3,852 research outputs found
Preliminary results of simulated vortex encounters by a twin-engine, commercial aircraft during final landing approach
Piloted simulations of encounters with vortices of various ages and degrees of attenuation were performed with the Visual Motion Simulator. In the simulations, a twin engine, commercial transport on final approach, encountered the modeled vortices of a four engine, wide body, commercial transport. The data show the effect of vortex age and attenuation on the severity of the initial upset, as well as the effect of the vortex encounters on the landing capability
Tip Splittings and Phase Transitions in the Dielectric Breakdown Model: Mapping to the DLA Model
We show that the fractal growth described by the dielectric breakdown model
exhibits a phase transition in the multifractal spectrum of the growth measure.
The transition takes place because the tip-splitting of branches forms a fixed
angle. This angle is eta dependent but it can be rescaled onto an
``effectively'' universal angle of the DLA branching process. We derive an
analytic rescaling relation which is in agreement with numerical simulations.
The dimension of the clusters decreases linearly with the angle and the growth
becomes non-fractal at an angle close to 74 degrees (which corresponds to eta=
4.0 +- 0.3).Comment: 4 pages, REVTex, 3 figure
Exact Multifractal Spectra for Arbitrary Laplacian Random Walks
Iterated conformal mappings are used to obtain exact multifractal spectra of
the harmonic measure for arbitrary Laplacian random walks in two dimensions.
Separate spectra are found to describe scaling of the growth measure in time,
of the measure near the growth tip, and of the measure away from the growth
tip. The spectra away from the tip coincide with those of conformally invariant
equilibrium systems with arbitrary central charge , with related
to the particular walk chosen, while the scaling in time and near the tip
cannot be obtained from the equilibrium properties.Comment: 4 pages, 3 figures; references added, minor correction
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Soft X-ray seeding studies for the SLAC Linac Coherent Light Source II
We present the results from studies of soft X-ray seeding options for the LCLS-II X-ray free electron laser (FEL) at SLAC. The LCLS-II will use superconducting accelerator technology to produce X-ray pulses at up to 1 MHz repetition rate using 4 GeV electron beams. If properly seeded, these pulses will be nearly fully coherent, and highly stable in photon energy, bandwidth, and intensity, thus enabling unique experiments with intense high-resolution soft X-rays. Given the expected electron beam parameters from start to end simulations and predicted FEL performance, our studies reveal echo enabled harmonic generation (EEHG) and soft X-ray self-seeding (SXRSS) as promising and complementary seeding methods. We find that SXRSS has the advantage of simplicity and will deliver 5-35 times higher spectral brightness than EEHG in the 1-2 nm range, but lacks some of the potential for phase-stable multipulse and multicolor FEL operations enabled by external laser seeding with EEHG
Almost Commuting Matrices, Localized Wannier Functions, and the Quantum Hall Effect
For models of non-interacting fermions moving within sites arranged on a
surface in three dimensional space, there can be obstructions to finding
localized Wannier functions. We show that such obstructions are -theoretic
obstructions to approximating almost commuting, complex-valued matrices by
commuting matrices, and we demonstrate numerically the presence of this
obstruction for a lattice model of the quantum Hall effect in a spherical
geometry. The numerical calculation of the obstruction is straightforward, and
does not require translational invariance or introducing a flux torus.
We further show that there is a index obstruction to approximating
almost commuting self-dual matrices by exactly commuting self-dual matrices,
and present additional conjectures regarding the approximation of almost
commuting real and self-dual matrices by exactly commuting real and self-dual
matrices. The motivation for considering this problem is the case of physical
systems with additional antiunitary symmetries such as time reversal or
particle-hole conjugation.
Finally, in the case of the sphere--mathematically speaking three almost
commuting Hermitians whose sum of square is near the identity--we give the
first quantitative result showing this index is the only obstruction to finding
commuting approximations. We review the known non-quantitative results for the
torus.Comment: 35 pages, 2 figure
The ground state of a class of noncritical 1D quantum spin systems can be approximated efficiently
We study families H_n of 1D quantum spin systems, where n is the number of
spins, which have a spectral gap \Delta E between the ground-state and
first-excited state energy that scales, asymptotically, as a constant in n. We
show that if the ground state |\Omega_m> of the hamiltonian H_m on m spins,
where m is an O(1) constant, is locally the same as the ground state
|\Omega_n>, for arbitrarily large n, then an arbitrarily good approximation to
the ground state of H_n can be stored efficiently for all n. We formulate a
conjecture that, if true, would imply our result applies to all noncritical 1D
spin systems. We also include an appendix on quasi-adiabatic evolutions.Comment: 9 pages, 1 eps figure, minor change
Social marketing and healthy eating : Findings from young people in Greece
This document is the Accepted Manuscript version. The final publication is available at Springer via http://dx.doi.org/10.1007/s12208-013-0112-xGreece has high rates of obesity and non-communicable diseases owing to poor dietary choices. This research provides lessons for social marketing to tackle the severe nutrition-related problems in this country by obtaining insight into the eating behaviour of young adults aged 18–23. Also, the main behavioural theories used to inform the research are critically discussed. The research was conducted in Athens. Nine focus groups with young adults from eight educational institutions were conducted and fifty-nine participants’ views towards eating habits, healthy eating and the factors that affect their food choices were explored. The study found that the participants adopted unhealthier nutritional habits after enrolment. Motivations for healthy eating were good health, appearance and psychological consequences, while barriers included lack of time, fast-food availability and taste, peer pressure, lack of knowledge and lack of family support. Participants reported lack of supportive environments when deciding on food choices. Based on the findings, recommendations about the development of the basic 4Ps of the marketing mix, as well as of a fifth P, for Policy are proposedPeer reviewe
Iterated Conformal Dynamics and Laplacian Growth
The method of iterated conformal maps for the study of Diffusion Limited
Aggregates (DLA) is generalized to the study of Laplacian Growth Patterns and
related processes. We emphasize the fundamental difference between these
processes: DLA is grown serially with constant size particles, while Laplacian
patterns are grown by advancing each boundary point in parallel, proportionally
to the gradient of the Laplacian field. We introduce a 2-parameter family of
growth patterns that interpolates between DLA and a discrete version of
Laplacian growth. The ultraviolet putative finite-time singularities are
regularized here by a minimal tip size, equivalently for all the models in this
family. With this we stress that the difference between DLA and Laplacian
growth is NOT in the manner of ultraviolet regularization, but rather in their
deeply different growth rules. The fractal dimensions of the asymptotic
patterns depend continuously on the two parameters of the family, giving rise
to a "phase diagram" in which DLA and discretized Laplacian growth are at the
extreme ends. In particular we show that the fractal dimension of Laplacian
growth patterns is much higher than the fractal dimension of DLA, with the
possibility of dimension 2 for the former not excluded.Comment: 13 pages, 12 figures, submitted to Phys. Rev.
Exponential Decay of Correlations Implies Area Law
We prove that a finite correlation length, i.e. exponential decay of
correlations, implies an area law for the entanglement entropy of quantum
states defined on a line. The entropy bound is exponential in the correlation
length of the state, thus reproducing as a particular case Hastings proof of an
area law for groundstates of 1D gapped Hamiltonians.
As a consequence, we show that 1D quantum states with exponential decay of
correlations have an efficient classical approximate description as a matrix
product state of polynomial bond dimension, thus giving an equivalence between
injective matrix product states and states with a finite correlation length.
The result can be seen as a rigorous justification, in one dimension, of the
intuition that states with exponential decay of correlations, usually
associated with non-critical phases of matter, are simple to describe. It also
has implications for quantum computing: It shows that unless a pure state
quantum computation involves states with long-range correlations, decaying at
most algebraically with the distance, it can be efficiently simulated
classically.
The proof relies on several previous tools from quantum information theory -
including entanglement distillation protocols achieving the hashing bound,
properties of single-shot smooth entropies, and the quantum substate theorem -
and also on some newly developed ones. In particular we derive a new bound on
correlations established by local random measurements, and we give a
generalization to the max-entropy of a result of Hastings concerning the
saturation of mutual information in multiparticle systems. The proof can also
be interpreted as providing a limitation on the phenomenon of data hiding in
quantum states.Comment: 35 pages, 6 figures; v2 minor corrections; v3 published versio
Tensor network states and geometry
Tensor network states are used to approximate ground states of local
Hamiltonians on a lattice in D spatial dimensions. Different types of tensor
network states can be seen to generate different geometries. Matrix product
states (MPS) in D=1 dimensions, as well as projected entangled pair states
(PEPS) in D>1 dimensions, reproduce the D-dimensional physical geometry of the
lattice model; in contrast, the multi-scale entanglement renormalization ansatz
(MERA) generates a (D+1)-dimensional holographic geometry. Here we focus on
homogeneous tensor networks, where all the tensors in the network are copies of
the same tensor, and argue that certain structural properties of the resulting
many-body states are preconditioned by the geometry of the tensor network and
are therefore largely independent of the choice of variational parameters.
Indeed, the asymptotic decay of correlations in homogeneous MPS and MERA for
D=1 systems is seen to be determined by the structure of geodesics in the
physical and holographic geometries, respectively; whereas the asymptotic
scaling of entanglement entropy is seen to always obey a simple boundary law --
that is, again in the relevant geometry. This geometrical interpretation offers
a simple and unifying framework to understand the structural properties of, and
helps clarify the relation between, different tensor network states. In
addition, it has recently motivated the branching MERA, a generalization of the
MERA capable of reproducing violations of the entropic boundary law in D>1
dimensions.Comment: 18 pages, 18 figure
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