1,862 research outputs found
Surface scaling behavior of isotropic Heisenberg systems: Critical exponents, structure factor, and profiles
The surface scaling behavior of classical isotropic Heisenberg magnets is
investigated by Monte - Carlo methods in d=3 dimensions for various values of
the surface - to - bulk coupling ratio J_1/J. For J_1/J <= 1.0 critical
behavior according to the ordinary surface universality class is found. New
estimates for magnetic surface exponents are presented and compared to older
estimates and their theoretical counterparts. For J_1/J >= 2.0 scaling is still
valid with effective exponents which depend on J_1/J. The surface structure
factor S_1(p,L) is investigated at bulk criticality as function of the momentum
transfer p parallel to the surface and the system size L. For J_1/J <= 1.0 and
J_1/J >= 2.0 the full p dependence of S_1(p,L) can be captured by generalized
shape functions to a remarkable accuracy. Profiles of the magnetization and the
energy density also confirm scaling, where for J_1/J <= 1.0 the ordinary
surface universality class is recovered and for J_1/J >= 2.0 scaling with J_1/J
dependent exponents is found. For J_1/J = 1.5 the system displays a striking
crossover behavior from spurious long - range surface order to the ordinary
surface universality class. For J_1/J >= 2.0 the effective scaling laws must be
interpreted as nonasymptotic and the value J_1/J = 1.5 marks a crossover
regime, in which the crossover from the nonasymptotic to the asymptotic
(ordinary) surface scaling behavior can be resolved within numerically
attainable system sizes.Comment: 14 pages RevTeX, 14 figures; to appear in Phys. Rev. B, Sept. 200
Anti-phase locking in a two-dimensional Josephson junction array
We consider theoretically phase locking in a simple two-dimensional Josephson
junction array consisting of two loops coupled via a joint line transverse to
the bias current. Ring inductances are supposed to be small, and special
emphasis is taken on the influence of external flux. Is is shown, that in the
stable oscillation regime both cells oscillate with a phase shift equal to
(i.e. anti-phase). This result may explain the low radiation output
obtained so far in two-dimensional Josephson junction arrays experimentally.Comment: 11 pages, REVTeX, 1 Postscript figure, Subm. to Appl. Phys. Let
Critical Casimir amplitudes for -component models with O(n)-symmetry breaking quadratic boundary terms
Euclidean -component theories whose Hamiltonians are O(n)
symmetric except for quadratic symmetry breaking boundary terms are studied in
films of thickness . The boundary terms imply the Robin boundary conditions
at the boundary
planes at and . Particular attention is paid
to the cases in which of the variables
take the special value corresponding to critical
enhancement while the remaining ones are subcritically enhanced. Under these
conditions, the semi-infinite system bounded by has a
multicritical point, called -special, at which an symmetric
critical surface phase coexists with the O(n) symmetric bulk phase, provided
is sufficiently large. The -dependent part of the reduced free energy
per area behaves as as at the bulk critical
point. The Casimir amplitudes are determined for small
in the general case where components are
critically enhanced at both boundary planes, components are
enhanced at one plane but satisfy asymptotic Dirichlet boundary conditions at
the respective other, and the remaining components satisfy asymptotic
Dirichlet boundary conditions at both . Whenever ,
these expansions involve integer and fractional powers with
(mod logarithms). Results to for general values of
, , and are used to estimate the
of 3D Heisenberg systems with surface spin anisotropies when , , and .Comment: Latex source file with 5 eps files; version with minor amendments and
corrected typo
The critical Casimir force and its fluctuations in lattice spin models: exact and Monte Carlo results
We present general arguments and construct a stress tensor operator for
finite lattice spin models. The average value of this operator gives the
Casimir force of the system close to the bulk critical temperature . We
verify our arguments via exact results for the force in the two-dimensional
Ising model, -dimensional Gaussian and mean spherical model with . On
the basis of these exact results and by Monte Carlo simulations for
three-dimensional Ising, XY and Heisenberg models we demonstrate that the
standard deviation of the Casimir force in a slab geometry confining a
critical substance in-between is , where is
the surface area of the plates, is the lattice spacing and is a
slowly varying nonuniversal function of the temperature . The numerical
calculations demonstrate that at the critical temperature the force
possesses a Gaussian distribution centered at the mean value of the force
, where is the distance between the
plates and is the (universal) Casimir amplitude.Comment: 21 pages, 7 figures, to appear in PR
Analysis of the Developmental Regulation of the Cyanogenic Compounds in Seedlings of Two Lines of \u3cem\u3eLinum usitatissimum\u3c/em\u3e L.
The developmental profiles and tissue distribution of the four cyanogenic compounds in seedlings of two developmentally contrasting inbred lines of flax (Linum usitatissimum L.) were examined using HPLC. During germination, the isoleucine-derived compound, neolinustatin, was hydrolysed faster in the more vigorous of the two lines. Furthermore, in this line, the neolinustatin content was higher in seeds and the accumulation of the other isoleucine-derived compound, lotaustralin, was also higher in the cotyledons of seedlings. In contrast, with one exception, the hydrolysis and accumulation of the valine-derived compounds, linustatin and linamarin, was the same in both lines. Differences in the levels of the compounds during germination, and in the hypocotyls, are interpreted as evidence for the involvement of transient levels of hydrogen cyanide in the autocatalytic regulation of ethylene production
Critical Casimir Effect in 3He-4He films
Universal aspects of the thermodynamic Casimir effect in wetting films of
3He-4He mixtures near their bulk tricritical point are studied within suitable
models serving as representatives of the corresponding universality class. The
effective forces between the boundaries of such films arising from the
confinement are calculated along isotherms at several fixed concentrations of
3He. Nonsymmetric boundary conditions impose nontrivial concentration profiles
leading to repulsive Casimir forces which exhibit a rich behavior of the
crossover between the tricritical point and the line of critical points. The
theoretical results agree with published experimental data and emphasize the
importance of logarithmic corrections.Comment: 12 pages, 4 figures, submitted to the Phys. Rev. Let
Phase diagram of a model for 3He-4He mixtures in three dimensions
A lattice model of 3He - 4He mixtures which takes into account the continuous
rotational symmetry O(2) of the superfluid degrees of freedom of 4He is studied
in the molecular-field approximation and by Monte Carlo simulations in three
dimensions. In contrast to its two-dimensional version, for reasonable values
of the interaction parameters the resulting phase diagram resembles that
observed experimentally for 3He - 4He mixtures, for which phase separation
occurs as a consequence of the superfluid transition. The corresponding
continuum Ginzburg-Landau model with two order parameters describing 3He- 4He
mixtures near tricriticality is derived from the considered lattice model. All
coupling constants appearing in the continuum model are explicitly expressed in
terms of the mean concentration of 4He, the temperature, and the microscopic
interaction parameters characterizing the lattice system.Comment: 32 pages, 12 figures, submitted to the Phys. Rev.
Casimir Forces at Tricritical Points: Theory and Possible Experiments
Using field-theoretical methods and exploiting conformal invariance, we study
Casimir forces at tricritical points exerted by long-range fluctuations of the
order-parameter field. Special attention is paid to the situation where the
symmetry is broken by the boundary conditions (extraordinary transition).
Besides the parallel-plate configuration, we also discuss the geometries of two
separate spheres and a single sphere near a planar wall, which may serve as a
model for colloidal particles immersed in a fluid. In the concrete case of
ternary mixtures a quantitative comparison with critical Casimir and van der
Waals forces shows that, especially with symmetry-breaking boundaries, the
tricritical Casimir force is considerably stronger than the critical one and
dominates also the competing van der Waals force.Comment: 18 pages, Latex, 3 postscript figures, uses Elsevier style file
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