2,088 research outputs found
Horava Gravity and Gravitons at a Conformal Point
Recently Horava proposed a renormalizable gravity theory with higher
derivatives by abandoning the Lorenz invariance in UV. Here, I study the Horava
model at , where an anisotropic Weyl symmetry exists in the UV
limit, in addition to the foliation-preserving diffeomorphism. By considering
linear perturbations around Minkowski vacuum, I show that the scalar graviton
mode is completely disappeared and only the usual tensor graviton modes remain
in the physical spectrum. The existence of the UV conformal symmetry is unique
to the theory with the detailed balance and it is quite probable that
be the UV fixed point. This situation is analogous to
, which is Lorentz invariant in the IR limit and is believed to be
the IR fixed point.Comment: Added comments and references, Accepted in GER
Quantum Vibrational Impurity Embedded in a One-dimensional Chain
We perform a fully quantum mechanical numerical calculation for the problem
of a single electron (or excitation) propagating in a N-site one-dimensional
chain in the presence of a single Holstein impurity. We compute the long-time
averaged probability for finding the electron on the impurity site as a
function of the nonlinearity parameter, defined in terms of the electron-phonon
coupling strength and the oscillator frequency. The results, in the
intermediate nonlinearity parameter range, differ substantially from the ones
obtained through the use of the discrete nonlinear Schroedinger equation, even
in the high-frequency regime.Comment: 8 pages, 4 figure
Electromagnetic properties of graphene junctions
A resonant chiral tunneling (CT) across a graphene junction (GJ) induced by
an external electromagnetic field (EF) is studied. Modulation of the electron
and hole wavefunction phases by the external EF during the CT
processes strongly impacts the CT directional diagram. Therefore the a.c.
transport characteristics of GJs depend on the EF polarization and frequency
considerably. The GJ shows great promises for various nanoelectronic
applications working in the THz diapason.Comment: 4 pages 3 figure
Two-particle localization and antiresonance in disordered spin and qubit chains
We show that, in a system with defects, two-particle states may experience
destructive quantum interference, or antiresonance. It prevents an excitation
localized on a defect from decaying even where the decay is allowed by energy
conservation. The system studied is a qubit chain or an equivalent spin chain
with an anisotropic () exchange coupling in a magnetic field. The chain
has a defect with an excess on-site energy. It corresponds to a qubit with the
level spacing different from other qubits. We show that, because of the
interaction between excitations, a single defect may lead to multiple localized
states. The energy spectra and localization lengths are found for
two-excitation states. The localization of excitations facilitates the
operation of a quantum computer. Analytical results for strongly anisotropic
coupling are confirmed by numerical studies.Comment: Updated version, 13 pages, 5 figures To appear in Phys. Rev. B (2003
Logistic regression for simulating damage occurrence on a fruit grading line
Many factors influence the incidence of mechanical damage in fruit handled on a grading line. This makes it difficult to address damage estimation from an analytical point of view. During fruit transfer from one element of a grading line to another, damage occurs as a combined effect of machinery roughness and the intrinsic susceptibility of fruit. This paper describes a method to estimate bruise probability by means of logistic regression, using data yielded by specific laboratory tests. Model accuracy was measured via the statistical significance of its parameters and its classification ability. The prediction model was then linked to a simulation model through which impacts and load levels, similar to those of real grading lines, could be generated. The simulation output sample size was determined to yield reliable estimations. The process makes it possible to derive a suitable line design and the type of fruit that should be handled to maintain bruise levels within European Union (EU) Standards. A real example with peaches was carried out with the aid of the software implementation SIMLIN®, developed by the authors and registered by Madrid Technical University. This kind of tool has been demanded by inter-professional associations and grading lines designers in recent year
Spontaneous Symmetry Breaking in Photonic Lattices: Theory and Experiment
We examine an example of spontaneous symmetry breaking in a double-well
waveguide with a symmetric potential. The ground state of the system beyond a
critical power becomes asymmetric. The effect is illustrated numerically, and
quantitatively analyzed via a Galerkin truncation that clearly shows the
bifurcation from a symmetric to an asymmetric steady state. This phenomenon is
also demonstrated experimentally when a probe beam is launched appropriately
into an optically induced photonic lattice in a photorefractive material.Comment: 4 pages, 3 figure
Functional output-controllability of time-invariant singular linear systems
In the space of finite-dimensional
singular linear continuous-time-invariant systems described in the form \begin{equation}\label{eq1}\left . \begin{array}{rl} E \dot x(t)&= Ax(t)+Bu(t)\\ y(t)&=Cx(t)\end{array}{\kern-1mm}\right \}\end{equation}
where , , , functional output-controllability character is considered. A simple test based in
the computation of the rank of a certain constant matrix that can be associated to the system is presentedPeer ReviewedPostprint (published version
Goos-H\"{a}nchen-like shifts for Dirac fermions in monolayer graphene barrier
We investigate the Goos-H\"{a}nchen-like shifts for Dirac fermions in
transmission through a monolayer graphene barrier. The lateral shifts, as the
functions of the barrier's width and the incidence angle, can be negative and
positive in Klein tunneling and classical motion, respectively. Due to their
relations to the transmission gap, the lateral shifts can be enhanced by the
transmission resonances when the incidence angle is less than the critical
angle for total reflection, while their magnitudes become only the order of
Fermi wavelength when the incidence angle is larger than the critical angle.
These tunable beam shifts can also be modulated by the height of potential
barrier and the induced gap, which gives rise to the applications in
graphene-based devices.Comment: 5 pages, 5 figure
Models of electron transport in single layer graphene
The main features of the conductivity of doped single layer graphene are
analyzed, and models for different scattering mechanisms are presented.Comment: 15 pages. Submitted to the Proceedings of the ULTI symposium on
Quantum Phenomena and Devices at Low Temperatures, Espoo, Finland, to be
published in the Journ. of Low. Temp. Phy
Uniqueness and Nondegeneracy of Ground States for in
We prove uniqueness of ground state solutions for the
nonlinear equation in , where
and for and for . Here denotes the fractional Laplacian
in one dimension. In particular, we generalize (by completely different
techniques) the specific uniqueness result obtained by Amick and Toland for
and in [Acta Math., \textbf{167} (1991), 107--126]. As a
technical key result in this paper, we show that the associated linearized
operator is nondegenerate;
i.\,e., its kernel satisfies .
This result about proves a spectral assumption, which plays a central
role for the stability of solitary waves and blowup analysis for nonlinear
dispersive PDEs with fractional Laplacians, such as the generalized
Benjamin-Ono (BO) and Benjamin-Bona-Mahony (BBM) water wave equations.Comment: 45 page
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