435 research outputs found
Critical Hysteresis from Random Anisotropy
Critical hysteresis in ferromagnets is investigated through a -component
spin model with random anisotropies, more prevalent experimentally than the
random fields used in most theoretical studies. Metastability, and the
tensorial nature of anisotropy, dictate its physics. Generically, random field
Ising criticality occurs, but other universality classes exist. In particular,
proximity to criticality may explain the discrepancy between
experiment and earlier theories. The uniaxial anisotropy constant, which can be
controlled in magnetostrictive materials by an applied stress, emerges as a
natural tuning parameter.Comment: four pages, revtex4; minor corrections in the text and typos
corrected (published version
Crises and collective socio-economic phenomena: simple models and challenges
Financial and economic history is strewn with bubbles and crashes, booms and
busts, crises and upheavals of all sorts. Understanding the origin of these
events is arguably one of the most important problems in economic theory. In
this paper, we review recent efforts to include heterogeneities and
interactions in models of decision. We argue that the Random Field Ising model
(RFIM) indeed provides a unifying framework to account for many collective
socio-economic phenomena that lead to sudden ruptures and crises. We discuss
different models that can capture potentially destabilising self-referential
feedback loops, induced either by herding, i.e. reference to peers, or
trending, i.e. reference to the past, and account for some of the phenomenology
missing in the standard models. We discuss some empirically testable
predictions of these models, for example robust signatures of RFIM-like herding
effects, or the logarithmic decay of spatial correlations of voting patterns.
One of the most striking result, inspired by statistical physics methods, is
that Adam Smith's invisible hand can badly fail at solving simple coordination
problems. We also insist on the issue of time-scales, that can be extremely
long in some cases, and prevent socially optimal equilibria to be reached. As a
theoretical challenge, the study of so-called "detailed-balance" violating
decision rules is needed to decide whether conclusions based on current models
(that all assume detailed-balance) are indeed robust and generic.Comment: Review paper accepted for a special issue of J Stat Phys; several
minor improvements along reviewers' comment
MgyNi1-y(Hx) thin films deposited by magnetron co-sputtering
In this work we have synthesised thin films of MgyNi1-y(Hx) metal and metal
hydride with y between 0 and 1. The films are deposited by magnetron
co-sputtering of metallic targets of Mg and Ni. Metallic MgyNi1-y films were
deposited with pure Ar plasma while MgyNi1-yHx hydride films were deposited
reactively with 30% H2 in the Ar plasma. The depositions were done with a fixed
substrate carrier, producing films with a spatial gradient in the Mg and Ni
composition. The combinatorial method of co-sputtering gives an insight into
the phase diagram of MgyNi1-y and MgyNi1-yHx, and allows us to investigate
structural, optical and electrical properties of the resulting alloys. Our
results show that reactive sputtering gives direct deposition of metal hydride
films, with high purity in the case of Mg~2NiH~4. We have observed limited
oxidation after several months of exposure to ambient conditions. MgyNi1-y and
MgyNi1-yHx films might be applied for optical control in smart windows, optical
sensors and as a semiconducting material for photovoltaic solar cells
Author Correction: Tree mode of death and mortality risk factors across Amazon forests (Nature Communications, (2020), 11, 1, (5515), 10.1038/s41467-020-18996-3)
The original version of this Article contained an error in Table 2, where the number of individuals in the “All Amazonia” row was reported as 11,6431 instead of 116,431. Also, the original version of this Article contained an error in the Methods, where the R2 for the proportion of broken/uprooted dead trees increase per year was reported as 0.12, the correct value being 0.06. The original version of this Article contained errors in the author affiliations. The affiliation of Gerardo A. Aymard C. with UNELLEZGuanare, Herbario Universitario (PORT), Portuguesa, Venezuela Compensation International Progress S.A. Ciprogress–Greenlife
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