4,530 research outputs found
Origin of the approximate universality of distributions in equilibrium correlated systems
We propose an interpretation of previous experimental and numerical
experiments, showing that for a large class of systems, distributions of global
quantities are similar to a distribution originally obtained for the
magnetization in the 2D-XY model . This approach, developed for the Ising
model, is based on previous numerical observations. We obtain an effective
action using a perturbative method, which successfully describes the order
parameter fluctuations near the phase transition. This leads to a direct link
between the D-dimensional Ising model and the XY model in the same dimension,
which appears to be a generic feature of many equilibrium critical systems and
which is at the heart of the above observations.Comment: To appear in Europhysics Letter
Competing types of quantum oscillations in the 2D organic conductor (BEDT-TTF)8Hg4Cl12(C6H5Cl)2
Interlayer magnetoconductance of the quasi-two dimensional organic metal
(BEDT-TTF)8Hg4Cl12(C6H5Cl)2 has been investigated in pulsed magnetic fields
extending up to 36 T and in the temperature range from 1.6 to 15 K. A complex
oscillatory spectrum, built on linear combinations of three basic frequencies
only is observed. These basic frequencies arise from the compensated closed
hole and electron orbits and from the two orbits located in between. The field
and temperature dependencies of the amplitude of the various oscillation series
are studied within the framework of the coupled orbits model of Falicov and
Stachowiak. This analysis reveals that these series result from the
contribution of either conventional Shubnikov-de Haas effect (SdH) or quantum
interference (QI), both of them being induced by magnetic breakthrough.
Nevertheless, discrepancies between experimental and calculated parameters
indicate that these phenomena alone cannot account for all of the data. Due to
its low effective mass, one of the QI oscillation series - which corresponds to
the whole first Brillouin zone area - is clearly observed up to 13 K.Comment: 8 pages, 8 figures. To be published in Phys. Rev.
Fermions and Disorder in Ising and Related Models in Two Dimensions
The aspects of phase transitions in the two-dimensional Ising models modified
by quenched and annealed site disorder are discussed in the framework of
fermionic approach based on the reformulation of the problem in terms of
integrals with anticommuting Grassmann variables.Comment: 11 pages, 1 table, no figures. The discussion is merely based on a
talk given at the International Bogoliubov Conference on Problems of
Theoretical and Mathematical Physics, MIRAS--JINR, Moscow--Dubna, Russia,
August 21--27, 200
Anomalous Lattice Vibrations of Single and Few-Layer MoS2
Molybdenum disulfide (MoS2) of single and few-layer thickness was exfoliated
on SiO2/Si substrate and characterized by Raman spectroscopy. The number of
S-Mo-S layers of the samples was independently determined by contact-mode
atomic-force microscopy. Two Raman modes, E12g and A1g, exhibited sensitive
thickness dependence, with the frequency of the former decreasing and that of
the latter increasing with thickness. The results provide a convenient and
reliable means for determining layer thickness with atomic-level precision. The
opposite direction of the frequency shifts, which cannot be explained solely by
van der Waals interlayer coupling, is attributed to Coulombic interactions and
possible stacking-induced changes of the intralayer bonding. This work
exemplifies the evolution of structural parameters in layered materials in
changing from the 3-dimensional to the 2-dimensional regime.Comment: 14 pages, 4 figure
Enhancement of de Haas-van Alphen Oscillation due to Spin in the Magnetic Breakdown System
The effects of the Zeeman term on the de Haas-van Alphen oscillation is
studied in the magnetic breakdown system. We find that the amplitude of the
oscillation with the frequencies of and are enhanced by the Zeeman term, while they are expected to be
reduced in the semiclassical theory. A possible interpretation of the
experiments in organic conductors is discussed.Comment: 4 pages,4 figures. Submitted to Journal of Physical Society of Japa
Stress Dependence of Exciton Relaxation Processes in Cu2O
A comprehensive study of the exciton relaxation processes in Cu2O has led to
some surprises. We find that the ortho-para conversion rate becomes slower at
high stress, and that the Auger nonradiative recombination rate increases with
stress, with apparently no Auger recombination at zero stress. These results
have important consequences for the pursuit of Bose-Einstein condensation of
excitons in a harmonic potential.Comment: 10 figures, 1 tabl
Universal Fluctuations in Correlated Systems
The probability density function (PDF) of a global measure in a large class
of highly correlated systems has been suggested to be of the same functional
form. Here, we identify the analytical form of the PDF of one such measure, the
order parameter in the low temperature phase of the 2D-XY model. We demonstrate
that this function describes the fluctuations of global quantities in other
correlated, equilibrium and non-equilibrium systems. These include a coupled
rotor model, Ising and percolation models, models of forest fires, sand-piles,
avalanches and granular media in a self organized critical state. We discuss
the relationship with both Gaussian and extremal statistics.Comment: 4 pages, 2 figure
Uniting statistical and individual-based approaches for animal movement modelling
<div><p>The dynamic nature of their internal states and the environment directly shape animals' spatial behaviours and give rise to emergent properties at broader scales in natural systems. However, integrating these dynamic features into habitat selection studies remains challenging, due to practically impossible field work to access internal states and the inability of current statistical models to produce dynamic outputs. To address these issues, we developed a robust method, which combines statistical and individual-based modelling. Using a statistical technique for forward modelling of the IBM has the advantage of being faster for parameterization than a pure inverse modelling technique and allows for robust selection of parameters. Using GPS locations from caribou monitored in Québec, caribou movements were modelled based on generative mechanisms accounting for dynamic variables at a low level of emergence. These variables were accessed by replicating real individuals' movements in parallel sub-models, and movement parameters were then empirically parameterized using Step Selection Functions. The final IBM model was validated using both k-fold cross-validation and emergent patterns validation and was tested for two different scenarios, with varying hardwood encroachment. Our results highlighted a functional response in habitat selection, which suggests that our method was able to capture the complexity of the natural system, and adequately provided projections on future possible states of the system in response to different management plans. This is especially relevant for testing the long-term impact of scenarios corresponding to environmental configurations that have yet to be observed in real systems.</p></div
Magnetic fluctuations in the classical XY model: the origin of an exponential tail in a complex system
We study the probability density function for the fluctuations of the
magnetic order parameter in the low temperature phase of the XY model of finite
size. In two-dimensions this system is critical over the whole of the low
temperature phase. It is shown analytically and without recourse to the scaling
hypothesis that, in this case, the distribution is non-Gaussian and of
universal form, independent of both system size and critical exponent .
An exact expression for the generating function of the distribution is
obtained, which is transformed and compared with numerical data from high
resolution molecular dynamics and Monte Carlo simulations. The calculation is
extended to general dimension and an exponential tail is found in all
dimensions less than four, despite the fact that critical fluctuations are
limited to D=2. These results are discussed in the light of similar behaviour
observed in models of interface growth and for dissipative systems driven into
a non-equilibrium steady state.Comment: 32 pages, 13 figures, 1 table. Few changes. To appear in Phys. Rev.
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