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Stochastic Vortex Dynamics in Two-Dimensional Easy Plane Ferromagnets: Multiplicative Versus Additive Noise

Description: We study how thermal fluctuations affect the dynamics of vortices in the two-dimensional anisotropic Heisenberg model depending on their additive or multiplicative character. Using a collective coordinate theory, we analytically show that multiplicative noise, arising from fluctuations in the local field term of the Landau-Lifshitz equations, and Langevin-like additive noise have the same effect on vortex dynamics (within a very plausible assumption consistent with the collective coordinate approach). This is a highly non-trivial result as multiplicative and additive noises usually modify the dynamics in very different ways. We also carry out numerical simulations of both versions of the model finding that they indeed give rise to very similar vortex dynamics.
Date: September 1, 1998
Creator: Kamppeter, T.; Mertens, F.G.; Moro, E.; Sanchez, A. & Bishop, A.R.
Partner: UNT Libraries Government Documents Department

Finite temperature spin-dynamics and phase transitions in spin-orbital models

Description: We study finite temperature properties of a generic spin-orbital model relevant to transition metal compounds, having coupled quantum Heisenberg-spin and Ising-orbital degrees of freedom. The model system undergoes a phase transition, consistent with that of a 2D Ising model, to an orbitally ordered state at a temperature set by short-range magnetic order. At low temperatures the orbital degrees of freedom freeze-out and the model maps onto a quantum Heisenberg model. The onset of orbital excitations causes a rapid scrambling of the spin spectral weight away from coherent spin-waves, which leads to a sharp increase in uniform magnetic susceptibility just below the phase transition, reminiscent of the observed behavior in the Fe-pnictide materials.
Date: April 29, 2010
Creator: Chen, C.-C.
Partner: UNT Libraries Government Documents Department

Multiple Walkers in the Wang-Landau Algorithm

Description: The mean cost for converging an estimated density of states using the Wang-Landau algorithm is measured for the Ising and Heisenberg models. The cost increases in a power-law fashion with the number of spins, with an exponent near 3 for one-dimensional models, and closer to 2.4 for two-dimensional models. The effect of multiple, simultaneous walkers on the cost is also measured. For the one-dimensional Ising model the cost can increase with the number of walkers for large systems. For both the Ising and Heisenberg models in two-dimensions, no adverse impact on the cost is observed. Thus multiple walkers is a strategy that should scale well in a parallel computing environment for many models of magnetic materials.
Date: December 28, 2005
Creator: Brown, G
Partner: UNT Libraries Government Documents Department

Temperature dependence of soliton diffusion in trans-polyacetylene

Description: The temperature dependence of 1-D diffusion rate of solitons in transpolyacetylene is determined by time-domain analysis of ESR measurements. The diffusion rate appears to obey a simple power law. Monte Carlo simulation of 1-D diffusion process in impure chains indicates that overall diffusion can be much slower than that without traps.
Date: July 1, 1997
Creator: Tang, J.; Norris, J.R. & Isoya, J.
Partner: UNT Libraries Government Documents Department

First-principles exchange interactions between ferro and antiferromagnetic films: Co on NiMn, a case study

Description: Heisenberg exchange parameters at the interface of antiferromagnetic NiMn with ferromagnetic Co are calculated from first-principles. The authors use a layer version of the Korringa-Kohn-Rostocker multiple scattering approach and an expression, which is based on the expansion of the band energy, to calculate the exchange parameters from the underlying electronic structure. For bulk systems, the parameter sets yield Curie temperatures that are in good agreement with experimental values. In the interface region, the inter-layer interactions in NiMn change significantly compared to the bulk while the intra-layer interactions are almost unchanged.
Date: December 31, 1997
Creator: Schulthess, T.C. & Butler, W.H.
Partner: UNT Libraries Government Documents Department

Neutron study of fracton excitations in percolating antiferromagnets

Description: The authors report the results of an inelastic neutron scattering experiment on nearly-percolating Heisenberg antiferromagnets (RbMn{sub c}Mg{sub 1{minus}o}F{sub 3}), in which the Mn concentrations (C = 0.31, 0.34 and 0.39) are very close to the percolation threshold (c{sub p} = 0.312). A broad peak superimposed on Ising-cluster excitations was observed throughout the Brillouin zone. The intensity of a broad peak increased on approaching the percolation threshold. The origin of this broad peak is attributed to the excitation of fractons in a percolating network.
Date: June 27, 1997
Creator: Ikeda, H.; Takahashi, M.; Fernandez-Baca, J.A. & Nicklow, R.M.
Partner: UNT Libraries Government Documents Department

Low energy spin-wave excitations in the bilayer manganite La{sub 1.2}Sr{sub 1.8}Mn{sub 2}O{sub 7}.

Description: Inelastic neutron scattering experiments were performed on a single crystal of the bilayer manganite La{sub 1.2}Sr{sub 1.8}Mn{sub 2}O{sub 7}. Low energy spin-wave excitations were observed along the c direction with a maximum energy of {approx} 0.5 meV at the zone boundary. The dispersion of these acoustic spin wave modes is modeled by a nearest-neighbor Heisenberg model with an inter-bilayer exchange interaction between neighboring spins in different bilayers of 0.048(1) meV and an anisotropy gap of {Delta} = 0.077(3) meV. These results confirm the two-dimensional nature of the spin-correlations in the bilayer manganites, with a ratio of the in-plane to inter-bilayer interaction of {approx}200. The temperature dependence of the energies and intensities of the spin wave excitations are in agreement with our earlier conclusion that the ferromagnetic transition is second-order.
Date: September 21, 1999
Creator: Rosenkranz, S.; Osborn, R.; Mitchell, J. F.; Vasiliu-Doloc, L.; Lynn, J. W. & Sinha, S. K.
Partner: UNT Libraries Government Documents Department

Chebyshev recursion methods: Kernel polynomials and maximum entropy

Description: The authors describe two Chebyshev recursion methods for calculations with very large sparse Hamiltonians, the kernel polynomial method (KPM) and the maximum entropy method (MEM). They are especially applicable to physical properties involving large numbers of eigenstates, which include densities of states, spectral functions, thermodynamics, total energies, as well as forces for molecular dynamics and Monte Carlo simulations. The authors apply Chebyshev methods to the electronic structure of Si, the thermodynamics of Heisenberg antiferromagnets, and a polaron problem.
Date: October 1, 1995
Creator: Silver, R.N.; Roeder, H.; Voter, A.F. & Kress, J.D.
Partner: UNT Libraries Government Documents Department

Cluster algorithms with empahsis on quantum spin systems

Description: The purpose of this lecture is to discuss in detail the generalized approach of Kawashima and Gubernatis for the construction of cluster algorithms. We first present a brief refresher on the Monte Carlo method, describe the Swendsen-Wang algorithm, show how this algorithm follows from the Fortuin-Kastelyn transformation, and re=interpret this transformation in a form which is the basis of the generalized approach. We then derive the essential equations of the generalized approach. This derivation is remarkably simple if done from the viewpoint of probability theory, and the essential assumptions will be clearly stated. These assumptions are implicit in all useful cluster algorithms of which we are aware. They lead to a quite different perspective on cluster algorithms than found in the seminal works and in Ising model applications. Next, we illustrate how the generalized approach leads to a cluster algorithm for world-line quantum Monte Carlo simulations of Heisenberg models with S = 1/2. More succinctly, we also discuss the generalization of the Fortuin- Kasetelyn transformation to higher spin models and illustrate the essential steps for a S = 1 Heisenberg model. Finally, we summarize how to go beyond S = 1 to a general spin, XYZ model.
Date: October 6, 1995
Creator: Gubernatis, J.E. & Kawashima, Naoki
Partner: UNT Libraries Government Documents Department

Reducing Memory Cost of Exact Diagonalization using Singular Value Decomposition

Description: We present a modified Lanczos algorithm to diagonalize lattice Hamiltonians with dramatically reduced memory requirements. The lattice of size N is partitioned into two subclusters. At each iteration the Lanczos vector is projected into a set of n{sub svd} smaller subcluster vectors using singular value decomposition. For low entanglement entropy S{sub ee}, (satisfied by short range Hamiltonians), we expect the truncation error to vanish as exp(-n{sup 1/S{sub ee}}{sub svd}). Convergence is tested for the Heisenberg model on Kagome clusters of up to 36 sites, with no symmetries exploited, using less than 15GB of memory. Generalization to multiple partitioning is discussed.
Date: November 4, 2011
Creator: Weinstein, Marvin; /SLAC; Auerbach, Assa; /Stanford U., Phys. Dept. /Technion; Chandra, V.Ravi & /Technion
Partner: UNT Libraries Government Documents Department