4,488 research outputs found
Non-equilibrium Relaxation Study of Ferromagnetic Transition in Double-Exchange Systems
Ferromagnetic transition in double-exchange systems is studied by
non-equilibrium relaxation technique combined with Monte Carlo calculations.
Critical temperature and critical exponents are estimated from relaxation of
the magnetic moment. The results are consistent with the previous Monte Carlo
results in thermal equilibrium. The exponents estimated by these independent
techniques suggest that the universality class of this transition is the same
as that of short-range interaction models but is different from the mean-field
one.Comment: 3 pages including 1 figure, submitted to J. Phys. Soc. Jp
Frustrated quantum Heisenberg ferrimagnetic chains
We study the ground-state properties of weakly frustrated Heisenberg
ferrimagnetic chains with nearest and next-nearest neighbor antiferromagnetic
exchange interactions and two types of alternating sublattice spins S_1 > S_2,
using 1/S spin-wave expansions, density-matrix renormalization group, and
exact- diagonalization techniques. It is argued that the zero-point spin
fluctuations completely destroy the classical commensurate- incommensurate
continuous transition. Instead, the long-range ferrimagnetic state disappears
through a discontinuous transition to a singlet state at a larger value of the
frustration parameter. In the ferrimagnetic phase we find a disorder point
marking the onset of incommensurate real-space short-range spin-spin
correlations.Comment: 16 pages (LaTex 2.09), 6 eps figure
A Model for the Analysis of Caries Occurrence in Primary Molar Tooth Surfaces
Recently methods of caries quantification in the primary dentition have moved away from summary ‘whole mouth’ measures at the individual level to methods based on generalised linear modelling (GLM) approaches or survival analysis approaches. However, GLM approaches based on logistic transformation fail to take into account the time-dependent process of tooth/surface survival to caries. There may also be practical difficulties associated with casting parametric survival-based approaches in a complex multilevel hierarchy and the selection of an optimal survival distribution, while non-parametric survival methods are not generally suitable for the assessment of supplementary information recorded on study participants. In the current investigation, a hybrid semi-parametric approach comprising elements of survival-based and GLM methodologies suitable for modelling of caries occurrence within fixed time periods is assessed, using an illustrative multilevel data set of caries occurrence in primary molars from a cohort study, with clustering of data assumed to occur at surface and tooth levels. Inferences of parameter significance were found to be consistent with previous parametric survival-based analyses of the same data set, with gender, socio-economic status, fluoridation status, tooth location, surface type and fluoridation status-surface type interaction significantly associated with caries occurrence. The appropriateness of the hierarchical structure facilitated by the hybrid approach was also confirmed. Hence the hybrid approach is proposed as a more appropriate alternative to primary caries modelling than non-parametric survival methods or other GLM-based models, and as a practical alternative to more rigorous survival-based methods unlikely to be fully accessible to most researchers
A Gravitational Instability-Driven Viscosity in Self-Gravitating Accretion Disks
We derive a viscosity from gravitational instability in self-gravitating
accretion disks, which has the required properties to account for the observed
fast formation of the first super-massive black holes in highly redshifted
quasars and for the cosmological evolution of the black hole-mass distribution.Comment: 14 pages, 1 figure, ApJ Letters (in press
Stable dynamics in forced systems with sufficiently high/low forcing frequency
We consider a class of parametrically forced Hamiltonian systems with
one-and-a-half degrees of freedom and study the stability of the dynamics when
the frequency of the forcing is relatively high or low. We show that, provided
the frequency of the forcing is sufficiently high, KAM theorem may be applied
even when the forcing amplitude is far away from the perturbation regime. A
similar result is obtained for sufficiently low frequency forcing, but in that
case we need the amplitude of the forcing to be not too large; however we are
still able to consider amplitudes of the forcing which are outside of the
perturbation regime. Our results are illustrated by means of numerical
simulations for the system of a forced cubic oscillator. In addition, we find
numerically that the dynamics are stable even when the forcing amplitude is
very large (beyond the range of validity of the analytical results), provided
the frequency of the forcing is taken correspondingly low.Comment: 12 pages, 3 figures, 2 table
Measurement of the analyzing power in pp elastic scattering in the peak CNI region at RHIC
We report the first measurements of the A_N absolute value and shape in the
-t range from 0.0015 to 0.010GeV/c^2 with a precision better than 0.005 for
each A_N data point using a polarized atomic hydrogen gas jet target and the
100 GeV RHIC proton beam.Comment: 4 pages, 5 figure
Frustrated spin model as a hard-sphere liquid
We show that one-dimensional topological objects (kinks) are natural degrees
of freedom for an antiferromagnetic Ising model on a triangular lattice. Its
ground states and the coexistence of spin ordering with an extensive
zero-temperature entropy can be easily understood in terms of kinks forming a
hard-sphere liquid. Using this picture we explain effects of quantum spin
dynamics on that frustrated model, which we also study numerically.Comment: 5 pages, 3 figure
Cluster variation method and disorder varieties of two-dimensional Ising-like models
I show that the cluster variation method, long used as a powerful hierarchy
of approximations for discrete (Ising-like) two-dimensional lattice models,
yields exact results on the disorder varieties which appear when competitive
interactions are put into these models. I consider, as an example, the
plaquette approximation of the cluster variation method for the square lattice
Ising model with nearest-neighbor, next-nearest-neighbor and plaquette
interactions, and, after rederiving known results, report simple closed-form
expressions for the pair and plaquette correlation functions.Comment: 10 revtex pages, 1 postscript figur
- …