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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

Low temperature spin wave dynamics in classical Heisenberg chains

Description: A detailed and quantitative study of the low-temperature spin-wave dynamics was made for the classical Heisenberg-coupled chain using computer simulation. Results for the spin-wave damping rates and the renormalization of the spin-wave frequencies are presented and compared with existing predictions.
Date: November 1, 1977
Creator: Heller, P. & Blume, M.
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

Dynamics of unbound vortices in the 2-dimensional XY and anisotropic Heisenberg models

Description: Assuming an ideal gas of vortices above the Kosterlitz-Thouless transition temperature, the dynamic form factors are calculated. For the in-plane correlations a Lorentzian central peak is predicted which is independent of the vortex size and shape. However, for the out-of-plane correlations the velocity dependence of the vortex structure is decisive for the occurrence of a Gaussian central peak. Both results are in good agreement with combined Monte Carlo-molecular dynamics simulations. 8 refs., 1 fig.
Date: January 1, 1988
Creator: Mertens, F.G.; Bishop, A.R.; Gouvea, M.E. & Wysin, G.M.
Partner: UNT Libraries Government Documents Department

The t-J model at small t/j: Numerical, perturbative, and supersymmetric results

Description: We discuss some recent results for one- and two-hole states in the t-J model at small t/J. These include numerical results (bandwidth determinations and accurate t/J values for 4 {times} 4 lattice one-hole ground-state level crossings), hopping-parameter perturbation theory (which gives the small-t/J one-hole bandwidth in terms of the static-vacancy ground state), and results at the supersymmetric point t/J = 1/2 (exact results for energies and bandwidths.) The perturbative results leads us to a new conjecture regarding the staggered magnetization of higher-spin states in the two-dimensional Heisenberg model. We also discuss extrapolation of small-t/J results to high-{Tc} parameter values; in the two-hole ground states we find (t/J){sup {lambda}} behavior in the rms hole-hole separation, and an extrapolation to t/J = 3 gives a bulk-limit rms hole-hole separation of {approx} 7{angstrom}. 18 refs., 6 figs.
Date: February 1, 1991
Creator: Barnes, T. (Oak Ridge National Lab., TN (USA). Center for Computationally Intensive Physics Tennessee Univ., Knoxville, TN (USA). Dept. of Physics)
Partner: UNT Libraries Government Documents Department

Magnons at the Curie temperature

Description: Random phase approximations (RPA) have bery successfully treated spin wave excitations in Heisenberg ferromagnets at low temperatures. The role played by these magnons at the order-disorder transition, however, has been a topic which has eluded RPA theories to date. In light of recent data, the idea of magnons at the Curie temperature and above has become more difficult to refute. This adds incentive to attempt to model interacting magnons at the Curie temperature. This work examines some attempts to formulate higher random phase approximations and discusses why they fail as they approach the transition temperature. A new interpretation of some work by Parmenter and the author is presented. The nature of the approximations made in that work is discussed, and an attempt is made to eliminate incorrect contributions from the energetically disfavored paramagnetic state to the correct magnon renormalization.
Date: January 1, 1981
Creator: Poling, D.A.
Partner: UNT Libraries Government Documents Department

Martensitic transformation of CsFeS/sub 2/ driven by the singlet to Neel state transition

Description: CsFeS/sub 2/ is regarded as an example of the quasi one-dimensional alternating antiferromagnetic Heisenberg chain above about 70K. In accordance with this picture, an energy gap of 10MeV was observed for the singlet to triplet excitation at a zone center near the transition temperature by our neutron measurements. Large excitation width implies a strong coupling of excitons to phonons, and LA phonons along <001> direction become ill-defined for q greater than 0.3. At about 70K, the first order transition occurs, whereby the singlet ground state changes to a Neel state and the simultaneous structural transformation takes place, which is a martensitic transformation. Mechanism for such martensitic transformation is discussed based on the similarity of the magnetic excitation and phonon behaviors between the present compound and the ..gamma.. Mn alloys with Fe and Cu, which are itinerant electron magnetic systems. 10 refs., 3 figs.
Date: January 1, 1985
Creator: Ito, Y.; Nishi, M.; Passell, L.; Majkrzak, C.F. & Shirane, G.
Partner: UNT Libraries Government Documents Department

Correlation theory applied to the static and dynamic properties of EuO and EuS

Description: The paramagnetic scattering was recently measured for EuO. It was found that spin-wave-like excitations develop for wave vectors approaching the zone boundary. The spectrum was found to be well described by damped harmonic oscillators (also called the two-pole-approximation). This approximation was used previously in the correlation theory primarily to calculate static properties. Selfconsistent dynamic and static calculations have been performed for EuO, which is an ideal Heisenberg magnet with significant second nearest neighbor interaction (J/sub 2/ = J/sub 1/5). The two-pole approximation describes accurately the correlation range, the static susceptibility and the qualitative behavior of the dynamic properties (i.e., the wave vector at which peaks appear in the spectrum as a function of temperature). However, in order to also obtain the correct frequency scale it is necessary to use a cut-off of the spectrum at high frequencies, which cannot be seen experimentally, but which significantly influences the frequency moments. It was found that the finite J/sub 2/ has significant importance for a comparison between theory and experiment. It is concluded that the calculation for a simple cubic n.n. magnet by Hubbard does not describe the EuO data accurately, neither with respect to lineshape nor frequency scale. Significant differences are to be expected between EuO and EuS having opposite sign for J/sub 2/.
Date: October 1, 1981
Creator: Lindgard, P.A.
Partner: UNT Libraries Government Documents Department

Spin-spin correlations in the one-dimensional spin-1/2 antiferromagnetic Heisenberg chain

Description: We have calculated the spin-spin correlations in the ground state of spin-1/2, one-dimensional, antiferromagnetic Heisenberg chain numerically. Using improved algorithms, we have been able to perform exact diagonalizations for lattices up to 30 sites and quantum Monte Carlo simulations for lattices up to 512 sites. This extensive range allows us to investigate the logarithmic dependence of spin-spin correlations. We present our results and discuss their implication. 11 refs., 1 fig., 2 tabs.
Date: January 1, 1990
Creator: Lin, H.Q. & Campbell, D.K.
Partner: UNT Libraries Government Documents Department