3,252 research outputs found
Dispersion interactions between semiconducting wires
The dispersion energy between extended molecular chains (or equivalently
infinite wires) with non-zero band gaps is generally assumed to be expressible
as a pair-wise sum of atom-atom terms which decay as . Using a model
system of two parallel wires with a variable band gap, we show that this is not
the case. The dispersion interaction scales as for large interwire
separations , as expected for an insulator, but as the band gap decreases
the interaction is greatly enhanced; while at shorter (but non-overlapping)
separations it approaches a power-law scaling given by , \emph{i.e.}
the dispersion interaction expected between \emph{metallic} wires. We
demonstrate that these effects can be understood from the increasing length
scale of the plasmon modes (charge fluctuations), and their increasing
contribution to the molecular dipole polarizability and the dispersion
interaction, as the band gaps are reduced. This result calls into question
methods which invoke locality assumptions in deriving dispersion interactions
between extended small-gap systems.Comment: 8 pages, 5 figure
A Full Configuration Interaction Perspective on the Homogeneous Electron Gas
Highly accurate results for the homogeneous electron gas (HEG) have only been
achieved to date within a diffusion Monte Carlo (DMC) framework. Here, we
introduce a newly developed stochastic technique, Full Configuration
Interaction Quantum Monte Carlo (FCIQMC), which samples the exact wavefunction
expanded in plane wave Slater determinants. Despite the introduction of a basis
set incompleteness error, we obtain a finite-basis energy which is
significantly, and variationally lower than any previously published work for
the 54-electron HEG at = 0.5 a.u., in a Hilbert space of
Slater determinants. At this value of , as well as of 1.0 a.u., we remove
the remaining basis set incompleteness error by extrapolation, yielding results
comparable or better than state-of-the-art DMC backflow energies. In doing so,
we demonstrate that it is possible to yield highly accurate results with the
FCIQMC method in sizable periodic systems.Comment: 4-page lette
Time Evolution of Decay Spectrum in K^0, K^0 bar -> pi^+ pi^- e^+ e^-
We consider the decay K^0 (K^0 bar) -> pi^+ pi^- e^+ e^- of a neutral K meson
prepared in a state of strangeness +1 (-1). The time evolution of the state
produces remarkable time-dependent effects in the angular distribution of the
pi^+ pi^- e^+ e^- system. These effects are correlated with the time-dependence
of the photon polarization in the radiative decay K^0 (K^0 bar) -> pi^+ pi^-
gamma. We study, in particular, the CP-odd, T-odd term in the distribution d
Gamma / d phi of the angle between the pi^+ pi^- and the e^+ e^- planes. We
also give the spectrum in the case that the decaying meson is an incoherent
mixture of K^0 and K^0 bar, and discuss the case of K_S regeneration in a K_L
beam.Comment: 16 pages, 8 figures, minor typos corrected, to appear in Phys. Lett.
Beam Halo Monitoring at CDF
Losses from the proton and antiproton beams of the Fermilab Tevatron have
been shown to produce a halo which contribute to backgrounds to physics signals
in the Collider Detector at Fermilab (CDF). To provide a measure of the beam
halo and losses, we have installed arrays of scintillation counters on both
sides of the CDF detector. We describe here the physical configuration of these
counters, their implementation and performance within the Fermilab Accelerator
Control Network (ACNET). We show correlations between these new devices and the
accelerator operating conditions. We point out that the use of these monitors
is leading to improvement in the accelerator operations and reduced backgrounds
in CDF.Comment: 27 pages, 17 figures, submitted to NIM
Direct CP violation in neutral kaon decays
The final result is presented of the NA48 Experiment performed at CERN SPS
neutral kaon beams on the direct CP violation parameter Re(epsilon'/epsilon),
as maesured from the decay rates of neutral kaons into two pions. The data
collected in years 1997-2001 yield the evidence for direct CP violation with
Re(epsilon'/epsilon)=(14.7+-2.2)10^-4. Description of expermental method and
systematics, comparison with world data and some discussion of implications for
theory are given.Comment: 5 pp., 3 figs, presented on behalf of NA48 Collaboration at PASCOS
2003 Conference, Mumbai, India, 2-8 Jan 2003, to appear in Praman
Identifiability of generalised Randles circuit models
The Randles circuit (including a parallel resistor and capacitor in series
with another resistor) and its generalised topology have widely been employed
in electrochemical energy storage systems such as batteries, fuel cells and
supercapacitors, also in biomedical engineering, for example, to model the
electrode-tissue interface in electroencephalography and baroreceptor dynamics.
This paper studies identifiability of generalised Randles circuit models, that
is, whether the model parameters can be estimated uniquely from the
input-output data. It is shown that generalised Randles circuit models are
structurally locally identifiable. The condition that makes the model structure
globally identifiable is then discussed. Finally, the estimation accuracy is
evaluated through extensive simulations
Explicitly correlated plane waves: Accelerating convergence in periodic wavefunction expansions
We present an investigation into the use of an explicitly correlated plane
wave basis for periodic wavefunction expansions at the level of second-order
M{\o}ller-Plesset perturbation theory (MP2). The convergence of the electronic
correlation energy with respect to the one-electron basis set is investigated
and compared to conventional MP2 theory in a finite homogeneous electron gas
model. In addition to the widely used Slater-type geminal correlation factor,
we also derive and investigate a novel correlation factor that we term
Yukawa-Coulomb. The Yukawa-Coulomb correlation factor is motivated by analytic
results for two electrons in a box and allows for a further improved
convergence of the correlation energies with respect to the employed basis set.
We find the combination of the infinitely delocalized plane waves and local
short-ranged geminals provides a complementary, and rapidly convergent basis
for the description of periodic wavefunctions. We hope that this approach will
expand the scope of discrete wavefunction expansions in periodic systems.Comment: 15 pages, 13 figure
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