7,062 research outputs found
Generalized Galilean Algebras and Newtonian Gravity
The non-relativistic versions of the generalized Poincar\'{e} algebras and
generalized -Lorentz algebras are obtained. This non-relativistic algebras
are called, generalized Galilean algebras type I and type II and denoted by
and
respectively. Using a generalized In\"{o}n\"{u}--Wigner contraction procedure
we find that the generalized Galilean algebras type I can be obtained from the
generalized Galilean algebras type II. The -expansion procedure allows us to
find the algebra from the Newton--Hooke
algebra with central extension. The procedure developed in Ref. \cite{newton}
allow us to show that the non-relativistic limit of the five dimensional
Einstein--Chern--Simons gravity is given by a modified version of the Poisson
equation. The modification could be compatible with the effects of Dark Matter,
which leads us to think that Dark Matter can be interpreted as a
non-relativistic limit of Dark Energy.Comment: 16 pages, no figures in 755 (2016) 433-43
Dynamics of quartz tuning fork force sensors used in scanning probe microscopy
We have performed an experimental characterization of the dynamics of
oscillating quartz tuning forks which are being increasingly used in scanning
probe microscopy as force sensors. We show that tuning forks can be described
as a system of coupled oscillators. Nevertheless, this description requires the
knowledge of the elastic coupling constant between the prongs of the tuning
fork, which has not yet been measured. Therefore tuning forks have been usually
described within the single oscillator or the weakly coupled oscillators
approximation that neglects the coupling between the prongs. We propose three
different procedures to measure the elastic coupling constant: an
opto-mechanical method, a variation of the Cleveland method and a thermal noise
based method. We find that the coupling between the quartz tuning fork prongs
has a strong influence on the dynamics and the measured motion is in remarkable
agreement with a simple model of coupled harmonic oscillators. The precise
determination of the elastic coupling between the prongs of a tuning fork
allows to obtain a quantitative relation between the resonance frequency shift
and the force gradient acting at the free end of a tuning fork prong.Comment: 16 pages, 6 figures, 2 Table
First-Principle Description of Correlation Effects in Layered Materials
We present a first-principles description of anisotropic materials
characterized by having both weak (dispersion-like) and strong covalent bonds,
based on the Adiabatic--Connection Fluctuation--Dissipation Theorem within
Density Functional Theory. For hexagonal boron nitride the in-plane and out of
plane bonding as well as vibrational dynamics are well described both at
equilibrium and when the layers are pulled apart. Also bonding in covalent and
ionic solids is described. The formalism allows to ping-down the deficiencies
of common exchange-correlation functionals and provides insight towards the
inclusion of dispersion interactions into the correlation functional.Comment: Accepted for publication in Physical Review Letter
Excitons in boron nitride nanotubes: dimensionality effects
We show that the optical absorption spectra of boron nitride (BN) nanotubes
are dominated by strongly bound excitons. Our first-principles calculations
indicate that the binding energy for the first and dominant excitonic peak
depends sensitively on the dimensionality of the system, varying from 0.7 eV in
bulk hexagonal BN via 2.1 eV in the single sheet of BN to more than 3 eV in the
hypothetical (2,2) tube. The strongly localized nature of this exciton dictates
the fast convergence of its binding energy with increasing tube diameter
towards the sheet value. The absolute position of the first excitonic peak is
almost independent of the tube radius and system dimensionality. This provides
an explanation for the observed "optical gap" constancy for different tubes and
bulk hBN [R. Arenal et al., to appear in Phys. Rev. Lett. (2005)].Comment: 5 pages, 2 figure
Pillars of creation amongst destruction: Star formation in molecular clouds near R136 in 30 Doradus
New sensitive CO(2-1) observations of the 30 Doradus region in the Large
Magellanic Cloud are presented. We identify a chain of three newly discovered
molecular clouds we name KN1, KN2 and KN3 lying within 2--14 pc in projection
from the young massive cluster R136 in 30 Doradus. Excited H 2.12m
emission is spatially coincident with the molecular clouds, but ionized
Br emission is not. We interpret these observations as the tails of
pillar-like structures whose ionized heads are pointing towards R136. Based on
infrared photometry, we identify a new generation of stars forming within this
structure.Comment: Accepted for publication in ApJ (includes 13 pages, 8 figures). For
higher resolution figures please see
http://www.das.uchile.cl/~vkalari/staplervk.pd
Quantum interference structures in the conductance plateaus of gold nanojunctions
The conductance of breaking metallic nanojunctions shows plateaus alternated
with sudden jumps, corresponding to the stretching of stable atomic
configurations and atomic rearrangements, respectively. We investigate the
structure of the conductance plateaus both by measuring the voltage dependence
of the plateaus' slope on individual junctions and by a detailed statistical
analysis on a large amount of contacts. Though the atomic discreteness of the
junction plays a fundamental role in the evolution of the conductance, we find
that the fine structure of the conductance plateaus is determined by quantum
interference phenomenon to a great extent.Comment: 4 pages, 4 figure
Symmetry and Temperature dependence of the Order parameter in MgB2 from point contact measurements
We have performed differential conductance versus voltage measurements of
Au/MgB2 point contacts. We find that the dominant component in the conductance
is due to Andreev reflection. The results are fitted to the theoretical model
of BTK for an s-wave symmetry from which we extract the value of the order
parameter (Delta) and its temperature dependence. From our results we also
obtain a lower experimental bound on the Fermi velocity in MgB2.Comment: 7 pages (Including figure captions) and 4 figure
Force-gradient-induced mechanical dissipation of quartz tuning fork force sensors used in atomic force microscopy
We have studied the dynamics of quartz tuning fork resonators used in atomic
force microscopy taking into account mechanical energy dissipation through the
attachment of the tuning fork base. We find that the tuning fork resonator
quality factor changes even for the case of a purely elastic sensor-sample
interaction. This is due to the effective mechanical imbalance of the tuning
fork prongs induced by the sensor-sample force gradient which in turn has an
impact on the dissipation through the attachment of the resonator base. This
effect may yield a measured dissipation signal that can be different to the one
exclusively related to the dissipation between the sensor and the sample. We
also find that there is a second order term in addition to the linear
relationship between the sensor-sample force gradient and the resonance
frequency shift of the tuning fork that is significant even for force gradients
usually present in atomic force microscopy which are in the range of tens of
N/m.Comment: 9 pages, 3 figures and supplemental informatio
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