2,445 research outputs found
Accurate molecular energies by extrapolation of atomic energies using an analytic quantum mechanical model
Using a new analytic quantum mechanical method based on Slater's Xalpha
method, we show that a fairly accurate estimate of the total energy of a
molecule can be obtained from the exact energies of its constituent atoms. The
mean absolute error in the total energies thus determined for the G2 set of 56
molecules is about 16 kcal/mol, comparable to or better than some popular pure
and hybrid density functional models.Comment: 5 pages, REVTE
Zener transitions between dissipative Bloch bands. II: Current Response at Finite Temperature
We extend, to include the effects of finite temperature, our earlier study of
the interband dynamics of electrons with Markoffian dephasing under the
influence of uniform static electric fields. We use a simple two-band
tight-binding model and study the electric current response as a function of
field strength and the model parameters. In addition to the Esaki-Tsu peak,
near where the Bloch frequency equals the damping rate, we find current peaks
near the Zener resonances, at equally spaced values of the inverse electric
field. These become more prominenent and numerous with increasing bandwidth (in
units of the temperature, with other parameters fixed). As expected, they
broaden with increasing damping (dephasing).Comment: 5 pages, LateX, plus 5 postscript figure
Fourier analyses of commensurability oscillations in Fibonacci lateral superlattices
Magnetotransport measurements have been performed on Fibonacci lateral
superlattices (FLSLs) -- two-dimensional electron gases subjected to a weak
potential modulation arranged in the Fibonacci sequence, LSLLSLS..., with
L/S=tau (the golden ratio). Complicated commensurability oscillation (CO) is
observed, which can be accounted for as a superposition of a series of COs each
arising from a sinusoidal modulation representing the characteristic length
scale of one of the self-similar generations in the Fibonacci sequence.
Individual CO components can be separated out from the magnetoresistance trace
by performing a numerical Fourier band-pass filter. From the analysis of the
amplitude of a single-component CO thus extracted, the magnitude of the
corresponding Fourier component in the potential modulation can be evaluated.
By examining all the Fourier contents observed in the magnetoresistance trace,
the profile of the modulated potential seen by the electrons can be
reconstructed with some remaining ambiguity about the interrelation of the
phase between different components.Comment: 11 pages, 10 figures, added references in Introduction, minor
revision
Quantitative analysis of regulatory flexibility under changing environmental conditions
The circadian clock controls 24-h rhythms in many biological processes, allowing appropriate timing of biological rhythms relative to dawn and dusk. Known clock circuits include multiple, interlocked feedback loops. Theory suggested that multiple loops contribute the flexibility for molecular rhythms to track multiple phases of the external cycle. Clear dawn- and dusk-tracking rhythms illustrate the flexibility of timing in Ipomoea nil. Molecular clock components in Arabidopsis thaliana showed complex, photoperiod-dependent regulation, which was analysed by comparison with three contrasting models. A simple, quantitative measure, Dusk Sensitivity, was introduced to compare the behaviour of clock models with varying loop complexity. Evening-expressed clock genes showed photoperiod-dependent dusk sensitivity, as predicted by the three-loop model, whereas the one- and two-loop models tracked dawn and dusk, respectively. Output genes for starch degradation achieved dusk-tracking expression through light regulation, rather than a dusk-tracking rhythm. Model analysis predicted which biochemical processes could be manipulated to extend dusk tracking. Our results reveal how an operating principle of biological regulators applies specifically to the plant circadian clock
Direct Minimization Generating Electronic States with Proper Occupation Numbers
We carry out the direct minimization of the energy functional proposed by
Mauri, Galli and Car to derive the correct self-consistent ground state with
fractional occupation numbers for a system degenerating at the Fermi level. As
a consequence, this approach enables us to determine the electronic structure
of metallic systems to a high degree of accuracy without the aid of level
broadening of the Fermi-distribution function. The efficiency of the method is
illustrated by calculating the ground-state energy of C and Si
molecules and the W(110) surface to which a tungsten adatom is adsorbed.Comment: 4 pages, 4 figure
On Fourier integral transforms for -Fibonacci and -Lucas polynomials
We study in detail two families of -Fibonacci polynomials and -Lucas
polynomials, which are defined by non-conventional three-term recurrences. They
were recently introduced by Cigler and have been then employed by Cigler and
Zeng to construct novel -extensions of classical Hermite polynomials. We
show that both of these -polynomial families exhibit simple transformation
properties with respect to the classical Fourier integral transform
Holstein polarons in a strong electric field: delocalized and stretched states
The coherent dynamics of a Holstein polaron in strong electric fields is
considered under different regimes. Using analytical and numerical analysis, we
show that even for small hopping constant and weak electron-phonon interaction,
the original discrete Wannier-Stark (WS) ladder electronic states are each
replaced by a semi-continuous band if a resonance condition is satisfied
between the phonon frequency and the ladder spacing. In this regime, the
original localized WS states can become {\em delocalized}, yielding both
`tunneling' and `stretched' polarons. The transport properties of such a system
would exhibit a modulation of the phonon replicas in typical tunneling
experiments. The modulation will reflect the complex spectra with
nearly-fractal structure of the semi-continuous band. In the off-resonance
regime, the WS ladder is strongly deformed, although the states are still
localized to a degree which depends on the detuning: Both the spacing between
the levels in the deformed ladder and the localization length of the resulting
eigenfunctions can be adjusted by the applied electric field. We also discuss
the regime beyond small hopping constant and weak coupling, and find an
interesting mapping to that limit via the Lang-Firsov transformation, which
allows one to extend the region of validity of the analysis.Comment: 10 pages, 13 figures, submitted to PR
Resonance Effects in the Nonadiabatic Nonlinear Quantum Dimer
The quantum nonlinear dimer consisting of an electron shuttling between the
two sites and in weak interaction with vibrations, is studied numerically under
the application of a DC electric field. A field-induced resonance phenomenon
between the vibrations and the electronic oscillations is found to influence
the electronic transport greatly. For initially delocalization of the electron,
the resonance has the effect of a dramatic increase in the transport. Nonlinear
frequency mixing is identified as the main mechanism that influences transport.
A characterization of the frequency spectrum is also presented.Comment: 7 pages, 6 figure
Periodic features in the Dynamic Structure Factor of the Quasiperiodic Period-doubling Lattice
We present an exact real-space renormalization group (RSRG) method for
evaluating the dynamic structure factor of an infinite one-dimensional
quasiperiodic period-doubling (PD) lattice. We observe that for every normal
mode frequency of the chain, the dynamic structure factor always
exhibits periodicity with respect to the wave vector and the presence of
such periodicity even in absence of translational invariance in the system is
quite surprising. Our analysis shows that this periodicity in
actually indicates the presence of delocalized phonon modes in the PD chain.
The Brillouin Zones of the lattice are found to have a hierarchical structure
and the dispersion relation gives both the acoustic as well as optical
branches. The phonon dispersion curves have a nested structure and we have
shown that it is actually the superposition of the dispersion curves of an
infinite set of periodic lattices.Comment: 9 pages, 3 postscript figures, REVTeX, To appear in Phys. Rev. B (1
February 1998-I
The oldest X-ray supernovae: X-ray emission from 1941C, 1959D, 1968D
We have studied the X-ray emission from four historical Type-II supernovae
(the newly-discovered 1941C in NGC 4631 and 1959D in NGC 7331; and 1968D, 1980K
in NGC 6946), using Chandra ACIS-S imaging. In particular, the first three are
the oldest ever found in the X-ray band, and provide constraints on the
properties of the stellar wind and circumstellar matter encountered by the
expanding shock at more advanced stages in the transition towards the remnant
phase. We estimate emitted luminosities ~ 5 x 10^{37} erg/s for SN 1941C, ~ a
few x 10^{37} erg/s for SN 1959D, ~ 2 x 10^{38} erg/s for SN 1968D, and ~ 4 x
10^{37} erg/s for SN 1980K, in the 0.3-8 keV band. X-ray spectral fits to SN
1968D suggest the presence of a harder component, possibly a power law with
photon index ~ 2, contributing ~ 10^{37} erg/s in the 2-10 keV band. We
speculate that it may be evidence of non-thermal emission from a Crab-like
young pulsar.Comment: 6 pages, accepted by ApJ. Revised version with a couple of added
references. Thanks to A. Kong and E. Schlegel for their comments. Credit to
Holt et al. (2003) for the X-ray discovery of SN 1968D, overlooked in other
recent catalog
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