35,363 research outputs found
Comment on "High Field Studies of Superconducting Fluctuations in High-Tc Cuprates. Evidence for a Small Gap distinct from the Large Pseudogap"
By using high magnetic field data to estimate the background conductivity,
Rullier-Albenque and coworkers have recently published [Phys.Rev.B 84, 014522
(2011)] experimental evidence that the in-plane paraconductivity in cuprates is
almost independent of doping. In this Comment we also show that, in contrast
with their claims, these useful data may be explained at a quantitative level
in terms of the Gaussian-Ginzburg-Landau approach for layered superconductors,
extended by Carballeira and coworkers to high reduced-temperatures by
introducing a total-energy cutoff [Phys.Rev.B 63, 144515 (2001)]. When
combined, these two conclusions further suggest that the paraconductivity in
cuprates is conventional, i.e., associated with fluctuating superconducting
pairs above the mean-field critical temperature.Comment: 9 pages, 1 figur
Perfect Sampling with Unitary Tensor Networks
Tensor network states are powerful variational ans\"atze for many-body ground
states of quantum lattice models. The use of Monte Carlo sampling techniques in
tensor network approaches significantly reduces the cost of tensor
contractions, potentially leading to a substantial increase in computational
efficiency. Previous proposals are based on a Markov chain Monte Carlo scheme
generated by locally updating configurations and, as such, must deal with
equilibration and autocorrelation times, which result in a reduction of
efficiency. Here we propose a perfect sampling scheme, with vanishing
equilibration and autocorrelation times, for unitary tensor networks -- namely
tensor networks based on efficiently contractible, unitary quantum circuits,
such as unitary versions of the matrix product state (MPS) and tree tensor
network (TTN), and the multi-scale entanglement renormalization ansatz (MERA).
Configurations are directly sampled according to their probabilities in the
wavefunction, without resorting to a Markov chain process. We also describe a
partial sampling scheme that can result in a dramatic (basis-dependent)
reduction of sampling error.Comment: 11 pages, 9 figures, renamed partial sampling to incomplete sampling
for clarity, extra references, plus a variety of minor change
Real-time support for high performance aircraft operation
The feasibility of real-time processing schemes using artificial neural networks (ANNs) is investigated. A rationale for digital neural nets is presented and a general processor architecture for control applications is illustrated. Research results on ANN structures for real-time applications are given. Research results on ANN algorithms for real-time control are also shown
The effective neutrino charge radius
It is shown that at one-loop order a neutrino charge radius (NCR) may be
defined, which is ultraviolet finite, does not depend on the gauge-fixing
parameter, nor on properties of the target other than its electric charge. This
is accomplished through the systematic decomposition of physical amplitudes
into effective self-energies, vertices, and boxes, which separately respect
electroweak gauge invariance. In this way the NCR stems solely from an
effective proper photon-neutrino one-loop vertex, which satisfies a naive,
QED-like Ward identity. The NCR so defined may be extracted from experiment, at
least in principle, by expressing a set of experimental electron-neutrino
cross-sections in terms of the finite NCR and two additional gauge- and
renormalization-group-invariant quantities, corresponding to the electroweak
effective charge and mixing angle.Comment: Talk given at EPS2003 - Aachen, Germany, July 2003; 3 pages, no
figure
Variational Monte Carlo with the Multi-Scale Entanglement Renormalization Ansatz
Monte Carlo sampling techniques have been proposed as a strategy to reduce
the computational cost of contractions in tensor network approaches to solving
many-body systems. Here we put forward a variational Monte Carlo approach for
the multi-scale entanglement renormalization ansatz (MERA), which is a unitary
tensor network. Two major adjustments are required compared to previous
proposals with non-unitary tensor networks. First, instead of sampling over
configurations of the original lattice, made of L sites, we sample over
configurations of an effective lattice, which is made of just log(L) sites.
Second, the optimization of unitary tensors must account for their unitary
character while being robust to statistical noise, which we accomplish with a
modified steepest descent method within the set of unitary tensors. We
demonstrate the performance of the variational Monte Carlo MERA approach in the
relatively simple context of a finite quantum spin chain at criticality, and
discuss future, more challenging applications, including two dimensional
systems.Comment: 11 pages, 12 figures, a variety of minor clarifications and
correction
A generalization of Bohr's Equivalence Theorem
Based on a generalization of Bohr's equivalence relation for general
Dirichlet series, in this paper we study the sets of values taken by certain
classes of equivalent almost periodic functions in their strips of almost
periodicity. In fact, the main result of this paper consists of a result like
Bohr's equivalence theorem extended to the case of these functions.Comment: Because of a mistake detected in one of the references, the previous
version of this paper has been modified by the authors to restrict the scope
of its application to the case of existence of an integral basi
Optical absorption and energy-loss spectra of aligned carbon nanotubes
Optical-absorption cross-sections and energy-loss spectra of aligned
multishell carbon nanotubes are investigated, on the basis of photonic
band-structure calculations. A local graphite-like dielectric tensor is
assigned to every point of the tubules, and the effective transverse dielectric
function of the composite is computed by solving Maxwell's equations in media
with tensor-like dielectric functions. A Maxwell-Garnett-like approach
appropriate to the case of infinitely long anisotropic tubules is also
developed. Our full calculations indicate that the experimentally measured
macroscopic dielectric function of carbon nanotube materials is the result of a
strong electromagnetic coupling between the tubes. An analysis of the
electric-field pattern associated with this coupling is presented, showing that
in the close-packed regime the incident radiation excites a very localized
tangential surface plasmon.Comment: 7 pages, 12 figures, to appear in Eur. Phys. J.
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