20,700 research outputs found
Interaction-assisted propagation of Coulomb-correlated electron-hole pairs in disordered semiconductors
A two-band model of a disordered semiconductor is used to analyze dynamical
interaction induced weakening of localization in a system that is accessible to
experimental verification. The results show a dependence on the sign of the
two-particle interaction and on the optical excitation energy of the
Coulomb-correlated electron-hole pair.Comment: 4 pages and 3 ps figure
Continuum states from time-dependent density functional theory
Linear response time-dependent density functional theory is used to study
low-lying electronic continuum states of targets that can bind an extra
electron. Exact formulas to extract scattering amplitudes from the
susceptibility are derived in one dimension. A single-pole approximation for
scattering phase shifts in three dimensions is shown to be more accurate than
static exchange for singlet electron-He scattering.Comment: 5 pages, 2 figures, J. Chem. Phys. accepte
Off-diagonal disorder in the Anderson model of localization
We examine the localization properties of the Anderson Hamiltonian with
additional off-diagonal disorder using the transfer-matrix method and
finite-size scaling. We compute the localization lengths and study the
metal-insulator transition (MIT) as a function of diagonal disorder, as well as
its energy dependence. Furthermore we investigate the different influence of
odd and even system sizes on the localization properties in quasi
one-dimensional systems. Applying the finite-size scaling approach in
conjunction with a nonlinear fitting procedure yields the critical parameters
of the MIT. In three dimensions, we find that the resulting critical exponent
of the localization length agrees with the exponent for the Anderson model with
pure diagonal disorder.Comment: 12 pages including 4 EPS figures, accepted for publication in phys.
stat. sol. (b
Power spectrum analysis of bursting cells in area MT in the behaving monkey
It is widely held that visual cortical neurons encode information primarily in their mean firing rates. Some proposals, however, emphasize the information potentially available in the temporal structure of spike trains (Optican and Richmond, 1987; Bialek et al., 1991), in particular with respect to stimulus-related synchronized oscillations in the 30–70 Hz range (Eckhorn et al., 1988; Gray et al., 1989; Kreiter and Singer, 1992) as well as via bursting cells (Cattaneo et al., 1981a; Bonds, 1992). We investigate the temporal fine structure of spike trains recorded in extrastriate area MT of the trained macaque monkey, a region that plays a major role in processing motion information. The data were recorded while the monkey performed a near- threshold direction discrimination task so that both physiological and psychophysical data could be obtained on the same set of trials (Britten et al., 1992). We identify bursting cells and quantify their properties, in particular in relation to the behavior of the animal. We compute the power spectrum and the distribution of interspike intervals (ISIs) associated with individual spike trains from 212 cells, averaging these quantities across similar trials. (1) About 33% of the cells have a relatively flat power spectrum with a dip at low temporal frequencies. We analytically derive the power spectrum of a Poisson process with refractory period and show that it matches the observed spectrum of these cells. (2) About 62% of the cells have a peak in the 20–60 Hz frequency band. In about 10% of all cells, this peak is at least twice the height of its base. The presence of such a peak strongly correlates with a tendency of the cell to respond in bursts, that is, two to four spikes within 2–8 msec. For 93% of cells, the shape of the power spectrum did not change dramatically with stimulus conditions. (3) Both the ISI distribution and the power spectrum of the vast majority of bursting cells are compatible with the notion that these cells fire Poisson-distributed bursts, with a burst-related refractory period. Thus, for our stimulus conditions, no explicitly oscillating neuronal process is required to yield a peak in the power spectrum. (4) We found no statistically significant relationship between the peak in the power spectrum and psychophysical measures of the monkeys' performance on the direction discrimination task
Optical absorption of non-interacting tight-binding electrons in a Peierls-distorted chain at half band-filling
In this first of three articles on the optical absorption of electrons in
half-filled Peierls-distorted chains we present analytical results for
non-interacting tight-binding electrons. We carefully derive explicit
expressions for the current operator, the dipole transition matrix elements,
and the optical absorption for electrons with a cosine dispersion relation of
band width and dimerization parameter . New correction
(``''-)terms to the current operator are identified. A broad band-to-band
transition is found in the frequency range whose shape
is determined by the joint density of states for the upper and lower Peierls
subbands and the strong momentum dependence of the transition matrix elements.Comment: 17 pages REVTEX 3.0, 2 postscript figures; hardcopy versions before
May 96 are obsolete; accepted for publication in The Philosophical Magazine
Protein Supplements for Beef Calves on Winter Range
These trials were to compare the effect of different sources and levels of supplemental protein on the winter and subsequent summer gains of beef calves grazing native range at the Fort Robinson Beef Cattle Research Station, Crawford, Nebraska
Determining the Contribution of Epidermal Cell Shape to Petal Wettability Using Isogenic Antirrhinum Lines
This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited
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