10 research outputs found
Geometric frustration in the mixed layer pnictide oxides
We present results from a Monte Carlo investigation of a simple bilayer model
with geometrically frustrated interactions similar to those found in the mixed
layer pnictide oxides Our model is
composed of two inequivalent square lattices with nearest neighbor intra- and
interlayer interactions. We find a ground state composed of two independent
N\'{e}el ordered layers when the interlayer exchange is an order of magnitude
weaker than the intralayer exchange, as suggested by experiment. We observe
this result independent of the number of layers in our model. We find evidence
for local orthogonal order between the layers, but it occurs in regions of
parameter space that are not experimentally realized. We conclude that
frustration caused by nearest neighbor interactions in the mixed layer pnictide
oxides is not sufficient to explain the long--range orthogonal order that is
observed experimentally, and that it is likely that other terms (e.g., local
anisotropies) in the Hamiltonian are required to explain the magnetic behavior.Comment: Revetex, 4 pages, 3 figures, to appear in the proceedings of "HFM
2000" (Waterloo, June 2000); submitted to Can. J. Phy
Geometric frustration in the mixed layer pnictide oxides
We present results from a Monte Carlo investigation of a simple bilayer model
with geometrically frustrated interactions similar to those found in the mixed
layer pnictide oxides Our model is
composed of two inequivalent square lattices with nearest neighbor intra- and
interlayer interactions. We find a ground state composed of two independent
N\'{e}el ordered layers when the interlayer exchange is an order of magnitude
weaker than the intralayer exchange, as suggested by experiment. We observe
this result independent of the number of layers in our model. We find evidence
for local orthogonal order between the layers, but it occurs in regions of
parameter space that are not experimentally realized. We conclude that
frustration caused by nearest neighbor interactions in the mixed layer pnictide
oxides is not sufficient to explain the long--range orthogonal order that is
observed experimentally, and that it is likely that other terms (e.g., local
anisotropies) in the Hamiltonian are required to explain the magnetic behavior.Comment: Revetex, 4 pages, 3 figures, to appear in the proceedings of "HFM
2000" (Waterloo, June 2000); submitted to Can. J. Phy
Ill-Behaved Convergence of a Model of the Gd3Ga5O12 Garnet Antiferromagnet with Truncated Magnetic Dipole-Dipole Interactions
Previous studies have found that calculations which consider long-range
magnetic dipolar interactions truncated at a finite cut-off distance Rc predict
spurious (unphysical) long-range ordered phases for Ising and Heisenberg
systems on the pyrochlore lattice. In this paper we show that, similar to these
two cases, calculations that use truncated dipolar interactions to model the
Gd3Ga5O12 garnet antiferromagnet also predict unphysical phases with
incommensurate ordering wave vector q_ord that is very sensitive to the dipolar
cut-off distance Rc.Comment: 7 pages, 2 color figures; Proceedings of the HFM2006 conference, to
appear in a special issue of J. Phys.: Condens. Matte
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
Understanding Paramagnetic Spin Correlations in the Spin-Liquid Pyrochlore Tb2Ti2O7
Recent elastic and inelastic neutron scattering studies of the highly
frustrated pyrochlore antiferromagnet Tb2Ti2O7 have shown some very intriguing
features that cannot be modeled by the local classical Ising model,
naively expected to describe this system at low temperatures. Using the random
phase approximation to take into account fluctuations between the ground state
doublet and the first excited doublet, we successfully describe the elastic
neutron scattering pattern and dispersion relations in Tb2Ti2O7,
semi-quantitatively consistent with experimental observations.Comment: revtex4, 4 pages, 1 Color+ 2 BW figure
Sign change of the extended s-wave pairing vertex in the dynamic Hubbard model : a quantum Monte Carlo study
8 pagesInternational audienceThe “dynamic” Hubbard Hamiltonian describes interacting fermions on a lattice whose on-site repulsion is modulated by a coupling to a fluctuating bosonic field. We investigate one such model, introduced by Hirsch, using the determinant quantum Monte Carlo method. Our key result is that the extended s-wave pairing vertex, repulsive in the usual static Hubbard model, becomes attractive as the coupling to the fluctuating Bose field increases. The sign problem prevents us from exploring a low enough temperature to see if a superconducting transition occurs. We also observe a stabilization of antiferromagnetic correlations and the Mott gap near half-filling, and a near linear behavior of the energy as a function of particle density which indicates a tendency toward phase separation