277 research outputs found
Anisotropy effects in a mixed quantum-classical Heisenberg model in two dimensions
We analyse a specific two dimensional mixed spin Heisenberg model with
exchange anisotropy, by means of high temperature expansions and Monte Carlo
simulations. The goal is to describe the magnetic properties of the compound
(NBu_{4})_{2}Mn_{2}[Cu(opba)]_{3}\cdot 6DMSO\cdot H_{2}O which exhibits a
ferromagnetic transition at . Extrapolating our analysis on the
basis of renormalisation group arguments, we find that this transition may
result from a very weak anisotropy effect.Comment: 8 pages, 10 Postscript figure
Thermodynamics of a mixed quantum-classical Heisenberg model in two dimensions
We study the planar antiferromagnetic Heisenberg model on a decorated
hexagonal lattice, involving both classical spins (occupying the vertices) and
quantum spins (occupying the middle of the links). This study is motivated by
the description of a recently synthesized molecular magnetic compound. First,
we trace out the spin 1/2 degrees of freedom to obtain a fully classical model
with an effective ferromagnetic interaction. Then, using high temperature
expansions and Monte Carlo simulations, we analyse its thermal and magnetic
properties. We show that it provides a good quantitative description of the
magnetic susceptibility of the molecular magnet in its paramagnetic phase.Comment: Revtex, 6 pages, 4 included postscript figures, fig.1 upon request to
[email protected] . To appear in J. of Physic C (condensed matter
Phenomenology of the Pentaquark Antidecuplet
We consider the mass splittings and strong decays of members of the
lowest-lying pentaquark multiplet, which we take to be a parity-odd
antidecuplet. We derive useful decompositions of the quark model wave functions
that allow for easy computation of color-flavor-spin matrix elements. We
compute mass splittings within the antidecuplet including spin-color and
spin-isospin interactions between constituents and point out the importance of
hidden strangeness in rendering the nucleon-like states heavier than the S=1
state. Using recent experimental data on a possible S=1 pentaquark state, we
make decay predictions for other members of the antidecuplet.Comment: 12 pages LaTeX, 1 eps figur
Magnetic properties of exactly solvable doubly decorated Ising-Heisenberg planar models
Applying the decoration-iteration procedure, we introduce a class of exactly
solvable doubly decorated planar models consisting both of the Ising- and
Heisenberg-type atoms. Exact solutions for the ground state, phase diagrams and
basic physical quantities are derived and discussed. The detailed analysis of
the relevant quantities suggests the existence of an interesting quantum
antiferromagnetic phase in the system.Comment: 9 pages, 9 figures, submitted to Physical Review
Iatrogenic damage to the mandibular nerves as assessed by the masseter inhibitory reflex
Iatrogenic injury of the inferior alveolar or lingual nerves frequently leads to legal actions for damage and compensation for personal suffering. The masseter inhibitory reflex (MIR) is the most used neurophysiological tool for the functional assessment of the trigeminal mandibular division. Aiming at measuring the MIR sensitivity and specificity, we recorded this reflex after mental and tongue stimulations in a controlled, blinded study in 160 consecutive patients with sensory disturbances following dental procedures. The MIR latency was longer on the affected than the contralateral side (P < 0.0001). The overall specificity and sensitivity were 99 and 51%. Our findings indicate that MIR testing, showing an almost absolute specificity, reliably demonstrates nerve damage beyond doubt, whereas the relatively low sensitivity makes the finding of a normal MIR by no means sufficient to exclude nerve damage. Probably, the dysfunction of a small number of nerve fibres, insufficient to produce a MIR abnormality, may still engender important sensory disturbances. We propose that MIR testing, when used for legal purposes, be considered reliable in one direction only, i.e. abnormality does prove nerve damage, normality does not disprove it
Magnetic and thermal properties of 4f-3d ladder-type molecular compounds
We report on the low-temperature magnetic susceptibilities and specific heats
of the isostructural spin-ladder molecular complexes L[M(opba)]_{3\cdot
xDMSOHO, hereafter abbreviated with LM (where L =
La, Gd, Tb, Dy, Ho and M = Cu, Zn). The results show that the Cu containing
complexes (with the exception of LaCu) undergo long range magnetic
order at temperatures below 2 K, and that for GdCu this ordering is
ferromagnetic, whereas for TbCu and DyCu it is probably
antiferromagnetic. The susceptibilities and specific heats of TbCu
and DyCu above have been explained by means of a model
taking into account nearest as well as next-nearest neighbor magnetic
interactions. We show that the intraladder L--Cu interaction is the predominant
one and that it is ferromagnetic for L = Gd, Tb and Dy. For the cases of Tb, Dy
and Ho containing complexes, strong crystal field effects on the magnetic and
thermal properties have to be taken into account. The magnetic coupling between
the (ferromagnetic) ladders is found to be very weak and is probably of dipolar
origin.Comment: 13 pages, 15 figures, submitted to Phys. Rev.
A Naturally Narrow Positive Parity Theta^+
We present a consistent color-flavor-spin-orbital wave function for a
positive parity Theta^+ that naturally explains the observed narrowness of the
state. The wave function is totally symmetric in its flavor-spin part and
totally antisymmetric in its color-orbital part. If flavor-spin interactions
dominate, this wave function renders the positive parity Theta^+ lighter than
its negative parity counterpart. We consider decays of the Theta^+ and compute
the overlap of this state with the kinematically allowed final states. Our
results are numerically small. We note that dynamical correlations between
quarks are not necessary to obtain narrow pentaquark widths.Comment: 10 pages, 1 figure, Revtex4, two-column format, version to be
published in Phys. Rev. D, includes numerical estimates of decay width
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