2,175 research outputs found
Using molecular similarity to construct accurate semiempirical electron structure theories
Ab initio electronic structure methods give accurate results for small
systems, but do not scale well to large systems. Chemical insight tells us that
molecular functional groups will behave approximately the same way in all
molecules, large or small. This molecular similarity is exploited in
semiempirical methods, which couple simple electronic structure theories with
parameters for the transferable characteristics of functional groups. We propse
that high-level calculations on small molecules provide a rich source of
parametrization data. In principle, we can select a functional group, generate
a large amount of ab initio data on the group in various small-molecule
environments, and "mine" this data to build a sophisticated model for the
group's behavior in large molecules. This work details such a model for
electron correlation: a semiempirical, subsystem-based correlation functional
that predicts a subsystem's two-electron density as a functional of its
one-electron density. This model is demonstrated on two small systems: chains
of linear, minimal-basis (H-H)5, treated as a sum of four overlapping (H-H)2
subsystems; and the aldehyde group of a set of HOC-R molecules. The results
provide an initial demonstration of the feasibility of this approach.Comment: The following article appeared in the Journal of Chemical Physics,
121 (12), 5635-5645 (2004) and may be found at http://jcp.aip.org
A Thousand and One Nova Outbursts
Multicycle nova evolution models have been calculated over the past twenty
years, the number being limited by numerical constraints. Here we present a
long-term evolution code that enables a continuous calculation through an
unlimited number of nova cycles for an unlimited evolution time, even up to (or
exceeding) a Hubble time. Starting with two sets of the three independent nova
parameters -- the white dwarf mass, the temperature of its isothermal core, and
the rate of mass transfer on to it -- we have followed the evolution of two
models, with initial masses of 1 and 0.65 solar masses, accretion rates
(constant throughout each calculation) of 1e-11 and 1e-9 solar-masses/yr, and
relatively high initial temperatures (as they are likely to be at the onset of
the outburst phase), through over 1000 and over 3000 cycles, respectively. The
results show that although on the short-term consecutive outbursts are almost
identical, on the long-term scale the characteristics change. This is mainly
due to the changing core temperature, which decreases very similarly to that of
a cooling white dwarf for a time, but at a slower rate thereafter. As the white
dwarf's mass continually decreases, since both models lose more mass than they
accrete, the central pressure decreases accordingly. The outbursts on the
massive white dwarf change gradually from fast to moderately fast, and the
other characteristics (velocity, abundance ratios, isotopic ratios) change,
too. Very slowly, a steady state is reached, where all characteristics, both in
quiescence and in outburst, remain almost constant. For the less massive white
dwarf accreting at a high rate, outbursts are similar throughout the evolution.Comment: To be published in MNRA
Quantum lithography by coherent control of classical light pulses
The smallest spot in optical lithography and microscopy is generally limited
by diffraction. Quantum lithography, which utilizes interference between groups
of N entangled photons, was recently proposed to beat the diffraction limit by
a factor N. Here we propose a simple method to obtain N photons interference
with classical pulses that excite a narrow multiphoton transition, thus
shifting the "quantum weight" from the electromagnetic field to the
lithographic material. We show how a practical complete lithographic scheme can
be developed and demonstrate the underlying principles experimentally by
two-photon interference in atomic Rubidium, to obtain focal spots that beat the
diffraction limit by a factor of 2.Comment: 6 pages, 4 figures, Submitted to Opt. Expres
WISeREP - An Interactive Supernova Data Repository
We have entered an era of massive data sets in astronomy. In particular, the
number of supernova (SN) discoveries and classifications has substantially
increased over the years from few tens to thousands per year. It is no longer
the case that observations of a few prototypical events encapsulate most
spectroscopic information about SNe, motivating the development of modern tools
to collect, archive, organize and distribute spectra in general, and SN spectra
in particular. For this reason we have developed the Weizmann Interactive
Supernova data REPository - WISeREP - an SQL-based database (DB) with an
interactive web-based graphical interface. The system serves as an archive of
high quality SN spectra, including both historical (legacy) data as well as
data that is accumulated by ongoing modern programs. The archive provides
information about objects, their spectra, and related meta-data. Utilizing
interactive plots, we provide a graphical interface to visualize data, perform
line identification of the major relevant species, determine object redshifts,
classify SNe and measure expansion velocities. Guest users may view and
download spectra or other data that have been placed in the public domain.
Registered users may also view and download data that are proprietary to
specific programs with which they are associated. The DB currently holds >8000
spectra, of which >5000 are public; the latter include published spectra from
the Palomar Transient Factory, all of the SUSPECT archive, the
Caltech-Core-Collapse Program, the CfA SN spectra archive and published spectra
from the UC Berkeley SNDB repository. It offers an efficient and convenient way
to archive data and share it with colleagues, and we expect that data stored in
this way will be easy to access, increasing its visibility, usefulness and
scientific impact.Comment: To be published in PASP. WISeREP:
http://www.weizmann.ac.il/astrophysics/wiserep
Quantum critical origin of the superconducting dome in SrTiO
We investigate the origin of superconductivity in doped SrTiO (STO) using
a combination of density functional and strong coupling theories within the
framework of quantum criticality. Our density functional calculations of the
ferroelectric soft mode frequency as a function of doping reveal a crossover
from quantum paraelectric to ferroelectric behavior at a doping level
coincident with the experimentally observed top of the superconducting dome.
Based on this finding, we explore a model in which the superconductivity in STO
is enabled by its proximity to the ferroelectric quantum critical point and the
soft mode fluctuations provide the pairing interaction on introduction of
carriers. Within our model, the low doping limit of the superconducting dome is
explained by the emergence of the Fermi surface, and the high doping limit by
departure from the quantum critical regime. We predict that the highest
critical temperature will increase and shift to lower carrier doping with
increasing O isotope substitution, a scenario that is experimentally
verifiable.Comment: 4 pages + supplemental, 3 + 2 figure
Ghost imaging with a single detector
We experimentally demonstrate pseudothermal ghost imaging and ghost
diffraction using only a single single-pixel detector. We achieve this by
replacing the high resolution detector of the reference beam with a computation
of the propagating field, following a recent proposal by Shapiro [J. H.
Shapiro, arXiv:0807.2614 (2008)]. Since only a single detector is used, this
provides an experimental evidence that pseudothermal ghost imaging does not
rely on non-local quantum correlations. In addition, we show the
depth-resolving capability of this ghost imaging technique.Comment: See video at http://www.weizmann.ac.il/home/feori/Misc.html Comments
are welcom
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