7,132 research outputs found
Finite Amplitude Method for Charge-Changing Transitions in Axially-Deformed Nuclei
We describe and apply a version of the finite amplitude method for obtaining
the charge-changing nuclear response in the quasiparticle random phase
approximation. The method is suitable for calculating strength functions and
beta-decay rates, both allowed and forbidden, in axially-deformed open-shell
nuclei. We demonstrate the speed and versatility of the code through a
preliminary examination of the effects of tensor terms in Skyrme functionals on
beta decay in a set of spherical and deformed open-shell nuclei. Like the
isoscalar pairing interaction, the tensor terms systematically increase allowed
beta-decay rates. This finding generalizes previous work in semimagic nuclei
and points to the need for a comprehensive study of time-odd terms in nuclear
density functionals.Comment: 11 pages, 8 figures, submitted to Physical Review
Self-consistent Skyrme QRPA for use in axially-symmetric nuclei of arbitrary mass
We describe a new implementation of the quasiparticle random phase
approximation (QRPA) in axially-symmetric deformed nuclei with Skyrme and
volume-pairing energy-density functionals. After using a variety of tests to
demonstrate the accuracy of the code in ^{24,26}Mg and ^{16}O, we report the
first fully self-consistent application of the Skyrme QRPA to a heavy deformed
nucleus, calculating strength distributions for several K^pi in ^{172}Yb. We
present energy-weighted sums, properties of gamma-vibrational and low-energy
K^pi=0^+ states, and the complete isovector E1 strength function. The QRPA
calculation reproduces the properties of the low-lying 2^+ states as well or
better than it typically does in spherical nuclei.Comment: 5 pages, 6 figure
Neutrino capture by r-process waiting-point nuclei
We use the Quasiparticle Random Phase Approximation to include the effects of
low-lying Gamow-Teller and first forbidden strength in neutrino capture by very
neutron-rich nuclei with N = 50, 82, or 126. For electron neutrinos in what is
currently considered the most likely r-process site the capture cross sections
are two or more times previous estimates. We briefly discuss the reliability of
our calculations and their implications for nucleosynthesis.Comment: 9 pages, 4 figure
Changes in r-process abundances at late times
We explore changes in abundance patterns that occur late in the r process. As
the neutrons available for capture begin to disappear, a quasiequilibrium
funnel shifts material into the large peaks at A=130 and A=195, and into the
rare-earth "bump" at A=160. A bit later, after the free-neutron abundance has
dropped and beta-decay has begun to compete seriously with neutron capture, the
peaks can widen. The degree of widening depends largely on neutron-capture
rates near closed neutron shells and relatively close to stability. We identify
particular nuclei the capture rates of which should be examined experimentally,
perhaps at a radioactive beam facility.Comment: 8 pages, 14 figures included in tex
High-Dimensional Inference with the generalized Hopfield Model: Principal Component Analysis and Corrections
We consider the problem of inferring the interactions between a set of N
binary variables from the knowledge of their frequencies and pairwise
correlations. The inference framework is based on the Hopfield model, a special
case of the Ising model where the interaction matrix is defined through a set
of patterns in the variable space, and is of rank much smaller than N. We show
that Maximum Lik elihood inference is deeply related to Principal Component
Analysis when the amp litude of the pattern components, xi, is negligible
compared to N^1/2. Using techniques from statistical mechanics, we calculate
the corrections to the patterns to the first order in xi/N^1/2. We stress that
it is important to generalize the Hopfield model and include both attractive
and repulsive patterns, to correctly infer networks with sparse and strong
interactions. We present a simple geometrical criterion to decide how many
attractive and repulsive patterns should be considered as a function of the
sampling noise. We moreover discuss how many sampled configurations are
required for a good inference, as a function of the system size, N and of the
amplitude, xi. The inference approach is illustrated on synthetic and
biological data.Comment: Physical Review E: Statistical, Nonlinear, and Soft Matter Physics
(2011) to appea
Observation of quantum-Hall effect in gated epitaxial graphene grown on SiC (0001)
Epitaxial graphene films were formed on the Si-face of semi-insulating 4H-SiC
substrates by a high temperature sublimation process. A high-k gate stack on
epitaxial graphene is realized by inserting a fully oxidized nanometer thin
aluminum film as a seeding layer followed by an atomic-layer deposition
process. The electrical properties of epitaxial graphene films are sustained
after gate stack formation without significant degradation. At low
temperatures, the quantum-Hall effect in Hall resistance is observed along with
pronounced Shubnikov-de Hass oscillations in diagonal magneto-resistance of
gated epitaxial graphene on SiC (0001).Comment: 2 new references adde
Initial Sulfonylurea Use and Subsequent Insulin Therapy in Older Subjects with Type 2 Diabetes Mellitus
BACKGROUND: In type 2 diabetes mellitus (T2DM), progressive loss of beta cell function over time requires treatment intensification and eventually initiation of insulin for many patients. Relative to metformin, a greater rate of decline in beta cell function over time has been observed with sulfonylurea treatment. The present study examined the association between initial monotherapy with metformin or sulfonylurea and subsequent initiation of insulin in older subjects with T2DM. METHODS: In a retrospective cohort study using the GE electronic medical record database, eligible subjects with T2DM included those â„65Â years who received their first prescription of sulfonylurea or metformin as initial monotherapy between January 1, 2003 to December 31, 2008. The follow-up period lasted to the end of 2009 or the subjectâs latest data available. Insulin initiation was determined by prescription records. Logistic regression analysis evaluated the likelihood of insulin addition. A Cox regression model estimated time to initiation of insulin. Differences in baseline characteristics were controlled for using propensity score matching. RESULTS: Overall, 12,036 subjects were included in the analysis. Mean age was 75Â years and 50% were male. Subjects who initiated with sulfonylurea had a significantly (PÂ <Â 0.001) higher incidence of insulin addition (2.8% vs. 1.4%) compared to those initiated with metformin within 1Â year of follow-up. The likelihood of initiating insulin was higher in subjects initiated with sulfonylurea than with metformin (adjusted odds ratio 1.82, 95% confidence interval [CI] 1.40â2.38; PÂ <Â 0.001). Sulfonylurea use was also significantly associated with a shorter time to insulin use compared to metformin (adjusted hazards ratio 2.10, 95% CI 1.83â2.39; PÂ <Â 0.001). CONCLUSION: In a cohort of older subjects with T2DM initiating antihyperglycemic therapy, new users of sulfonylurea monotherapy were more likely to receive insulin therapy and received it earlier than those starting with metformin
AC Magnetotransport in Reentrant Insulating Phases of Two-dimensional Electrons near 1/5 and 1/3 Landau fillings
We have measured high frequency magnetotransport of a high quality
two-dimensional electron system (2DES) near the reentrant insulating phase
(RIP) at Landau fillings () between 1/5 and 2/9. The
magneto\textit{conductivity} in the RIP has resonant behavior around 150 MHz,
showing a \textit{peak} at 0.21. Our data support the interpretation
of the RIP as due to some pinned electron solid. We have also investigated a
narrowly confined 2DES recently found to have a RIP at 1/31/2 and we
have revealed features, not seen in DC transport, that suggest some intriguing
interplay between the 1/3 FQHE and RIP.Comment: 4 pages and 1 figure (amsart format), 16th International Conference
on High Magnetic Fields in Semiconductor Physics (SemiMag16), August 2-6,
2004, Tallahasse
Evidence for Two Different Solid Phases of Two Dimensional Electrons in High Magnetic Fields
We have performed RF spectroscopy on very high quality two dimensional
electron systems in the high magnetic field insulating phase, usually
associated with a Wigner solid (WS) pinned by disorder. We have found two
different resonances in the frequency dependent real diagonal conductivity
spectrum and we interpret them as coming from \textit{two} different pinned
solid phases (labeled as "WS-A" and "WS-B"). The resonance of WS-A is
observable for Landau level filling 2/9 (but absent around the
=1/5 fractional quantum Hall effect (FQHE)); it then \textit{crosses over}
for 0.18 to the different WS-B resonance which dominates the spectrum
at 0.125. Moreover, WS-A resonance is found to show dispersion with
respect to the size of transmission line, indicating that WS-A has a large
correlation length (exceeding 100 m); in contrast no such behavior
is found for WS-B. We suggest that quantum correlations such as those
responsible for FQHE may play an important role in giving rise to such
different solids.Comment: 4 pages, 3 figure
Configuration Mixing within the Energy Density Functional Formalism: Removing Spurious Contributions from Non-Diagonal Energy Kernels
Multi-reference calculations along the lines of the Generator Coordinate
Method or the restoration of broken symmetries within the nuclear Energy
Density Functional (EDF) framework are becoming a standard tool in nuclear
structure physics. These calculations rely on the extension of a
single-reference energy functional, of the Gogny or the Skyrme types, to
non-diagonal energy kernels. There is no rigorous constructive framework for
this extension so far. The commonly accepted way proceeds by formal analogy
with the expressions obtained when applying the generalized Wick theorem to the
non-diagonal matrix element of a Hamilton operator between two product states.
It is pointed out that this procedure is ill-defined when extended to EDF
calculations as the generalized Wick theorem is taken outside of its range of
applicability. In particular, such a procedure is responsible for the
appearance of spurious divergences and steps in multi-reference EDF energies,
as was recently observed in calculations restoring particle number or angular
momentum. In the present work, we give a formal analysis of the origin of this
problem for calculations with and without pairing, i.e. constructing the
density matrices from either Slater determinants or quasi-particle vacua. We
propose a correction to energy kernels that removes the divergences and steps,
and which is applicable to calculations based on any symmetry restoration or
generator coordinate. The method is formally illustrated for particle number
restoration and is specified to configuration mixing calculations based on
Slater determinants.Comment: 27 pages, 1 figure, accepted for publication in PR
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