13,661 research outputs found
Domain State Model for Exchange Bias
Monte Carlo simulations of a system consisting of a ferromagnetic layer
exchange coupled to a diluted antiferromagnetic layer described by a classical
spin model show a strong dependence of the exchange bias on the degree of
dilution in agreement with recent experimental observations on Co/CoO bilayers.
These simulations reveal that diluting the antiferromagnet leads to the
formation of domains in the volume of the antiferromagnet carrying a remanent
surplus magnetization which causes and controls exchange bias. To further
support this domain state model for exchange bias we study in the present paper
the dependence of the bias field on the thickness of the antiferromagnetic
layer. It is shown that the bias field strongly increases with increasing film
thickness and eventually goes over a maximum before it levels out for large
thicknesses. These findings are in full agreement with experiments.Comment: 8 pages latex, 3 postscript figure
Modeling exchange bias microscopically
Exchange bias is a horizontal shift of the hysteresis loop observed for a
ferromagnetic layer in contact with an antiferromagnetic layer. Since exchange
bias is related to the spin structure of the antiferromagnet, for its
fundamental understanding a detailed knowledge of the physics of the
antiferromagnetic layer is inevitable. A model is investigated where domains
are formed in the volume of the AFM stabilized by dilution. These domains
become frozen during the initial cooling procedure carrying a remanent net
magnetization which causes and controls exchange bias. Varying the anisotropy
of the antiferromagnet we find a nontrivial dependence of the exchange bias on
the anisotropy of the antiferromagnet.Comment: 7 pages, 5 figure
Transversity Distribution and Polarized Fragmentation Function from Semi-inclusive Pion Electroproduction
A method is discussed to determine the hitherto unknown u-quark transversity
distribution from a planned HERMES measurement of a single-spin asymmetry in
semi-inclusive pion electroproduction off a transversely polarized target.
Assuming u-quark dominance, the measurement yields the shapes of the
transversity distribution and of the ratio of a polarized and the unpolarized
u-quark fragmentation functions. The unknown relative normalization can be
obtained by identifying the transversity distribution with the well-known
helicity distribution at large x. The systematic uncertainty of the method is
dominated by the assumption of u-quark dominance.Comment: 5 pages, 5 figures, revised version as will be published in EPJ
From synaptic interactions to collective dynamics in random neuronal networks models: critical role of eigenvectors and transient behavior
The study of neuronal interactions is currently at the center of several
neuroscience big collaborative projects (including the Human Connectome, the
Blue Brain, the Brainome, etc.) which attempt to obtain a detailed map of the
entire brain matrix. Under certain constraints, mathematical theory can advance
predictions of the expected neural dynamics based solely on the statistical
properties of such synaptic interaction matrix. This work explores the
application of free random variables (FRV) to the study of large synaptic
interaction matrices. Besides recovering in a straightforward way known results
on eigenspectra of neural networks, we extend them to heavy-tailed
distributions of interactions. More importantly, we derive analytically the
behavior of eigenvector overlaps, which determine stability of the spectra. We
observe that upon imposing the neuronal excitation/inhibition balance, although
the eigenvalues remain unchanged, their stability dramatically decreases due to
strong non-orthogonality of associated eigenvectors. It leads us to the
conclusion that the understanding of the temporal evolution of asymmetric
neural networks requires considering the entangled dynamics of both
eigenvectors and eigenvalues, which might bear consequences for learning and
memory processes in these models. Considering the success of FRV analysis in a
wide variety of branches disciplines, we hope that the results presented here
foster additional application of these ideas in the area of brain sciences.Comment: 24 pages + 4 pages of refs, 8 figure
Neutrino Interactions In Color-Flavor-Locked Dense Matter
At high density, diquarks could condense in the vacuum with the QCD color
spontaneously broken. Based on the observation that the symmetry breaking
pattern involved in this phenomenon is essentially the same as that of the
Pati-Salam model with broken electroweak--color SU(3) group, we determine the
relevant electroweak interactions in the color-flavor locked (CFL) phase in
high density QCD. We briefly comment on the possible implications on the
cooling of neutron stars.Comment: 13 pages. LaTeX. Talk given at the First KIAS Workshop on
Astrophysics, Seoul, May 2000; V2. references added. comments on cooling
change
Correlation of Positive and Negative Reciprocity Fails to Confer an Evolutionary Advantage: Phase Transitions to Elementary Strategies
Economic experiments reveal that humans value cooperation and fairness. Punishing unfair behavior is therefore common, and according to the theory of strong reciprocity, it is also directly related to rewarding cooperative behavior. However, empirical data fail to confirm that positive and negative reciprocity are correlated. Inspired by this disagreement, we determine whether the combined application of reward and punishment is evolutionarily advantageous. We study a spatial public goods game, where in addition to the three elementary strategies of defection, rewarding, and punishment, a fourth strategy that combines the latter two competes for space. We find rich dynamical behavior that gives rise to intricate phase diagrams where continuous and discontinuous phase transitions occur in succession. Indirect territorial competition, spontaneous emergence of cyclic dominance, as well as divergent fluctuations of oscillations that terminate in an absorbing phase are observed. Yet, despite the high complexity of solutions, the combined strategy can survive only in very narrow and unrealistic parameter regions. Elementary strategies, either in pure or mixed phases, are much more common and likely to prevail. Our results highlight the importance of patterns and structure in human cooperation, which should be considered in future experiments
Nonabelian Berry Phases in Baryons
We show how generic nonabelian gauge fields can be induced in baryons when a
hierarchy of fast degrees of freedom is integrated out. We identify them with
nonabelian Berry potentials and discuss their role in transmuting quantum
numbers in bag and soliton models of baryons. The resulting baryonic spectra
for both light and heavy quark systems are generic and resemble closely the
excitation spectrum of diatomic molecules. The symmetry restoration in the
system, i.e., the electronic rotational invariance in diatomic molecules, the
heavy-quark symmetry in heavy baryons etc. is interpreted in terms of the
vanishing of nonabelian Berry potentials that otherwise govern the hyperfine
splitting.Comment: Latex 35 pages (2 figures not added, will be faxed if requested),
NTG-92-2
Asymptotic boundary forms for tight Gabor frames and lattice localization domains
We consider Gabor localization operators defined by two
parameters, the generating function of a tight Gabor frame
, parametrized by the elements of a
given lattice , i.e. a discrete cocompact subgroup
of , and a lattice localization domain
with its boundary consisting of line segments connecting points of .
We find an explicit formula for the boundary form
, the normalized limit of the projection
functional
,
where are the eigenvalues of the localization
operators applied to dilated domains , is an
integer and is the area of the fundamental domain of the
lattice .Comment: 35 page
- …