1,127 research outputs found
A condition for first order phase transitions in quantum mechanical tunneling models
A criterion is derived for the determination of parameter domains of first
order phase transitions in quantum mechanical tunneling models. The criterion
is tested by application to various models, in particular to some which have
been used recently to explore spin tunneling in macroscopic particles. In each
case agreement is found with previously heuristically determined domains.Comment: 13 pages, 5 figure
Cluster-mining: An approach for determining core structures of metallic nanoparticles from atomic pair distribution function data
We present a novel approach for finding and evaluating structural models of
small metallic nanoparticles. Rather than fitting a single model with many
degrees of freedom, the approach algorithmically builds libraries of
nanoparticle clusters from multiple structural motifs, and individually fits
them to experimental PDFs. Each cluster-fit is highly constrained. The
approach, called cluster-mining, returns all candidate structure models that
are consistent with the data as measured by a goodness of fit. It is highly
automated, easy to use, and yields models that are more physically realistic
and result in better agreement to the data than models based on cubic
close-packed crystallographic cores, often reported in the literature for
metallic nanoparticles
Finite Temperature Theory of Metastable Anharmonic Potentials
The decay rate for a particle in a metastable cubic potential is investigated
in the quantum regime by the Euclidean path integral method in semiclassical
approximation. The imaginary time formalism allows one to monitor the system as
a function of temperature. The family of classical paths, saddle points for the
action, is derived in terms of Jacobian elliptic functions whose periodicity
sets the energy-temperature correspondence. The period of the classical
oscillations varies monotonically with the energy up to the sphaleron, pointing
to a smooth crossover from the quantum to the activated regime. The softening
of the quantum fluctuation spectrum is evaluated analytically by the theory of
the functional determinants and computed at low up to the crossover. In
particular, the negative eigenvalue, causing an imaginary contribution to the
partition function, is studied in detail by solving the Lam\`{e} equation which
governs the fluctuation spectrum. For a heavvy particle mass, the decay rate
shows a remarkable temperature dependence mainly ascribable to a low lying soft
mode and, approaching the crossover, it increases by a factor five over the
predictions of the zero temperature theory. Just beyond the peak value, the
classical Arrhenius behavior takes over. A similar trend is found studying the
quartic metastable potential but the lifetime of the latter is longer by a
factor ten than in a cubic potential with same parameters. Some formal
analogies with noise-induced transitions in classically activated metastable
systems are discussed.Comment: European Physical Journal B EDP Sciences, Societ`a Italiana di
Fisica, Springer-Verlag 200
Recommended from our members
Identifying Sources of Children’s Consumption of Junk Food in Boston After-School Programs, April–May 2011
Introduction: Little is known about how the nutrition environment in after-school settings may affect children’s dietary intake. We measured the nutritional quality of after-school snacks provided by programs participating in the National School Lunch Program or the Child and Adult Care Food Program and compared them with snacks brought from home or purchased elsewhere (nonprogram snacks). We quantified the effect of nonprogram snacks on the dietary intake of children who also received program-provided snacks during after-school time. Our study objective was to determine how different sources of snacks affect children’s snack consumption in after-school settings. Methods: We recorded snacks served to and brought in by 298 children in 18 after-school programs in Boston, Massachusetts, on 5 program days in April and May 2011. We measured children’s snack consumption on 2 program days using a validated observation protocol. We then calculated within-child change-in-change models to estimate the effect of nonprogram snacks on children’s dietary intake after school. Results: Nonprogram snacks contained more sugary beverages and candy than program-provided snacks. Having a nonprogram snack was associated with significantly higher consumption of total calories (+114.7 kcal, P < .001), sugar-sweetened beverages (+0.5 oz, P = .01), desserts (+0.3 servings, P < .001), and foods with added sugars (+0.5 servings; P < .001) during the snack period. Conclusion: On days when children brought their own after-school snack, they consumed more salty and sugary foods and nearly twice as many calories than on days when they consumed only program-provided snacks. Policy strategies limiting nonprogram snacks or setting nutritional standards for them in after-school settings should be explored further as a way to promote child health
Volume stabilization in a warped flux compactification model
We investigate the stability of the extra dimensions in a warped, codimension
two braneworld that is based upon an Einstein-Maxwell-dilaton theory with a
non-vanishing scalar field potential. The braneworld solution has two 3-branes,
which are located at the positions of the conical singularities. For this type
of brane solution the relative positions of the branes (the shape modulus) is
determined via the tension-deficit relations, if the brane tensions are fixed.
However, the volume of the extra dimensions (the volume modulus) is not fixed
in the context of the classical theory, implying we should take quantum
corrections into account. Hence, we discuss the one-loop effective potential of
the volume modulus for a massless, minimally coupled scalar field.Comment: 25 pages, 8 figures, typos correcte
Vacuum energy in a spherically symmetric background field
The vacuum energy of a scalar field in a spherically symmetric background
field is considered. It is expressed through the Jost function of the
corresponding scattering problem. The renormalization is discussed in detail
and performed using the uniform asymptotic expansion of the Jost function. The
method is demonstrated in a simple explicit example.Comment: 14 pages, 1 Postscript figur
Spectral Energy Distributions of Local Luminous And Ultraluminous Infrared Galaxies
Luminous and ultraluminous infrared galaxies ((U)LIRGs) are the most extreme
star forming galaxies in the universe. The local (U)LIRGs provide a unique
opportunity to study their multi-wavelength properties in detail for comparison
to their more numerous counterparts at high redshifts. We present common large
aperture photometry at radio through X-ray wavelengths, and spectral energy
distributions (SEDs) for a sample of 53 nearby LIRGs and 11 ULIRGs spanning log
(LIR/Lsun) = 11.14-12.57 from the flux-limited Great Observatories All-sky LIRG
Survey (GOALS). The SEDs for all objects are similar in that they show a broad,
thermal stellar peak and a dominant FIR thermal dust peak, where nuLnu(60um) /
nuLnu(V) increases from ~2-30 with increasing LIR. When normalized at
IRAS-60um, the largest range in the luminosity ratio,
R(lambda)=log[nuLnu(lambda)/nuLnu(60um)] observed over the full sample is seen
in the Hard X-rays (HX=2-10 keV). A small range is found in the Radio (1.4GHz),
where the mean ratio is largest. Total infrared luminosities, LIR(8-1000um),
dust temperatures, and dust masses were computed from fitting thermal dust
emission modified blackbodies to the mid-infrared (MIR) through submillimeter
SEDs. The new results reflect an overall ~0.02 dex lower luminosity than the
original IRAS values. Total stellar masses were computed by fitting stellar
population synthesis models to the observed near-infrared (NIR) through
ultraviolet (UV) SEDs. Mean stellar masses are found to be log(M/Msun) =
10.79+/-0.40. Star formation rates have been determined from the infrared
(SFR_IR~45Msun/yr) and from the monochromatic UV luminosities
(SFR_UV~1.3Msun/yr), respectively. Multiwavelength AGN indicators have be used
to select putative AGN: about 60% of the ULIRGs would have been classified as
an AGN by at least one of the selection criteria.Comment: 39 pages, including 12 figures and 11 tables; accepted for
publication in ApJ
- …