4,070 research outputs found
Non-degenerate colorings in the Brook's Theorem
Let and be two integers. We will call a proper coloring
of the graph a \textit{-nondegenerate}, if for any vertex of
with degree at least there are at least vertices of different colors
adjacent to it. In our work we prove the following result, which generalizes
Brook's Theorem. Let and be a graph without cliques on
vertices and the degree of any vertex in this graph is not greater than .
Then for every integer there is a proper -nondegenerate vertex
-coloring of , where During the primary proof,
some interesting corollaries are derived.Comment: 18 pages, 10 figure
Universal quantized spin-Hall conductance fluctuation in graphene
We report a theoretical investigation of quantized spin-Hall conductance
fluctuation of graphene devices in the diffusive regime. Two graphene models
that exhibit quantized spin-Hall effect (QSHE) are analyzed. Model-I is with
unitary symmetry under an external magnetic field but with zero
spin-orbit interaction, . Model-II is with symplectic symmetry where
B=0 but . Extensive numerical calculations indicate that the two
models have exactly the same universal QSHE conductance fluctuation value
regardless of the symmetry. Qualitatively different from the
conventional charge and spin universal conductance distributions, in the
presence of edge states the spin-Hall conductance shows an one-sided log-normal
distribution rather than a Gaussian distribution. Our results strongly suggest
that the quantized spin-Hall conductance fluctuation belongs to a new
universality class
Weakly nonlinear quantum transport: an exactly solvable model
We have studied the weakly non-linear quantum transport properties of a
two-dimensional quantum wire which can be solved exactly. The non-linear
transport coefficients have been calculated and interesting physical properties
revealed. In particular we found that as the incoming electron energy
approaches a resonant point given by energy , where the transport is
characterized by a complete reflection, the second order non-linear conductance
changes its sign. This has interesting implications to the current-voltage
characteristics. We have also investigated the establishment of the gauge
invariance condition. We found that for systems with a finite scattering
region, correction terms to the theoretical formalism are needed to preserve
the gauge invariance. These corrections were derived analytically for this
model.Comment: 15 pages, LaTeX, submitted to Phys. Rev.
Scales of Mass Generation for Quarks, Leptons and Majorana Neutrinos
We study 2 --> n inelastic fermion-(anti)fermion scattering into multiple
longitudinal weak gauge bosons and derive universal upper bounds on the scales
of fermion mass generation by imposing unitarity of the S-matrix. We place new
upper limits on the scales of fermion mass generation, independent of the
electroweak symmetry breaking scale. We find that the strongest 2 --> n limits
fall in a narrow range, 3-170 TeV (with n=2-24), depending on the observed
fermion masses.Comment: Phys. Rev. Lett.(in press), minor rewordin
Deconstruction and Elastic pi pi Scattering in Higgsless Models
We study elastic pion-pion scattering in global linear moose models and apply
the results to a variety of Higgsless models in flat and AdS space using the
Equivalence Theorem. In order to connect the global moose to Higgsless models,
we first introduce a block-spin transformation which corresponds, in the
continuum, to the freedom to perform coordinate transformations in the
Higgsless model. We show that it is possible to make an "f-flat" deconstruction
in which all of the f-constants f_j of the linear moose model are identical;
the phenomenologically relevant f-flat models are those in which the coupling
constants of the groups at either end of the moose are small - corresponding to
the global linear moose. In studying pion-pion scattering, we derive various
sum rules, including one analogous to the KSRF relation, and use them in
evaluating the low-energy and high-energy forms of the leading elastic partial
wave scattering amplitudes. We obtain elastic unitarity bounds as a function of
the mass of the lightest KK mode and discuss their physical significance.Comment: 33 pages, JHEP3. Minor typos correcte
Double Type-II Seesaw, Baryon Asymmetry and Dark Matter for Cosmic e^\pm Excesses
We construct a new realization of type-II seesaw for neutrino masses and
baryon asymmetry by extending the standard model with one light and two heavy
singlet scalars besides one Higgs triplet. The heavy singlets pick up small
vacuum expectation values to give a suppressed trilinear coupling between the
triplet and doublet Higgs bosons after the light singlet drives the spontaneous
breaking of lepton number. The Higgs triplet can thus remain light and be
accessible at the LHC. The lepton number conserving decays of the heavy
singlets can generate a lepton asymmetry stored in the Higgs triplet to account
for the matter-antimatter asymmetry in the Universe. We further introduce
stable gauge bosons from a hidden sector, which obtain masses and annihilate
into the Higgs triplet after spontaneous breaking of the associated non-Abelian
gauge symmetry. With Breit-Wigner enhancement, the stable gauge bosons can
simultaneously explain the relic density of dark matter and the cosmic
positron/electron excesses.Comment: 9 pages, 4 figures, minor rewording, final PRD version (in Press
New application of decomposition of U(1) gauge potential:Aharonov-Bohm effect and Anderson-Higgs mechanism
In this paper we study the Aharonov-Bohm (A-B) effect and Anderson-Higgs
mechanism in Ginzburg-Landau model of superconductors from the perspective of
the decomposition of U(1) gauge potential. By the Helmholtz theorem, we derive
exactly the expression of the transverse gauge potential in A-B
experiment, which is gauge-invariant and physical. For the case of a bulk
superconductor, we find that the gradient of the total phase field
provides the longitudinal component , which reflects the
Anderson-Higgs mechanism. For the case of a superconductor ring, the gradient
of the longitudinal phase field provides the longitudinal component
, while the transverse phase field produces
new physical effects such as the flux quantization inside a superconducting
ring.Comment: 6 pages, no figures, final version to appear in Modern Physics
Letters
Energetics of oxygen-octahedra rotations in perovskite oxides from first principles
We use first-principles methods to study oxygen-octahedra rotations in ABO3
perovskite oxides. We focus on the short-period, perfectly antiphase or
in-phase, tilt patterns that characterize most compounds and control their
physical (e.g., conductive, magnetic) properties. Based on an analytical form
of the relevant potential energy surface, we discuss the conditions for the
stability of polymorphs presenting different tilt patterns, and obtain
numerical results for a collection of thirty-five representative materials. Our
results reveal the mechanisms responsible for the frequent occurrence of a
particular structure that combines antiphase and in-phase rotations, i.e., the
orthorhombic Pbnm phase displayed by about half of all perovskite oxides and by
many non-oxidic perovskites. The Pbnm phase benefits from the simultaneous
occurrence of antiphase and in-phase tilt patterns that compete with each
other, but not as strongly as to be mutually exclusive. We also find that
secondary antipolar modes, involving the A cations, contribute to weaken the
competition between different tilts and play a key role in their coexistence.
Our results thus confirm and better explain previous observations for
particular compounds. Interestingly, we also find that strain effects, which
are known to be a major factor governing phase competition in related (e.g.,
ferroelectric) perovskite oxides, play no essential role as regards the
relative stability of different rotational polymorphs. Further, we discuss why
the Pbnm structure stops being the ground state in two opposite limits, for
large and small A cations, showing that very different effects become relevant
in each case. Our work thus provides a comprehensive discussion on these
all-important and abundant materials, which will be useful to better understand
existing compounds as well as to identify new strategies for materials
engineering
Direct tunneling through high- amorphous HfO: effects of chemical modification
We report first principles modeling of quantum tunneling through amorphous
HfO dielectric layer of metal-oxide-semiconductor (MOS) nanostructures in
the form of n-Si/HfO/Al. In particular we predict that chemically modifying
the amorphous HfO barrier by doping N and Al atoms in the middle region -
far from the two interfaces of the MOS structure, can reduce the
gate-to-channel tunnel leakage by more than one order of magnitude. Several
other types of modification are found to enhance tunneling or induce
substantial band bending in the Si, both are not desired from leakage point of
view. By analyzing transmission coefficients and projected density of states,
the microscopic physics of electron traversing the tunnel barrier with or
without impurity atoms in the high- dielectric is revealed.Comment: 5 pages, 5 figure
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