283 research outputs found
Efficient calculation of the antiferromagnetic phase diagram of the 3D Hubbard model
The Dynamical Cluster Approximation with Betts clusters is used to calculate
the antiferromagnetic phase diagram of the 3D Hubbard model at half filling.
Betts clusters are a set of periodic clusters which best reflect the properties
of the lattice in the thermodynamic limit and provide an optimal finite-size
scaling as a function of cluster size. Using a systematic finite-size scaling
as a function of cluster space-time dimensions, we calculate the
antiferromagnetic phase diagram. Our results are qualitatively consistent with
the results of Staudt et al. [Eur. Phys. J. B 17 411 (2000)], but require the
use of much smaller clusters: 48 compared to 1000
Statistical mechanical description of liquid systems in electric field
We formulate the statistical mechanical description of liquid systems for
both polarizable and polar systems in an electric field in the
-ensemble, which is the pendant to the thermodynamic description in
terms of the free energy at constant potential. The contribution of the
electric field to the configurational integral in
the -ensemble is given in an exact form as a factor in the
integrand of . We calculate the contribution of the
electric field to the Ornstein-Zernike formula for the scattering function in
the -ensemble. As an application we determine the field induced
shift of the critical temperature for polarizable and polar liquids, and show
that the shift is upward for polarizable liquids and downward for polar
liquids.Comment: 6 page
Phase transition in a 2-dimensional Heisenberg model
We investigate the two-dimensional classical Heisenberg model with a
nonlinear nearest-neighbor interaction
V(s,s')=2K[(1+s.s')/2 ]^p.
The analogous nonlinear interaction for the XY model was introduced by
Domany, Schick, and Swendsen, who find that for large p the Kosterlitz-Thouless
transition is preempted by a first-order transition. Here we show that, whereas
the standard (p=1) Heisenberg model has no phase transition, for large enough p
a first-order transition appears. Both phases have only short range order, but
with a correlation length that jumps at the transition.Comment: 6 pages, 5 encapsulated postscript figures; to appear in Physical
Review Letter
The 1/D Expansion for Classical Magnets: Low-Dimensional Models with Magnetic Field
The field-dependent magnetization m(H,T) of 1- and 2-dimensional classical
magnets described by the -component vector model is calculated analytically
in the whole range of temperature and magnetic fields with the help of the 1/D
expansion. In the 1-st order in 1/D the theory reproduces with a good accuracy
the temperature dependence of the zero-field susceptibility of antiferromagnets
\chi with the maximum at T \lsim |J_0|/D (J_0 is the Fourier component of the
exchange interaction) and describes for the first time the singular behavior of
\chi(H,T) at small temperatures and magnetic fields: \lim_{T\to 0}\lim_{H\to 0}
\chi(H,T)=1/(2|J_0|)(1-1/D) and \lim_{H\to 0}\lim_{T\to 0}
\chi(H,T)=1/(2|J_0|)
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
Theory of paramagnetic scattering in highly frustrated magnets with long-range dipole-dipole interactions: The case of the Tb2Ti2O7, pyrochlore antiferromagnet
Highly frustrated antiferromagnets composed of magnetic rare-earth moments
are currently attracting much experimental and theoretical interest. Rare-earth
ions generally have small exchange interactions and large magnetic moments.
This makes it necessary to understand in detail the role of long-range magnetic
dipole-dipole interactions in these systems, in particular in the context of
spin-spin correlations that develop in the paramagnetic phase, but are often
unable to condense into a conventional long-range magnetic ordered phase. This
scenario is most dramatically emphasized in the frustrated pyrochlore
antiferromagnet material Tb2Ti207 which does not order down to 50 mK despite an
antiferromagnetic Curie-Weiss temperature Tcw ~ -20 K. In this paper we report
results from mean-field theory calculations of the paramagnetic elastic
neutron-scattering in highly frustrated magnetic systems with long-range
dipole-dipole interactions, focusing on the Tb2Ti207 system. Modeling Tb2Ti207
as an antiferromagnetic Ising pyrochlore, we find that the mean-field
paramagnetic scattering is inconsistent with the experimentally observed
results. Through simple symmetry arguments we demonstrate that the observed
paramagnetic correlations in Tb2Ti207 are precluded from being generated by any
spin Hamiltonian that considers only Ising spins, but are qualitatively
consistent with Heisenberg-like moments. Explicit calculations of the
paramagnetic scattering pattern for both Ising and Heisenberg models,
which include finite single-ion anisotropy, support these claims. We offer
suggestions for reconciling the need to restore spin isotropy with the Ising
like structure suggested by the single-ion properties of Tb3+.Comment: Revtex4, 18 pages, 3 eps figures (2 color figures). Change in title
and emphasis on Tb2Ti2O7 only. Spin-ice material removed, to appear in a
later publicatio
Fundamental measure theory for lattice fluids with hard core interactions
We present the extension of Rosenfeld's fundamental measure theory to lattice
models by constructing a density functional for d-dimensional mixtures of
parallel hard hypercubes on a simple hypercubic lattice. The one-dimensional
case is exactly solvable and two cases must be distinguished: all the species
with the same lebgth parity (additive mixture), and arbitrary length parity
(nonadditive mixture). At the best of our knowledge, this is the first time
that the latter case is considered. Based on the one-dimensional exact
functional form, we propose the extension to higher dimensions by generalizing
the zero-dimensional cavities method to lattice models. This assures the
functional to have correct dimensional crossovers to any lower dimension,
including the exact zero-dimensional limit. Some applications of the functional
to particular systems are also shown.Comment: 22 pages, 7 figures, needs IOPP LaTeX styles file
Thermodynamic properties of ferromagnetic mixed-spin chain systems
Using a combination of high-temperature series expansion, exact
diagonalization and quantum Monte Carlo, we perform a complementary analysis of
the thermodynamic properties of quasi-one-dimensional mixed-spin systems with
alternating magnetic moments. In addition to explicit series expansions for
small spin quantum numbers, we present an expansion that allows a direct
evaluation of the series coefficients as a function of spin quantum numbers.
Due to the presence of excitations of both acoustic and optical nature, the
specific heat of a mixed-spin chain displays a double-peak-like structure,
which is more pronounced for ferromagnetic than for antiferromagnetic
intra-chain exchange. We link these results to an analytically solvable
half-classical limit. Finally, we extend our series expansion to incorporate
the single-ion anisotropies relevant for the molecular mixed-spin ferromagnetic
chain material MnNi(NO)(ethylenediamine), with alternating
spins of magnitude 5/2 and 1. Including a weak inter-chain coupling, we show
that the observed susceptibility allows for an excellent fit, and the
extraction of microscopic exchange parameters.Comment: 8 pages including 7 figures, submitted to Phys. Rev. B; series
extended to 29th. QMC adde
Power-law correlations and orientational glass in random-field Heisenberg models
Monte Carlo simulations have been used to study a discretized Heisenberg
ferromagnet (FM) in a random field on simple cubic lattices. The spin variable
on each site is chosen from the twelve [110] directions. The random field has
infinite strength and a random direction on a fraction x of the sites of the
lattice, and is zero on the remaining sites. For x = 0 there are two phase
transitions. At low temperatures there is a [110] FM phase, and at intermediate
temperature there is a [111] FM phase. For x > 0 there is an intermediate phase
between the paramagnet and the ferromagnet, which is characterized by a
|k|^(-3) decay of two-spin correlations, but no true FM order. The [111] FM
phase becomes unstable at a small value of x. At x = 1/8 the [110] FM phase has
disappeared, but the power-law correlated phase survives.Comment: 8 pages, 12 Postscript figure
Double Exchange Alone Does Not Explain the Resistivity of
The system with has
traditionally been modelled with a ``double exchange'' Hamiltonian, in which it
is assumed that the only relevant physics is the tendency of carrier hopping to
line up neighboring spins. We present a solution of the double exchange model,
show it is incompatible with many aspects of the resistivity data, and propose
that a strong electron-phonon interaction arising from a Jahn-Teller splitting
of the outer Mn d-level plays a crucial role.Comment: Figure available via concentional mail. Contact
[email protected]
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