90 research outputs found
Langevin Simulations of a Long Range Electron Phonon Model
We present a Quantum Monte Carlo (QMC) study, based on the Langevin equation,
of a Hamiltonian describing electrons coupled to phonon degrees of freedom. The
bosonic part of the action helps control the variation of the field in
imaginary time. As a consequence, the iterative conjugate gradient solution of
the fermionic action, which depends on the boson coordinates, converges more
rapidly than in the case of electron-electron interactions, such as the Hubbard
Hamiltonian. Fourier Acceleration is shown to be a crucial ingredient in
reducing the equilibration and autocorrelation times. After describing and
benchmarking the method, we present results for the phase diagram focusing on
the range of the electron-phonon interaction. We delineate the regions of
charge density wave formation from those in which the fermion density is
inhomogeneous, caused by phase separation. We show that the Langevin approach
is more efficient than the Determinant QMC method for lattice sizes and that it therefore opens a potential path to problems including,
for example, charge order in the 3D Holstein model
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
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