452 research outputs found
Ising antiferromagnet with mobile, pinned and quenched defects
Motivated by recent experiments on (Sr,Ca,La)_14 Cu_24 O_41, a
two-dimensional Ising antiferromagnet with mobile, locally pinned and quenched
defects is introduced and analysed using mainly Monte Carlo techniques. The
interplay between the arrangement of the defects and the magnetic ordering as
well as the effect of an external field are studied.Comment: 10 pages, 6 figures. Condensed Matter Physics (Festschrift in honour
of R. Folk
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.
Two-dimensional anisotropic Heisenberg antiferromagnet in a field
The classical, square lattice, uniaxially anisotropic Heisenberg
antiferromagnet in a magnetic field parallel to the easy axis is studied using
Monte Carlo techniques. The model displays a long-range ordered
antiferromagnetic, an algebraically ordered spin-flop, and a paramagnetic
phase. The simulations indicate that a narrow disordered phase intervenes
between the ordered phases down to quite low temperatures. Results are compared
to previous, partially conflicting findings on related classical models as well
as the quantum variant with spin S=1/2.Comment: 8 pages, 9 figure
Comment on "Frustrating interactions and broadened magnetic interactions in the edge-sharing CuO_2 chains in La_5 Ca_9 Cu_24 O_41"
Using Monte Carlo techniques, we show that the two--dimensional anisotropic
Heisenberg model reproducing nicely inelastic neutron scattering measurements
on La_5 Ca_9 Cu_24 O_41 (Matsuda et al. [Phys. Rev. B 68, 060406(R) (2003)])
seems to be insufficient to describe correctly measurements on thermodynamic
quantities like the magnetization or the susceptibility. Possible reasons for
the discrepancy are suggested.Comment: 3 pages, 2 EPS figures; part (ii) rewritten, some typos corrected;
final version that has been accepted for publication in Phys. Rev.
Ising model with periodic pinning of mobile defects
A two-dimensional Ising model with short-range interactions and mobile
defects describing the formation and thermal destruction of defect stripes is
studied. In particular, the effect of a local pinning of the defects at the
sites of straight equidistant lines is analysed using Monte Carlo simulations
and the transfer matrix method. The pinning leads to a long-range ordered
magnetic phase at low temperatures. The dependence of the phase transition
temperature, at which the defect stripes are destabilized, on the pinning
strength is determined. The transition seems to be of first order, with and
without pinning.Comment: 7 pages, 7 figure
Phase diagrams of a classical two-dimensional Heisenberg antiferromagnet with single-ion anisotropy
A classical variant of the two-dimensional anisotropic Heisenberg model
reproducing inelastic neutron scattering experiments on La_5 Ca_9 Cu_24 O_41
[M. Matsuda et al., Phys.Rev. B 68, 060406(R) (2003)] is analysed using mostly
Monte Carlo techniques. Phase diagrams with external fields parallel and
perpendicular to the easy axis of the anisotropic interactions are determined,
including antiferromagnetic and spin-flop phases. Mobile spinless defects, or
holes, are found to form stripes which bunch, debunch and break up at a phase
transition. A parallel field can lead to a spin-flop phase.Comment: 9 pages, 9 figures; final version as accepted by Phys. Rev. B (Fig. 5
replaced, added remarks in Secs. I, III, and V
Interfacial adsorption phenomena of the three-dimensional three-state Potts model
We study the interfacial adsorption phenomena of the three-state
ferromagnetic Potts model on the simple cubic lattice by the Monte Carlo
method. Finite-size scaling analyses of the net-adsorption yield the evidence
of the phase transition being of first-order and .Comment: 14 page
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.
Quantum Monte Carlo scheme for frustrated Heisenberg antiferromagnets
When one tries to simulate quantum spin systems by the Monte Carlo method,
often the 'minus-sign problem' is encountered. In such a case, an application
of probabilistic methods is not possible. In this paper the method has been
proposed how to avoid the minus sign problem for certain class of frustrated
Heisenberg models. The systems where this method is applicable are, for
instance, the pyrochlore lattice and the Heisenberg model. The method
works in singlet sector. It relies on expression of wave functions in dimer
(pseudo)basis and writing down the Hamiltonian as a sum over plaquettes. In
such a formulation, matrix elements of the exponent of Hamiltonian are
positive.Comment: 19 LaTeX pages, 6 figures, 1 tabl
Surface critical behavior of two-dimensional dilute Ising models
Ising models with nearest-neighbor ferromagnetic random couplings on a square
lattice with a (1,1) surface are studied, using Monte Carlo techniques and
star-tiangle transformation method. In particular, the critical exponent of the
surface magnetization is found to be close to that of the perfect model,
beta_s=1/2. The crossover from surface to bulk critical properties is
discussed.Comment: 6 pages in RevTex, 3 ps figures, to appear in Journal of Stat. Phy
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