37,980 research outputs found
The resistible effects of Coulomb interaction on nucleus-vapor phase coexistence
We explore the effects of Coulomb interaction upon the nuclear liquid vapor
phase transition. Because large nuclei (A>60) are metastable objects, phases,
phase coexistence, and phase transitions cannot be defined with any generality
and the analogy to liquid vapor is ill-posed for these heavy systems. However,
it is possible to account for the Coulomb interaction in the decay rates and
obtain the coexistence phase diagram for the corresponding uncharged system.Comment: 5 pages, 5 figure
Probability distribution of the order parameter in the directed percolation universality class
The probability distributions of the order parameter for two models in the
directed percolation universality class were evaluated. Monte Carlo simulations
have been performed for the one-dimensional generalized contact process and the
Domany-Kinzel cellular automaton. In both cases, the density of active sites
was chosen as the order parameter. The criticality of those models was obtained
by solely using the corresponding probability distribution function. It has
been shown that the present method, which has been successfully employed in
treating equilibrium systems, is indeed also useful in the study of
nonequilibrium phase transitions.Comment: 6 pages, 4 figure
Compound nuclear decay and the liquid to vapor phase transition: a physical picture
Analyses of multifragmentation in terms of the Fisher droplet model (FDM) and
the associated construction of a nuclear phase diagram bring forth the problem
of the actual existence of the nuclear vapor phase and the meaning of its
associated pressure. We present here a physical picture of fragment production
from excited nuclei that solves this problem and establishes the relationship
between the FDM and the standard compound nucleus decay rate for rare particles
emitted in first-chance decay. The compound thermal emission picture is
formally equivalent to a FDM-like equilibrium description and avoids the
problem of the vapor while also explaining the observation of Boltzmann-like
distribution of emission times. In this picture a simple Fermi gas thermometric
relation is naturally justified and verified in the fragment yields and time
scales. Low energy compound nucleus fragment yields scale according to the FDM
and lead to an estimate of the infinite symmetric nuclear matter critical
temperature between 18 and 27 MeV depending on the choice of the surface energy
coefficient of nuclear matter.Comment: Five page two column pages, four figures, submitted to Phys. Rev.
Critical adsorption and critical Casimir forces for geometrically structured confinements
We study the behavior of fluids, confined by geometrically structured
substrates, upon approaching a critical point at T = Tc in their bulk phase
diagram. As generic substrate structures periodic arrays of wedges and ridges
are considered. Based on general renormalization group arguments we calculate,
within mean field approximation, the universal scaling functions for order
parameter profiles of a fluid close to a single structured substrate and
discuss the decay of its spatial variation into the bulk. We compare the excess
adsorption at corrugated substrates with the one at planar walls. The
confinement of a critical fluid by two walls generates effective critical
Casimir forces between them. We calculate corresponding universal scaling
functions for the normal critical Casimir force between a flat and a
geometrically structured substrate as well as the lateral critical Casimir
force between two identically patterned substrates.Comment: 25 pages, 21 figure
Pulsed versus DC I-V characteristics of resistive manganites
We report on pulsed and DC I-V characteristics of polycrystalline samples of
three charge-ordered manganites, Pr_{2/3}Ca_{1/3}MnO_3, Pr_{1/2}Ca_{1/2}MnO_3,
Bi_{1/2}Sr_{1/2}MnO_3 and of a double-perovskite Sr_2MnReO_6, in a temperature
range where their ohmic resistivity obeys the Efros-Shklovskii variable range
hopping relation. For all samples, the DC I(V) exhibits at high currents
negative differential resistance and hysteresis, which mask a perfectly ohmic
or a moderately nonohmic conductivity obtained by pulsed measurements. This
demonstrates that the widely used DC I-V measurements are usually misleading.Comment: 6 pages, 4 figures. Accepted for publication to AP
Inter-grain tunneling in the half-metallic double-perovskites SrBB'O (BB'-- FeMo, FeRe, CrMo, CrW, CrRe
The zero-field conductivities () of the polycrystaline title
materials, are governed by inter-grain transport. In the majority of cases
their (T) can be described by the "fluctuation induced tunneling"
model. Analysis of the results in terms of this model reveals two remarkable
features: 1. For \emph{all} SrFeMoO samples of various microstructures,
the tunneling constant (barrier width inverse decay-length of the
wave-function) is 2, indicating the existence of an intrinsic insulating
boundary layer with a well defined electronic (and magnetic) structure. 2. The
tunneling constant for \emph{all} cold-pressed samples decreases linearly with
increasing magnetic-moment/formula-unit.Comment: 10 pages, 2 tables, 3 figure
2d frustrated Ising model with four phases
In this paper we consider a 2d random Ising system on a square lattice with
nearest neighbour interactions. The disorder is short range correlated and
asymmetry between the vertical and the horizontal direction is admitted. More
precisely, the vertical bonds are supposed to be non random while the
horizontal bonds alternate: one row of all non random horizontal bonds is
followed by one row where they are independent dichotomic random variables. We
solve the model using an approximate approach that replace the quenched average
with an annealed average under the constraint that the number of frustrated
plaquettes is keep fixed and equals that of the true system. The surprising
fact is that for some choices of the parameters of the model there are three
second order phase transitions separating four different phases:
antiferromagnetic, glassy-like, ferromagnetic and paramagnetic.Comment: 17 pages, Plain TeX, uses Harvmac.tex, 4 ps figures, submitted to
Physical Review
Plume mapping and isotopic characterisation of anthropogenic methane sources
Methane stable isotope analysis, coupled with mole fraction measurement, has been used to link isotopic signature to methane emissions from landfill sites, coal mines and gas leaks in the United Kingdom. A mobile Picarro G2301 CRDS (Cavity Ring-Down Spectroscopy) analyser was installed on a vehicle, together with an anemometer and GPS receiver, to measure atmospheric methane mole fractions and their relative location while driving at speeds up to 80 kph. In targeted areas, when the methane plume was intercepted, air samples were collected in Tedlar bags, for delta C-13-CH4 isotopic analysis by CF-GC-IRMS (Continuous Flow Gas Chromatography-Isotope Ratio Mass Spectrometry). This method provides high precision isotopic values, determining delta C-13-CH4 to +/- 0.05 per mil. The bulk signature of the methane plume into the atmosphere from the whole source area was obtained by Keeling plot analysis, and a delta C-13 -CH4 signature, with the relative uncertainty, allocated to each methane source investigated. Both landfill and natural gas emissions in SE England have tightly constrained isotopic signatures. The averaged delta C-13-CH4 for landfill sites is -58 +/- 3%o. The delta C-13-CH4 signature for gas leaks is also fairly constant around -36 +/- 2 parts per thousand, a value characteristic of homogenised North Sea supply. In contrast, signatures for coal mines in N. England and Wales fall in a range of -51.2 +/- 0.3 parts per thousand to 30.9 +/- 1.4 parts per thousand, but can be tightly constrained by region. The study demonstrates that CRDS-based mobile methane measurement coupled with off-line high precision isotopic analysis of plume samples is an efficient way of characterising methane sources. It shows that iiotopic measurements allow type identification, and possible location of previously unknown methane sources. In modelling studies this measurement provides an independent constraint to determine the contributions of different sources to the regional methane budget and in the verification of inventory source distribution. (C) 2015 Elsevier Ltd. All rights reserved
Random Vibrational Networks and Renormalization Group
We consider the properties of vibrational dynamics on random networks, with
random masses and spring constants. The localization properties of the
eigenstates contrast greatly with the Laplacian case on these networks. We
introduce several real-space renormalization techniques which can be used to
describe this dynamics on general networks, drawing on strong disorder
techniques developed for regular lattices. The renormalization group is capable
of elucidating the localization properties, and provides, even for specific
network instances, a fast approximation technique for determining the spectra
which compares well with exact results.Comment: 4 pages, 3 figure
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