278 research outputs found
Mean-field phase diagrams of compounds
Magnetic-field -- temperature phase diagrams of the axial
next-nearest-neighbor Ising model are calculated within the framework of a
Landau-type expansion of the free energy derived from molecular-field theory.
Good qualitative agreement is found with recently reported results on
body-centered-tetragonal . This work is expected to also be relevant
for related compounds.Comment: J1K 2R1 8 pages (RevTex 3.0), 2 figures available upon request,
Report# CRPS-94-0
The critical Binder cumulant in a two--dimensional anisotropic Ising model with competing interaction
The Binder cumulant at the phase transition of Ising models on square
lattices with ferromagnetic couplings between nearest neighbors and with
competing antiferromagnetic couplings between next--nearest neighbors, along
only one diagonal, is determined using Monte Carlo techniques. In the phase
diagram a disorder line occurs separating regions with monotonically decaying
and with oscillatory spin--spin correlations. Findings on the variation of the
critical cumulant with the ratio of the two interaction strengths are compared
to related recent results based on renormalization group calculations.Comment: 4 pages, 4 figure
Quenched charge disorder in CuO2 spin chains: Experimental and numerical studies
We report on measurements of the magnetic response of the anisotropic CuO_2
spin chains in lightly hole-doped La_x (Ca,Sr)_14-x Cu_24 O_41, x>=5. The
experimental data suggest that in magnetic fields B >~ 4T (applied along the
easy axis) the system is characterized by short-range spin order and
quasi-static (quenched) charge disorder. The magnetic susceptibility chi(B)
shows a broad anomaly, which we interpret as the remnant of a spin-flop
transition. To corroborate this idea, we present Monte Carlo simulations of a
classical, anisotropic Heisenberg model with randomly distributed, static
holes. Our numerical results clearly show that the spin-flop transition of the
pure model (without holes) is destroyed and smeared out due to the disorder
introduced by the quasi-static holes. Both the numerically calculated
susceptibility curves chi(B) and the temperature dependence of the position of
the anomaly are in qualitative agreement with the experimental data.Comment: 10 pages, REVTeX4. 11 figures; v2: Fig.2 replaced, small changes in
Figs.1 and 11; minor revisons in Sec. III.C; accepted by Phys. Rev.
Ising magnets with mobile defects
Motivated by recent experiments on cuprates with low-dimensional magnetic
interactions, a new class of two-dimensional Ising models with short-range
interactions and mobile defects is introduced and studied. The non-magnetic
defects form lines, which, as temperature increases, first meander and then
become unstable. Using Monte Carlo simulations and analytical low- and
high-temperature considerations, the instability of the defect stripes is
monitored for various microscopic and thermodynamic quantities in detail for a
minimal model, assuming some of the couplings to be indefinitely strong. The
robustness of the findings against weakening the interactions is discussed as
well
Critical Binder cumulant for isotropic Ising models on square and triangular lattices
Using Monte Carlo techniques, the critical Binder cumulant U* of isotropic
nearest-neighbour Ising models on square and triangular lattices is studied.
For rectangular shapes, employing periodic boundary conditions, U* is found to
show the same dependence on the aspect ratio for both lattice types. Similarly,
applying free boundary conditions for systems with square as well as circular
shapes for both lattices, the simulational findings are also consistent with
the suggestion that, for isotropic Ising models with short-range interactions,
U* depends on the shape and the boundary condition, but not on the lattice
structure.Comment: 7 pages, 4 figures, submitted to J. Stat. Mec
Relaxation of Surface Profiles by Evaporation Dynamics
We present simulations of the relaxation towards equilibrium of one
dimensional steps and sinusoidal grooves imprinted on a surface below its
roughening transition. We use a generalization of the hypercube stacking model
of Forrest and Tang, that allows for temperature dependent
next-nearest-neighbor interactions. For the step geometry the results at T=0
agree well with the t^(1/4) prediction of continuum theory for the spreading of
the step. In the case of periodic profiles we modify the mobility for the tips
of the profile and find the approximate solution of the resulting free boundary
problem to be in reasonable agreement with the T=0 simulations.Comment: 6 pages, Revtex, 5 Postscript figures, to appear in PRB 15, October
199
Critical Binder cumulant in two-dimensional anisotropic Ising models
The Binder cumulant at the phase transition of Ising models on square
lattices with various ferromagnetic nearest and next-nearest neighbour
couplings is determined using mainly Monte Carlo techniques. We discuss the
possibility to relate the value of the critical cumulant in the isotropic,
nearest neighbour and in the anisotropic cases to each other by means of a
scale transformation in rectangular geometry, to pinpoint universal and
nonuniversal features.Comment: 7 pages, 4 figures, submitted to J. Phys.
Monte Carlo Study of Mixed-Spin S=(1/2,1) Ising Ferrimagnets
We investigate Ising ferrimagnets on square and simple-cubic lattices with
exchange couplings between spins of values S=1/2 and S=1 on neighbouring sites
and an additional single-site anisotropy term on the S=1 sites. Based mainly on
a careful and comprehensive Monte Carlo study, we conclude that there is no
tricritical point in the two--dimensional case, in contradiction to mean-field
predictions and recent series results. However, evidence for a tricritical
point is found in the three-dimensional case. In addition, a line of
compensation points is found for the simple-cubic, but not for the square
lattice.Comment: 14 pages, 11 figure
Boundary critical behaviour of two-dimensional random Ising models
Using Monte Carlo techniques and a star-triangle transformation, Ising models
with random, 'strong' and 'weak', nearest-neighbour ferromagnetic couplings on
a square lattice with a (1,1) surface are studied near the phase transition.
Both surface and bulk critical properties are investigated. In particular, the
critical exponents of the surface magnetization, 'beta_1', of the correlation
length, 'nu', and of the critical surface correlations, 'eta_{\parallel}', are
analysed.Comment: 16 pages in ioplppt style, 7 ps figures, submitted to J. Phys.
Critical behavior of the pure and random-bond two-dimensional triangular Ising ferromagnet
We investigate the effects of quenched bond randomness on the critical
properties of the two-dimensional ferromagnetic Ising model embedded in a
triangular lattice. The system is studied in both the pure and disordered
versions by the same efficient two-stage Wang-Landau method. In the first part
of our study we present the finite-size scaling behavior of the pure model, for
which we calculate the critical amplitude of the specific heat's logarithmic
expansion. For the disordered system, the numerical data and the relevant
detailed finite-size scaling analysis along the lines of the two well-known
scenarios - logarithmic corrections versus weak universality - strongly support
the field-theoretically predicted scenario of logarithmic corrections. A
particular interest is paid to the sample-to-sample fluctuations of the random
model and their scaling behavior that are used as a successful alternative
approach to criticality.Comment: 10 pages, 8 figures, slightly revised version as accepted for
publication in Phys. Rev.
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