216 research outputs found
Controlled self-assembly of periodic and aperiodic cluster crystals
Soft particles are known to overlap and form stable clusters that
self-assemble into periodic crystalline phases with density-independent lattice
constants. We use molecular dynamics simulations in two dimensions to
demonstrate that, through a judicious design of an isotropic pair potential,
one can control the ordering of the clusters and generate a variety of phases,
including decagonal and dodecagonal quasicrystals. Our results confirm
analytical predictions based on a mean-field approximation, providing insight
into the stabilization of quasicrystals in soft macromolecular systems, and
suggesting a practical approach for their controlled self-assembly in
laboratory realizations using synthesized soft-matter particles.Comment: Supplemental Material can be obtained through the author's website
at: http://www.tau.ac.il/~ronlif/pubs/ClusterCrystals-Supp.pd
Colourings of planar quasicrystals
The investigation of colour symmetries for periodic and aperiodic systems
consists of two steps. The first concerns the computation of the possible
numbers of colours and is mainly combinatorial in nature. The second is
algebraic and determines the actual colour symmetry groups. Continuing previous
work, we present the results of the combinatorial part for planar patterns with
n-fold symmetry, where n=7,9,15,16,20,24. This completes the cases with values
of n such that Euler's totient function of n is less than or equal to eight.Comment: Talk presented by Max Scheffer at Quasicrystals 2001, Sendai
(September 2001). 6 pages, including two colour figure
Hypnosis and meditation: Vehicles of attention and suggestion
Although hypnosis and meditation represent distinct domains of practice, they appear to overlap in phenomenology, cognitive mechanisms, neural substrates, and potential therapeutic merits. Whereas numerous studies have documented the beneficial impact of these approaches, few have harnessed these distinctive phenomena together, either clinically or as a means of illuminating cognitive questions. This paper introduces the theme of the present issue and discusses the potential value of yoking empirical studies of hypnosis and meditation. The marriage of these seemingly disparate yet overlapping practices promises to improve our scientific understanding of each as well as unravel their underlying mechanisms. On the one hand, albeit largely overlooked by researchers studying meditation, the intimate relationship between attention and suggestion holds important implications for both theoretical models and therapeutic applications of contemplative practice. On the other hand, hypnosis and meditation serve as complementary vehicles for elucidating salient topics in cognitive neuroscience, including the neural underpinnings of perception and cognitive control, and the governing of deeply-ingrained processes. Binding these approaches to the science of attention and suggestion paves the road to a more nuanced appreciation of hypnosis and meditation while fostering novel therapeutic prospects and improving our understanding of consciousness and cognition
Fermi Surface Reconstruction by Dynamic Magnetic Fluctuations
We demonstrate that nearly critical quantum magnetic fluctuations in strongly
correlated electron systems can change the Fermi surface topology and also lead
to spin charge separation (SCS) in two dimensions. To demonstrate these effects
we consider a small number of holes injected into the bilayer antiferromagnet.
The system has a quantum critical point (QCP) which separates magnetically
ordered and disordered phases. We demonstrate that in the physically
interesting regime there is a magnetically driven Lifshitz point (LP) inside
the magnetically disordered phase. At the LP the topology of the hole Fermi
surface is changed. We also demonstrate that in this regime the hole spin and
charge necessarily separate when approaching the QCP. The considered model
sheds light on generic problems concerning the physics of the cuprates.Comment: updated version, accepted to PR
Collective modes of an Anisotropic Quark-Gluon Plasma II
We continue our exploration of the collective modes of an anisotropic quark
gluon plasma by extending our previous analysis to arbitrary Riemann sheets. We
demonstrate that in the presence of momentum-space anisotropies in the parton
distribution functions there are new relevant singularities on the neighboring
unphysical sheets. We then show that for sufficiently strong anisotropies that
these singularities move into the region of spacelike momentum and their effect
can extend down to the physical sheet. In order to demonstrate this explicitly
we consider the polarization tensor for gluons propagating parallel to the
anisotropy direction. We derive analytic expressions for the gluon structure
functions in this case and then analytically continue them to unphysical
Riemann sheets. Using the resulting analytic continuations we numerically
determine the position of the unphysical singularities. We then show that in
the limit of infinite contraction of the distribution function along the
anisotropy direction that the unphysical singularities move onto the physical
sheet and result in real spacelike modes at large momenta for all
"out-of-plane" angles of propagation.Comment: 13 pages, 8 figure
The Climate of Neurofeedback: Scientific Rigour and the Perils of Ideology
Over the last six decades, an in-group with ideological and financial stakes has been conducting sub-par research to develop an ostensibly effective clinical intervention: EEG-neurofeedback. More recently, however, a string of independent studies featuring increased scientific rigour and tighter experimental controls has challenged the foundation on which EEG-neurofeedback stands. Earlier this year, Brain published one of the most robust EEG-neurofeedback experiments to date (Schabus et al., 2017), which sparked a flurry of correspondence concerning the therapeutic value of neurofeedback (Fovet et al., 2017; Schabus, 2017); notably, a parallel discussion continues in Lancet Psychiatry (Micoulaud-Franchi and Fovet, 2016; Thibault and Raz, 2016a; Schönenberg et al., 2017). However, to effectively interpret the pro and con viewpoints, one must appreciate the peculiar culture surrounding the field of EEG-neurofeedback. The present breezy piece provides little-discussed yet highly relevant contextual information often absent from formal papers and technical reports
Neurofeedback or Neuroplacebo?
This scientific commentary refers to ‘Better than sham? A double-blind placebo-controlled neurofeedback study in primary insomnia’, by Schabus et al.. (doi:10.1093/brain/awx011)
Numerical solution of the Boltzmann equation for the collective modes of trapped Fermi gases
We numerically solve the Boltzmann equation for trapped fermions in the
normal phase using the test-particle method. After discussing a couple of tests
in order to estimate the reliability of the method, we apply it to the
description of collective modes in a spherical harmonic trap. The numerical
results are compared with those obtained previously by taking moments of the
Boltzmann equation. We find that the general shape of the response function is
very similar in both methods, but the relaxation time obtained from the
simulation is significantly longer than that predicted by the method of
moments. It is shown that the result of the method of moments can be corrected
by including fourth-order moments in addition to the usual second-order ones
and that this method agrees very well with our numerical simulations.Comment: 13 pages, 8 figures, accepted for publication in Phys. Rev.
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