28,162 research outputs found
Exactly-solvable coupled-channel potential models of atom-atom magnetic Feshbach resonances from supersymmetric quantum mechanics
Starting from a system of radial Schr\"odinger equations with a vanishing
potential and finite threshold differences between the channels, a coupled exactly-solvable potential model is obtained with the help of a
single non-conservative supersymmetric transformation. The obtained potential
matrix, which subsumes a result obtained in the literature, has a compact
analytical form, as well as its Jost matrix. It depends on
unconstrained parameters and on one upper-bounded parameter, the factorization
energy. A detailed study of the model is done for the case: a
geometrical analysis of the zeros of the Jost-matrix determinant shows that the
model has 0, 1 or 2 bound states, and 0 or 1 resonance; the potential
parameters are explicitly expressed in terms of its bound-state energies, of
its resonance energy and width, or of the open-channel scattering length, which
solves schematic inverse problems. As a first physical application,
exactly-solvable atom-atom interaction potentials are constructed,
for cases where a magnetic Feshbach resonance interplays with a bound or
virtual state close to threshold, which results in a large background
scattering length.Comment: 19 pages, 15 figure
Restricted Wiedemann-Franz law and vanishing thermoelectric power in one-dimensional conductors
In one-dimensional (1D) conductors with linear E-k dispersion (Dirac systems)
intrabranch thermalization is favored by elastic electron-electron interaction
in contrast to electron systems with a nonlinear (parabolic) dispersion. We
show that under external electric fields or thermal gradients the carrier
populations of different branches, treated as Fermi gases, have different
temperatures as a consequence of self-consistent carrier-heat transport.
Specifically, in the presence of elastic phonon scattering, the Wiedemann-Franz
law is restricted to each branch with its specific temperature and is
characterized by twice the Lorenz number. In addition thermoelectric power
vanishes due to electron-hole symmetry, which is validated by experiment.Comment: 10 pages, 2 figure
The Pattern of Correlated X-ray Timing and Spectral Behavior in GRS 1915+105
From data obtained from the PCA in the 2-11 keV and 11-30.5 keV energy range,
GRS 1915+105 is seen during RXTE observations between 1996 May and October on
two separate branches in a hardness intensity diagram. On the hard branch, GRS
1915+105 exhibits narrow quasi-periodic oscillations ranging from 0.5 to 6 Hz
with . The QPOs are observed over intensities
ranging from about 6,000 to 20,000 counts s in the 2 - 12.5 keV energy
band, indicating a strong dependence on source intensity. Strong harmonics are
seen, especially, at lower frequencies. As the QPO frequency increases, the
harmonic feature weakens and disappears. On the soft branch, narrow QPOs are
absent and the low frequency component of the power density spectrum is
approximated by a power-law, with index for low count rates and
for high count rates (\gta 18000 cts/s). Occasionally, a broad
peaked feature in the 1-6 Hz frequency range is also observed on this branch.
The source was probably in the very high state similar to those of other black
hole candidates. Thermal-viscous instabilities in accretion disk models do not
predict the correlation of the narrow QPO frequency and luminosity unless the
fraction of luminosity from the disk decreases with the total luminosity.Comment: ApJ Lett accepte
Statistical Curse of the Second Half Rank
In competitions involving many participants running many races the final rank
is determined by the score of each participant, obtained by adding its ranks in
each individual race. The "Statistical Curse of the Second Half Rank" is the
observation that if the score of a participant is even modestly worse than the
middle score, then its final rank will be much worse (that is, much further
away from the middle rank) than might have been expected. We give an
explanation of this effect for the case of a large number of races using the
Central Limit Theorem. We present exact quantitative results in this limit and
demonstrate that the score probability distribution will be gaussian with
scores packing near the center. We also derive the final rank probability
distribution for the case of two races and we present some exact formulae
verified by numerical simulations for the case of three races. The variant in
which the worst result of each boat is dropped from its final score is also
analyzed and solved for the case of two races.Comment: 16 pages, 10 figure
Vibrational spectroscopy of H2+: precise evaluation of the Zeeman effect
We present an accurate computation of the g-factors of the hyperfine states
of the hydrogen molecular ion H2+. The results are in good agreement with
previous experiments, and can be tested further by rf spectroscopy. Their
implication for high-precision two-photon vibrational spectroscopy of H2+ is
also discussed. It is found that the most intense hyperfine components of
two-photon lines benefit from a very small Zeeman splitting
Measuring Extinction Curves of Lensing Galaxies
We critique the method of constructing extinction curves of lensing galaxies
using multiply imaged QSOs. If one of the two QSO images is lightly reddened or
if the dust along both sightlines has the same properties then the method works
well and produces an extinction curve for the lensing galaxy. These cases are
likely rare and hard to confirm. However, if the dust along each sightline has
different properties then the resulting curve is no longer a measurement of
extinction. Instead, it is a measurement of the difference between two
extinction curves. This "lens difference curve'' does contain information about
the dust properties, but extracting a meaningful extinction curve is not
possible without additional, currently unknown information. As a quantitative
example, we show that the combination of two Cardelli, Clayton, & Mathis (CCM)
type extinction curves having different values of R(V) will produce a CCM
extinction curve with a value of R(V) which is dependent on the individual R(V)
values and the ratio of V band extinctions. The resulting lens difference curve
is not an average of the dust along the two sightlines. We find that lens
difference curves with any value of R(V), even negative values, can be produced
by a combination of two reddened sightlines with different CCM extinction
curves with R(V) values consistent with Milky Way dust (2.1 < R(V) < 5.6). This
may explain extreme values of R(V) inferred by this method in previous studies.
But lens difference curves with more normal values of R(V) are just as likely
to be composed of two dust extinction curves with R(V) values different than
that of the lens difference curve. While it is not possible to determine the
individual extinction curves making up a lens difference curve, there is
information about a galaxy's dust contained in the lens difference curves.Comment: 15 pages, 4 figues, ApJ in pres
Joint density-functional theory for electronic structure of solvated systems
We introduce a new form of density functional theory for the {\em ab initio}
description of electronic systems in contact with a molecular liquid
environment. This theory rigorously joins an electron density-functional for
the electrons of a solute with a classical density-functional theory for the
liquid into a single variational principle for the free energy of the combined
system. A simple approximate functional predicts, without any fitting of
parameters to solvation data, solvation energies as well as state-of-the-art
quantum-chemical cavity approaches, which require such fitting.Comment: Fixed typos and minor updates to tex
- …