1,219 research outputs found
Nonlinear dynamics of coupled transverse-rotational waves in granular chains
The nonlinear dynamics of coupled waves in one-dimensional granular chains with and without a substrate
is theoretically studied accounting for quadratic nonlinearity. The multiple time scale method is used to derive
the nonlinear dispersion relations for infinite granular chains and to obtain the wave solutions for semiinfinite
systems. It is shown that the sum-frequency and difference-frequency components of the coupled
transverse-rotational waves are generated due to their nonlinear interactions with the longitudinal wave.
Nonlinear resonances are not present in the chain with no substrate where these frequency components have
low amplitudes and exhibit beating oscillations. In the chain positioned on a substrate two types of nonlinear
resonances are predicted. At resonance, the fundamental frequency wave amplitudes decrease and the
generated frequency component amplitudes increase along the chain, accompanied by the oscillations due to
the wave numbers asynchronism. The results confirm the possibility of a highly efficient energy transfer
between the waves of different frequencies, which could find applications in the design of acoustic devices
for energy transfer and energy rectification
No black hole information puzzle in a relational universe
The introduction of a relational time in quantum gravity naturally implies
that pure quantum states evolve into mixed quantum states. We show, using a
recently proposed concrete implementation, that the rate at which pure states
naturally evolve into mixed ones is faster than that due to collapsing into a
black hole that later evaporates. This is rather remarkable since the
fundamental mechanism for decoherence is usually very weak. Therefore the
``black hole information puzzle'' is rendered de-facto unobservable.Comment: 4 pages, no figures, revte
Spin dependent point potentials in one and three dimensions
We consider a system realized with one spinless quantum particle and an array
of spins 1/2 in dimension one and three. We characterize all the
Hamiltonians obtained as point perturbations of an assigned free dynamics in
terms of some ``generalized boundary conditions''. For every boundary condition
we give the explicit formula for the resolvent of the corresponding
Hamiltonian. We discuss the problem of locality and give two examples of spin
dependent point potentials that could be of interest as multi-component
solvable models.Comment: 15 pages, some misprints corrected, one example added, some
references modified or adde
Super-roughening versus intrinsic anomalous scaling of surfaces
In this paper we study kinetically rough surfaces which display anomalous
scaling in their local properties such as roughness, or height-height
correlation function. By studying the power spectrum of the surface and its
relation to the height-height correlation, we distinguish two independent
causes for anomalous scaling. One is super-roughening (global roughness
exponent larger than or equal to one), even if the spectrum behaves non
anomalously. Another cause is what we term an intrinsically anomalous spectrum,
in whose scaling an independent exponent exists, which induces different
scaling properties for small and large length scales (that is, the surface is
not self-affine). In this case, the surface does not need to be super-rough in
order to display anomalous scaling. In both cases, we show how to extract the
independent exponents and scaling relations from the correlation functions, and
we illustrate our analysis with two exactly solvable examples. One is the
simplest linear equation for molecular beam epitaxy , well known to display
anomalous scaling due to super-roughening. The second example is a random
diffusion equation, which features anomalous scaling independent of the value
of the global roughness exponent below or above one.Comment: 9 pages, 6 figures, Revtex (uses epsfig), Phys. Rev. E, submitte
Strong anisotropy in two-dimensional surfaces with generic scale invariance: Gaussian and related models
Among systems that display generic scale invariance, those whose asymptotic
properties are anisotropic in space (strong anisotropy, SA) have received a
relatively smaller attention, specially in the context of kinetic roughening
for two-dimensional surfaces. This is in contrast with their experimental
ubiquity, e.g. in the context of thin film production by diverse techniques.
Based on exact results for integrable (linear) cases, here we formulate a SA
Ansatz that, albeit equivalent to existing ones borrowed from equilibrium
critical phenomena, is more naturally adapted to the type of observables that
are measured in experiments on the dynamics of thin films, such as one and
two-dimensional height structure factors. We test our Ansatz on a paradigmatic
nonlinear stochastic equation displaying strong anisotropy like the Hwa-Kardar
equation [Phys. Rev. Lett. 62, 1813 (1989)], that was initially proposed to
describe the interface dynamics of running sand piles. A very important role to
elucidate its SA properties is played by an accurate (Gaussian) approximation
through a non-local linear equation that shares the same asymptotic properties
Operator mixing and three-point functions in N=4 SYM
We study the three-point functions between two BPS and one non-BPS local
gauge invariant operators in N=4 Super Yang-Mills theory. In particular we
show, in explicit 1-loop examples, that the operator mixing discussed in
arXiv:0810.0499 plays an important role in the computations of the correlators
and is necessary to cancel contributions that would violate the constraints
following from the superconformal and the bonus U(1)_Y symmetries. We analyse
the same type of correlators also at strong coupling by using the BMN limit of
the AdS_5xS^5 string theory. Again the mixing between states with different
types of impurities is crucial to ensure the cancellation of various amplitudes
that would violate the constraints mentioned above. However, on the string
side, we also find some examples of interactions between one non-BPS and two
BPS states that do not satisfy expectations based on the superconformal and the
bonus U(1)_Y symmetries.Comment: 28 pages, 5 figure
Power Allocation, Relay Selection, and User Pairing for Cooperative NOMA Systems with Rate Fairness
Funding Information: This research was partially funded by FundacĀøĆ£o para a CiĆŖncia e Tecnologia under the Projects CoSHARE (PTDC/EEI-TEL/30709/2017), UIDB/04111/2020, IF/00325/2015, and UIDB/50008/2020.Assuming a cooperative non-orthogonal multiple access (NOMA) system with rate fairness in a scenario with multiple users and arbitrary relays, this paper investigates adaptive power allocation (PA), relay selection (RS), and user pairing (UP) policies. Specifically, two adaptive PA optimization problems are formulated, one at the base station (BS) and another at the selected relays. Closed-form expressions for the power allocation factors are derived as well as an algorithm that provides the optimal solution at the BS. In order to show the superiority of the proposed study, our results are compared with other benchmark schemes in terms of outage probability, Jain's fairness index, and average sum rate.publishersversionpublishe
The large cosmological constant approximation to classical and quantum gravity: model examples
We have recently introduced an approach for studying perturbatively classical
and quantum canonical general relativity. The perturbative technique appears to
preserve many of the attractive features of the non-perturbative quantization
approach based on Ashtekar's new variables and spin networks. With this
approach one can find perturbatively classical observables (quantities that
have vanishing Poisson brackets with the constraints) and quantum states
(states that are annihilated by the quantum constraints). The relative ease
with which the technique appears to deal with these traditionally hard problems
opens several questions about how relevant the results produced can possibly
be. Among the questions is the issue of how useful are results for large values
of the cosmological constant and how the approach can deal with several
pathologies that are expected to be present in the canonical approach to
quantum gravity. With the aim of clarifying these points, and to make our
construction as explicit as possible, we study its application in several
simple models. We consider Bianchi cosmologies, the asymmetric top, the coupled
harmonic oscillators with constant energy density and a simple quantum
mechanical system with two Hamiltonian constraints. We find that the technique
satisfactorily deals with the pathologies of these models and offers promise
for finding (at least some) results even for small values of the cosmological
constant. Finally, we briefly sketch how the method would operate in the full
four dimensional quantum general relativity case.Comment: 21 pages, RevTex, 2 figures with epsfi
Sea anemones may thrive in a high CO2 world
Increased seawater pCO 2, and in turn 'ocean acidification' (OA), is predicted to profoundly impact marine ecosystem diversity and function this century. Much research has already focussed on calcifying reef-forming corals (Class: Anthozoa) that appear particularly susceptible to OA via reduced net calcification. However, here we show that OA-like conditions can simultaneously enhance the ecological success of non-calcifying anthozoans, which not only play key ecological and biogeochemical roles in present day benthic ecosystems but also represent a model organism should calcifying anthozoans exist as less calcified (soft-bodied) forms in future oceans. Increased growth (abundance and size) of the sea anemone (Anemonia viridis) population was observed along a natural CO 2 gradient at Vulcano, Italy. Both gross photosynthesis (P G) and respiration (R) increased with pCO 2 indicating that the increased growth was, at least in part, fuelled by bottom up (CO 2 stimulation) of metabolism. The increase of P G outweighed that of R and the genetic identity of the symbiotic microalgae (Symbiodinium spp.) remained unchanged (type A19) suggesting proximity to the vent site relieved CO 2 limitation of the anemones' symbiotic microalgal population. Our observations of enhanced productivity with pCO 2, which are consistent with previous reports for some calcifying corals, convey an increase in fitness that may enable non-calcifying anthozoans to thrive in future environments, i.e. higher seawater pCO 2. Understanding how CO 2-enhanced productivity of non- (and less-) calcifying anthozoans applies more widely to tropical ecosystems is a priority where such organisms can dominate benthic ecosystems, in particular following localized anthropogenic stress. Ā© 2012 Blackwell Publishing Ltd
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