113 research outputs found
Mathematical Structure of Magnons in Quantum Ferromagnets
We provide the mathematical structure and a simple, transparent and rigorous
derivation of the magnons as elementary quasi-particle excitations at low
temperatures and in the infinite spin limit for a large class of Heisenberg
ferromagnets. The magnon canonical variables are obtained as fluctuation
operators in the infinite spin limit. Their quantum character is governed by
the size of the magnetization
Non-Extensive Bose-Einstein Condensation Model
The imperfect Boson gas supplemented with a gentle repulsive interaction is
completely solved. In particular it is proved that it has non-extensive
Bose-Einstein condensation, i.e., there is condensation without macroscopic
occupation of the ground state (k=0) level
BEC for a Coupled Two-type Hard Core Bosons Model
We study a solvable model of two types hard core Bose particles. A complete
analysis is given of its equilibrium states including the proof of existence of
Bose-Einstein condensation. The plasmon frequencies and the quantum normal
modes corresponding to these frequencies are rigorously constructed. In
particular we show a two-fold degeneracy of these frequencies. We show that all
this results from spontaneous gauge symmetry breakdown
Transport of interface states in the Heisenberg chain
We demonstrate the transport of interface states in the one-dimensional
ferromagnetic Heisenberg model by a time dependent magnetic field. Our analysis
is based on the standard Adiabatic Theorem. This is supplemented by a numerical
analysis via the recently developed time dependent DMRG method, where we
calculate the adiabatic constant as a function of the strength of the magnetic
field and the anisotropy of the interaction.Comment: minor revision, final version; 13 pages, 4 figure
Neuropsychological deficits in disordered screen use behaviours : A systematic review and meta-analysis
Over the last few decades, excessive and disordered screen use has become more prevalent, prompting investigations into its associated consequences. The extent to which disordered screen use behaviours impact neuropsychological functioning has been reportedly mixed and at times inconsistent. This review sought to synthesise the literature and estimate the magnitude of overall cognitive impairment across a wide range of disordered screen use behaviours. We also sought to determine the cognitive domains most impacted, and whether the observed impairments were moderated by the classification of screen-related behaviours (i.e., Internet or gaming) or the format of cognitive test administration (i.e., paper-and-pencil or computerised). A systematic search of databases (Embase, PsycINFO, MEDLINE) identified 43 cross-sectional articles that assessed neuropsychological performance in disordered screen use populations, 34 of which were included in the meta-analysis. A random-effects meta-analysis revealed significant small/medium (g = .38) cognitive deficits for individuals with disordered screen use behaviours relative to controls. The most affected cognitive domain with a significant medium effect size (g = .50) was attention and focus followed by a significant reduction in executive functioning (g = .31). The classification of disordered screen use behaviours into Internet or gaming categories or the format of cognitive testing did not moderate these deficits. Additionally, excluding disordered social media use in an exploratory analysis had little effect on the observed outcomes. This study highlights a number of methodological considerations that may have contributed to disparate findings and shows that disordered screen use can significantly impact cognitive performance. Recommendations for future research are also discussed. Data for this study can be found at https://osf.io/upeha/
Module networks revisited: computational assessment and prioritization of model predictions
The solution of high-dimensional inference and prediction problems in
computational biology is almost always a compromise between mathematical theory
and practical constraints such as limited computational resources. As time
progresses, computational power increases but well-established inference
methods often remain locked in their initial suboptimal solution. We revisit
the approach of Segal et al. (2003) to infer regulatory modules and their
condition-specific regulators from gene expression data. In contrast to their
direct optimization-based solution we use a more representative centroid-like
solution extracted from an ensemble of possible statistical models to explain
the data. The ensemble method automatically selects a subset of most
informative genes and builds a quantitatively better model for them. Genes
which cluster together in the majority of models produce functionally more
coherent modules. Regulators which are consistently assigned to a module are
more often supported by literature, but a single model always contains many
regulator assignments not supported by the ensemble. Reliably detecting
condition-specific or combinatorial regulation is particularly hard in a single
optimum but can be achieved using ensemble averaging.Comment: 8 pages REVTeX, 6 figure
The Canonical Perfect Bose Gas in Casimir Boxes
We study the problem of Bose-Einstein condensation in the perfect Bose gas in
the canonical ensemble, in anisotropically dilated rectangular parallelpipeds
(Casimir boxes). We prove that in the canonical ensemble for these anisotropic
boxes there is the same type of generalized Bose-Einstein condensation as in
the grand-canonical ensemble for the equivalent geometry. However the amount of
condensate in the individual states is different in some cases and so are the
fluctuations.Comment: 23 page
Quantum spin systems at positive temperature
We develop a novel approach to phase transitions in quantum spin models based
on a relation to their classical counterparts. Explicitly, we show that
whenever chessboard estimates can be used to prove a phase transition in the
classical model, the corresponding quantum model will have a similar phase
transition, provided the inverse temperature and the magnitude of the
quantum spins \CalS satisfy \beta\ll\sqrt\CalS. From the quantum system we
require that it is reflection positive and that it has a meaningful classical
limit; the core technical estimate may be described as an extension of the
Berezin-Lieb inequalities down to the level of matrix elements. The general
theory is applied to prove phase transitions in various quantum spin systems
with \CalS\gg1. The most notable examples are the quantum orbital-compass
model on and the quantum 120-degree model on which are shown to
exhibit symmetry breaking at low-temperatures despite the infinite degeneracy
of their (classical) ground state.Comment: 47 pages, version to appear in CMP (style files included
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