. We observe important qualitative changes in these
coefficients and in the related localization lengths due to ineffectiveness of
the evanescent modes for transmission and reflection in the various scattering
channels. In particular, the localization lengths are generally significantly
larger than the values obtained when evanescent modes are absent. Effects
associated with disorder mediated coupling between propagating and evanescent
modes are shown to be suppressed by quantum interference effects, in lowest
order for weak disorder
Exact transmission moments in one-dimensional weak localization and single-parameter scaling
We obtain for the first time the expressions for the mean and the variance of
the transmission coefficient for an Anderson chain in the weak localization
regime, using exact expansions of the complex transmission- and reflection
coefficients to fourth order in the weakly disordered site energies. These
results confirm the validity of single-parameter scaling theory in a domain
where the higher transmission cumulants may be neglected. We compare our
results with earlier results for transmission cumulants in the weak
localization domain based on the phase randomization hypothesis
Absence of Fragmentation in Two-Dimensional Bose-Einstein Condensation
We investigate the possibility that the BEC-like phenomena recently detected
on two-dimensional finite trapped systems consist of fragmented condensates. We
derive and diagonalize the one-body density matrix of a two-dimensional
isotropically trapped Bose gas at finite temperature. For the ideal gas, the
procedure reproduces the exact harmonic-oscillator eigenfunctions and the Bose
distribution. We use a new collocation-minimization method to study the
interacting gas in the Hartree-Fock approximation and obtain a ground-state
wavefunction and condensate fraction consistent with those obtained by other
methods. The populations of the next few eigenstates increase at the expense of
the ground state but continue to be negligible; this supports the conclusion
that two-dimensional BEC is into a single state.Comment: 6 pages, 1 figur
Localization fom conductance in few-channel disordered wires
We study localization in two- and three channel quasi-1D systems using
multichain tight-binding Anderson models with nearest-neighbour interchain
hopping. In the three chain case we discuss both the case of free- and that of
periodic boundary conditions between the chains. The finite disordered wires
are connected to ideal leads and the localization length is defined from the
Landauer conductance in terms of the transmission coefficients matrix. The
transmission- and reflection amplitudes in properly defined quantum channels
are obtained from S-matrices constructed from transfer matrices in Bloch wave
bases for the various quasi-1D systems. Our exact analytic expressions for
localization lengths for weak disorder reduce to the Thouless expression for 1D
systems in the limit of vanishing interchain hopping. For weak interchain
hopping the localization length decreases with respect to the 1D value in all
three cases. In the three-channel cases it increases with interchain hopping
over restricted domains of large hopping
Relationship between Planthoppers (\u3ci\u3eNilaparvata lugens\u3c/i\u3e and \u3ci\u3eSogatella furcifera\u3c/i\u3e) and Rice Diseases
The locational preference of the brown planthopper (BPH) Nilaparvata lugens
(Still) and the whitebacked plant hopper (WBPH) Sogatella furcifera (Horvath) was studied on rice cultivars IR22 and IR36 as an integral part of subsequent research on insect-fungal pathogen relationships. The BPH was observed to stay consistently on the basal portion while the WBPH showed a general preference for the upper portion regardless of varieties, rice growth stages and insect population density levels. The habitat preference of both species (BPH and WBPH) was found not to be affected by the presence of the other species when both species are present on the same host plant.
Five rice cultivars with different reactions to BPH biotype 2 were used in the study on BPH-Rhizoctonia solani relationship: IR22 and TN1 (susceptible); Triveni and ASD7 (moderately resistant); and IR42 (resistant). Test plants were inoculated with R. solani (Kuhn) 3~4days after insect infestation. Sheath blight disease severity/incidence was significantly higher in the treatment where BPH+R. solani were together than in the treatment with only the pathogen. Symptom expression of the disease in the BPH-pathogen combination was faster and mycelial growth was more profuse inducing the formation of more infection structures. Regardless of varietal reaction to BPH biotype 2, the degree of hopperburn was significantly higher in the combination of the two pests as compared with that of BPH alone. There could be a synergistic relationship between the insect pest and the pathogen indicated by a positive interaction between the two species
Localization length in Dorokhov's microscopic model of multichannel wires
We derive exact quantum expressions for the localization length for
weak disorder in two- and three chain tight-binding systems coupled by random
nearest-neighbour interchain hopping terms and including random energies of the
atomic sites. These quasi-1D systems are the two- and three channel versions of
Dorokhov's model of localization in a wire of periodically arranged atomic
chains. We find that for the considered systems with
, where is Thouless' quantum expression for the inverse
localization length in a single 1D Anderson chain, for weak disorder. The
inverse localization length is defined from the exponential decay of the
two-probe Landauer conductance, which is determined from an earlier transfer
matrix solution of the Schr\"{o}dinger equation in a Bloch basis. Our exact
expressions above differ qualitatively from Dorokhov's localization length
identified as the length scaling parameter in his scaling description of the
distribution of the participation ratio. For N=3 we also discuss the case where
the coupled chains are arranged on a strip rather than periodically on a tube.
From the transfer matrix treatment we also obtain reflection coefficients
matrices which allow us to find mean free paths and to discuss their relation
to localization lengths in the two- and three channel systems
Pattern selection as a nonlinear eigenvalue problem
A unique pattern selection in the absolutely unstable regime of driven,
nonlinear, open-flow systems is reviewed. It has recently been found in
numerical simulations of propagating vortex structures occuring in
Taylor-Couette and Rayleigh-Benard systems subject to an externally imposed
through-flow. Unlike the stationary patterns in systems without through-flow
the spatiotemporal structures of propagating vortices are independent of
parameter history, initial conditions, and system length. They do, however,
depend on the boundary conditions in addition to the driving rate and the
through-flow rate. Our analysis of the Ginzburg-Landau amplitude equation
elucidates how the pattern selection can be described by a nonlinear eigenvalue
problem with the frequency being the eigenvalue. Approaching the border between
absolute and convective instability the eigenvalue problem becomes effectively
linear and the selection mechanism approaches that of linear front propagation.
PACS: 47.54.+r,47.20.Ky,47.32.-y,47.20.FtComment: 18 pages in Postsript format including 5 figures, to appear in:
Lecture Notes in Physics, "Nonlinear Physics of Complex Sytems -- Current
Status and Future Trends", Eds. J. Parisi, S. C. Mueller, and W. Zimmermann
(Springer, Berlin, 1996
Two-dimensional atom trapping in field-induced adiabatic potentials
We show how to create a novel two-dimensional trap for ultracold atoms from a conventional magnetic trap. We achieve this by utilizing rf-induced adiabatic potentials to enhance the trapping potential in one direction. We demonstrate the loading process and discuss the experimental conditions under which it might be possible to prepare a 2D Bose condensate. A scheme for the preparation of coherent matterwave bubbles is also discussed
- …