22,169 research outputs found
On the spin-isospin decomposition of nuclear symmetry energy
The decomposition of nuclear symmetry energy into spin and isospin components
is discussed to elucidate the underlying properties of the NN bare interaction.
This investigation was carried out in the framework of the
Brueckner-Hartree-Fock theory of asymmetric nuclear matter with consistent two
and three body forces. It is shown the interplay among the various two body
channels in terms of isospin singlet and triplet components as well as spin
singlet and triplet ones. The broad range of baryon densities enables to study
the effects of three body force moving from low to high densities.Comment: 8 pages, 4 figure
Steps on current-voltage characteristics of a silicon quantum dot covered by natural oxide
Considering a double-barrier structure formed by a silicon quantum dot
covered by natural oxide with two metallic terminals, we derive simple
conditions for a step-like voltage-current curve. Due to standard chemical
properties, doping phosphorus atoms located in a certain domain of the dot form
geometrically parallel current channels. The height of the current step
typically equals to (1.2 pA)N, where N=0,1,2,3... is the number of doping atoms
inside the domain, and only negligibly depends on the actual position of the
dopants. The found conditions are feasible in experimentally available
structures.Comment: 4 pages, 3 figure
Comment on ``Scientific collaboration networks. II. Shortest paths, weighted networks, and centrality"
In this comment, we investigate a common used algorithm proposed by Newman
[M. E. J. Newman, Phys. Rev. E {\bf 64}, 016132(2001)] to calculate the
betweenness centrality for all vertices. The inaccurateness of Newman's
algorithm is pointed out and a corrected algorithm, also with O() time
complexity, is given. In addition, the comparison of calculating results for
these two algorithm aiming the protein interaction network of Yeast is shown.Comment: 3 pages, 2 tables, and 2 figure
The electric dipole moment of the neutron from 2+1 flavor lattice QCD
We compute the electric dipole moment d_n of the neutron from a fully
dynamical simulation of lattice QCD with 2+1 flavors of clover fermions and
nonvanishing theta term. The latter is rotated into the pseudoscalar density in
the fermionic action using the axial anomaly. To make the action real, the
vacuum angle theta is taken to be purely imaginary. The physical value of d_n
is obtained by analytic continuation. We find d_n = -3.8(2)(9) x 10^{-16}
[theta e cm], which, when combined with the experimental limit on d_n, leads to
the upper bound theta < 7.6 x 10^{-11}.Comment: 12 pages, 8 figures, matches PRL published versio
Conservation Properties in the Time-Dependent Hartree Fock Theory
We discuss the conservation of angular momentum in nuclear time-dependent
Hartree-Fock calculations for a numerical representation of wave functions and
potentials on a three-dimensional cartesian grid. Free rotation of a deformed
nucleus performs extremely well even for relatively coarse spatial grids. Heavy
ion collisions produce a highly excited compound system associated with
substantial nucleon emission. These emitted nucleons reach the bounds of the
numerical box which leads to a decrease of angular momentum. We discuss
strategies to distinguish the physically justified loss from numerical
artifacts.Comment: 4 page
Approximation Algorithms for the Capacitated Domination Problem
We consider the {\em Capacitated Domination} problem, which models a
service-requirement assignment scenario and is also a generalization of the
well-known {\em Dominating Set} problem. In this problem, given a graph with
three parameters defined on each vertex, namely cost, capacity, and demand, we
want to find an assignment of demands to vertices of least cost such that the
demand of each vertex is satisfied subject to the capacity constraint of each
vertex providing the service. In terms of polynomial time approximations, we
present logarithmic approximation algorithms with respect to different demand
assignment models for this problem on general graphs, which also establishes
the corresponding approximation results to the well-known approximations of the
traditional {\em Dominating Set} problem. Together with our previous work, this
closes the problem of generally approximating the optimal solution. On the
other hand, from the perspective of parameterization, we prove that this
problem is {\it W[1]}-hard when parameterized by a structure of the graph
called treewidth. Based on this hardness result, we present exact
fixed-parameter tractable algorithms when parameterized by treewidth and
maximum capacity of the vertices. This algorithm is further extended to obtain
pseudo-polynomial time approximation schemes for planar graphs
Quantum transport theory for nanostructures with Rashba spin-orbital interaction
We report on a general theory for analyzing quantum transport through devices
in the Metal-QD-Metal configuration where QD is a quantum dot or the device
scattering region which contains Rashba spin-orbital and electron-electron
interactions. The metal leads may or may not be ferromagnetic, they are assumed
to weakly couple to the QD region. Our theory is formulated by second
quantizing the Rashba spin-orbital interaction in spectral space (instead of
real space), and quantum transport is then analyzed within the Keldysh
nonequilibrium Green's function formalism. The Rashba interaction causes two
main effects to the Hamiltonian: (i) it gives rise to an extra spin-dependent
phase factor in the coupling matrix elements between the leads and the QD; (ii)
it gives rise to an inter-level spin-flip term but forbids any intra-level
spin-flips. Our formalism provides a starting point for analyzing many quantum
transport issues where spin-orbital effects are important. As an example, we
investigate transport properties of a Aharnov-Bohm ring in which a QD having
Rashba spin-orbital and e-e interactions is located in one arm of the ring. A
substantial spin-polarized conductance or current emerges in this device due to
a combined effect of a magnetic flux and the Rashba interaction. The direction
and strength of the spin-polarization are shown to be controllable by both the
magnetic flux and a gate voltage.Comment: 12 pages, 8 figure
Density Evolution in the New Modified Chaplygin Gas Model
In this paper, we have considered new modified Chaplygin gas (NMCG) model
which interpolates between radiation at early stage and CDM at late
stage. This model is regarded as a unification of dark energy and dark matter
(with general form of matter). We have derived the density parameters from the
equation of motion for the interaction between dark energy and dark matter.
Also we have studied the evolution of the various components of density
parameters.Comment: 6 Latex pages, 4 figures, RevTex styl
On the Threshold of Intractability
We study the computational complexity of the graph modification problems
Threshold Editing and Chain Editing, adding and deleting as few edges as
possible to transform the input into a threshold (or chain) graph. In this
article, we show that both problems are NP-complete, resolving a conjecture by
Natanzon, Shamir, and Sharan (Discrete Applied Mathematics, 113(1):109--128,
2001). On the positive side, we show the problem admits a quadratic vertex
kernel. Furthermore, we give a subexponential time parameterized algorithm
solving Threshold Editing in time,
making it one of relatively few natural problems in this complexity class on
general graphs. These results are of broader interest to the field of social
network analysis, where recent work of Brandes (ISAAC, 2014) posits that the
minimum edit distance to a threshold graph gives a good measure of consistency
for node centralities. Finally, we show that all our positive results extend to
the related problem of Chain Editing, as well as the completion and deletion
variants of both problems
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