13,560 research outputs found
Characteristics of light charged particle emission in the ternary fission of 250Cf and 252Cf at different excitation energies
The emission probabilities and the energy distributions of tritons, α and ^6He particles emitted in the spontaneous ternary fission (zero excitation energy) of ^250Cf and ^252Cf and in the cold neutron induced fission (excitation energy ≈ 6.5 MeV) of ^249Cf and 251Cf are determined. The particle identification was done with suited ΔE-E telescope detectors, at the IRMM (Geel, Belgium) for the spontaneous fission and at the ILL (Grenoble, France) for the neutron induced fission measurements. Hence particle emission characteristics of the fissioning systems ^250Cf and ^252Cf are obtained at zero and at about 6.5 MeV excitation energies. While the triton emission probability is hardly influenced by the excitation energy, the ^4He and ^6He emission probability in spontaneous fission is higher than for neutron induced fission. This can be explained by the strong influence of the cluster preformation probability on the ternary particle emission probability
Power dissipation for systems with junctions of multiple quantum wires
We study power dissipation for systems of multiple quantum wires meeting at a
junction, in terms of a current splitting matrix (M) describing the junction.
We present a unified framework for studying dissipation for wires with either
interacting electrons (i.e., Tomonaga-Luttinger liquid wires with Fermi liquid
leads) or non-interacting electrons. We show that for a given matrix M, the
eigenvalues of M^T M characterize the dissipation, and the eigenvectors
identify the combinations of bias voltages which need to be applied to the
different wires in order to maximize the dissipation associated with the
junction. We use our analysis to propose and study some microscopic models of a
dissipative junction which employ the edge states of a quantum Hall liquid.
These models realize some specific forms of the M-matrix whose entries depends
on the tunneling amplitudes between the different edges.Comment: 9 pages, 4 figures; made several minor changes; this is the published
versio
The Domination Number of Grids
In this paper, we conclude the calculation of the domination number of all
grid graphs. Indeed, we prove Chang's conjecture saying that for
every , .Comment: 12 pages, 4 figure
Quenching across quantum critical points in periodic systems: dependence of scaling laws on periodicity
We study the quenching dynamics of a many-body system in one dimension
described by a Hamiltonian that has spatial periodicity. Specifically, we
consider a spin-1/2 chain with equal xx and yy couplings and subject to a
periodically varying magnetic field in the z direction or, equivalently, a
tight-binding model of spinless fermions with a periodic local chemical
potential, having period 2q, where q is a natural number. For a linear quench
of the magnetic field strength (or potential strength) at rate 1/\tau across a
quantum critical point, we find that the density of defects thereby produced
scales as 1/\tau^{q/(q+1)}, deviating from the 1/\sqrt{\tau} scaling that is
ubiquitous to a range of systems. We analyze this behavior by mapping the
low-energy physics of the system to a set of fermionic two-level systems
labeled by the lattice momentum k undergoing a non-linear quench as well as by
performing numerical simulations. We also find that if the magnetic field is a
superposition of different periods, the power law depends only on the smallest
period for very large values of \tau although it may exhibit a cross-over at
intermediate values of \tau. Finally, for the case where a zz coupling is also
present in the spin chain, or equivalently, where interactions are present in
the fermionic system, we argue that the power associated with the scaling law
depends on a combination of q and interaction strength.Comment: 13 pages including 11 figure
Glassy transition and metastability in four-spin Ising model
Using Monte Carlo simulations we show that the three-dimensional Ising model
with four-spin (plaquette) interactions has some characteristic glassy
features. The model dynamically generates diverging energy barriers, which give
rise to slow dynamics at low temperature. Moreover, in a certain temperature
range the model possesses a metastable (supercooled liquid) phase, which is
presumably supported by certain entropy barriers. Although extremely strong,
metastability in our model is only a finite-size effect and sufficiently large
droplets of stable phase divert evolution of the system toward the stable
phase. Thus, the glassy transitions in this model is a dynamic transition,
preceded by a pronounced peak in the specific heat.Comment: extensively revised, with further simulations of metastability
properties, response to referees tactfully remove
Peristaltic Transport of a Couple Stress Fluid: Some Applications to Hemodynamics
The present paper deals with a theoretical investigation of the peristaltic
transport of a couple stress fluid in a porous channel. The study is motivated
towards the physiological flow of blood in the micro-circulatory system, by
taking account of the particle size effect. The velocity, pressure gradient,
stream function and frictional force of blood are investigated, when the
Reynolds number is small and the wavelength is large, by using appropriate
analytical and numerical methods. Effects of different physical parameters
reflecting porosity, Darcy number, couple stress parameter as well as amplitude
ratio on velocity profiles, pumping action and frictional force, streamlines
pattern and trapping of blood are studied with particular emphasis. The
computational results are presented in graphical form. The results are found to
be in good agreement with those of Shapiro et. al \cite{r25} that was carried
out for a non-porous channel in the absence of couple stress effect. The
present study puts forward an important observation that for peristaltic
transport of a couple stress fluid during free pumping when the couple stress
effect of the fluid/Darcy permeability of the medium, flow reversal can be
controlled to a considerable extent. Also by reducing the permeability it is
possible to avoid the occurrence of trapping phenomenon
Gravastar Solutions with Continuous Pressures and Equation of State
We study the gravitational vacuum star (gravastar) configuration as proposed
by other authors in a model where the interior de Sitter spacetime segment is
continuously extended to the exterior Schwarzschild spacetime. The multilayered
structure in previous papers is replaced by a continuous stress-energy tensor
at the price of introducing anisotropy in the (fluid) model of the gravastar.
Either with an ansatz for the equation of state connecting the radial and
tangential pressure or with a calculated equation of state with
non-homogeneous energy/fluid density, solutions are obtained which in all
aspects satisfy the conditions expected for an anisotropic gravastar. Certain
energy conditions have been shown to be obeyed and a polytropic equation of
state has been derived. Stability of the solution with respect to possible
axial perturbation is shown to hold.Comment: 19 pages, 9 figures. Latest version contains new and updated
references along with some clarifying remarks in the stability analysi
Efficacy of Organophosphorus Derivatives Containing Chalcones/Chalcone Semicarbazones Against Fungal Pathogens of Sugarcane
Ten newly synthesized organophosphorus derivatives containing substituted chalcones and
substituted chalcone semicarbazones were tested for their antifungal efficacy against Colletotrichum
falcatum, Fusarium oxysporum, Curvularia pallescens (all sugarcane pathogens). The O,O-diethylphosphate derivatives containing 2-chlorochalcone and 2-chlorochalcone semicarbazone exhibited 70-85% mycelial inhibition against all the test fungi at 1000 ppm. The screening results were correlated with structural features of the tested compounds
Hysteresis loops of Co-Pt perpendicular magnetic multilayers
We develop a phenomenological model to study magnetic hysteresis in two
samples designed as possible perpendicular recording media. A stochastic
cellular automata model captures cooperative behavior in the nucleation of
magnetic domains. We show how this simple model turns broad hysteresis loops
into loops with sharp drops like those observed in these samples, and explains
their unusual features. We also present, and experimentally verify, predictions
of this model, and suggest how insights from this model may apply more
generally.Comment: 4.5 pages, 5 figure
Renormalization group study of the conductances of interacting quantum wire systems with different geometries
We examine the effect of interactions between the electrons on the
conductances of some systems of quantum wires with different geometries. The
systems include a wire with a stub in the middle, a wire containing a ring
which can enclose a magnetic flux, and a system of four wires which are
connected in the middle through a fifth wire. Each of the wires is taken to be
a weakly interacting Tomonaga-Luttinger liquid, and scattering matrices are
introduced at all the junctions. Using a renormalization group method developed
recently for studying the flow of scattering matrices for interacting systems
in one dimension, we compute the conductances of these systems as functions of
the temperature and the wire lengths. We present results for all three regimes
of interest, namely, high, intermediate and low temperature. These correspond
respectively to the thermal coherence length being smaller than, comparable to
and larger than the smallest wire length in the different systems, i.e., the
length of the stub or each arm of the ring or the fifth wire. The
renormalization group procedure and the formulae used to compute the
conductances are different in the three regimes. We present a
phenomenologically motivated formalism for studying the conductances in the
intermediate regime where there is only partial coherence. At low temperatures,
we study the line shapes of the conductances versus the electron energy near
some of the resonances; the widths of the resonances go to zero with decreasing
temperature. Our results show that the conductances of various systems of
experimental interest depend on the temperature and lengths in a non-trivial
way when interactions are taken into account.Comment: Revtex, 17 pages including 15 figure
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