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Multigrid Monte Carlo methods

Description: This paper is intended to be a tutorial on multigrid Monte Carlo techniques, illustrated with two examples. Path-integral quantum Monte Carlo is seen to take only a finite amount of computer time even as the paths are discretized on infinitesimally small scales. A method for eliminating critical slowing down completely/emdash/even for models with discrete degrees of freedom, as in Potts models, or discrete excitations, such as isolated vortices in the XY model/emdash/is presented. 11 refs., 1 fig.
Date: January 1, 1988
Creator: Loh, E. Jr.
Partner: UNT Libraries Government Documents Department

Operation of a high-P/sub T/ spectrometer arm at 10/sup 33/ cm/sup -2/ sec/sup -1/ with particle identification

Description: In the Proceedings of the 1978 Isabelle Summer Study a high-pT spectrometer pair is presented as a prototype high-luminosity experiment. We consider here an updated version of this apparatus with the following questions in mind: (1) what rate capabilities are required to cope with L = 10/sup 33/cm/sup -2/sec/sup -1/; (2) what segmentation is needed to deal with the particle densities expected in high pT jets; and (3) is the resulting device within the reach of present technology. The current version of the device and the expected rates are presented, and the rates and segmentation of the components are discussed. The results of calculations related to event pile-up and triggering are presented. The main conclusion, that particle identification appears to be quite feasible at these rates, is discussed in detail. (WHK)
Date: January 1, 1983
Creator: Aronson, S.; Goldberg, M.; Holder, M. & Loh, E.
Partner: UNT Libraries Government Documents Department

Off-diagonal Coulomb interactions in the extended Peierls-Hubbard model: Exact diagonalization results

Description: For conjugated polymers, the extended Peierls-Hubbard model has been widely accepted as correctly incorporating the effects of both e-p and e-e interactions. In the absence of e-e interactions, the ground state of (CH)/sub x/ is the 2k/sub F/ bond-order wave (BOW) -- dimerization/bond alternation -- predicted by Peierls theorem. It is now well established that for the weak e-p coupling appropriate to (CH)/sub x/, the on-site Coulomb repulsion actually enhances dimerization up to fairly large values (U > 6t/sub 0/). These results are still widely regarded as counter to the conventional wisdom that Coulomb interactions should suppress the build-up of charge anywhere, on the sites or on the bonds. Since the extended Peierls-Hubbard model incorporates only (lattice) site-diagonal parts of the electron-electron interactions (U, V), and omits the off-diagonal bond-charge repulsions (W,X), it is natural to ask whether this model adequately describes the full consequences of e-e interactions. In the specific context of (CH)/sub x/, the potential importance of this omission is readily recognized. Intuitively, the bond-charge repulsion should suppress dimerization, since it opposes the build-up of charge on the bonds. Thus, the absence of W and X terms in the standard extended Peierls-Hubbard models suggests a priori that these models may artificially favor the continuation of dimerization in the half-filled band into the region of intermediate to strong Coulomb interaction. More generally, the omission of terms such as W and X raises significant questions about the appropriateness of Hubbard models for describing e-e interactions in the whole class of novel solid state materials. Thus this issue is extremely important and is, discussed further in this paper. 14 refs., 6 figs.
Date: January 1, 1989
Creator: Campbell, D.K.; Gammel, J.T. & Loh, E.Y. Jr.
Partner: UNT Libraries Government Documents Department

Strongly correlated quasi-one-dimensional bands: Ground states, optical absorption, and phonons

Description: Using the Lanczos method for exact diagonalization on systems up to 14 sites, combined with a novel phase randomization'' technique for extracting more information from these small systems, we investigate several aspects of the one-dimensional Peierls-Hubbard Hamiltonian, in the context of trans-polyacetylene: the dependence of the ground state dimerization on the strength of the electron-electron interactions, including the effects of off-diagonal'' Coulomb terms generally ignored in the Hubbard model; the phonon vibrational frequencies and dispersion relations, and the optical absorption properties, including the spectrum of absorptions as a function of photon energy. These three different observables provide considerable insight into the effects of electron-electron interactions on the properties of real materials and thus into the nature of strongly correlated electron systems. 29 refs., 11 figs.
Date: January 1, 1989
Creator: Campbell, D.K.; Gammel, J.T. & Loh, E.Y. Jr.
Partner: UNT Libraries Government Documents Department

The extended Peierls-Hubbard model: Off-diagonal model

Description: We investigated the effects of including ''off-diagonal'' terms/endash/nearest-neighbor bond-bond repulsion (W) and bond-site repulsion (X)/endash/in the extended Peierls-Hubbard model. As a specific illustration we study the ground state dimerization in one-dimensional, half-filled-band models that have been widely applied to conjugated polymers such as /ital trans/-polyacetylene and related charge density wave systems. 17 refs., 5 figs.
Date: January 1, 1988
Creator: Campbell, D.K.; Gammel, J.T. & Loh, E.Y. Jr.
Partner: UNT Libraries Government Documents Department

Lanczos diagonalizations of the 1-D Peierls-Hubbard model

Description: In studies of interacting electrons in reduced dimensions'' one is trapped between the Scylla of exponential growth of the number of states in any exact many-body basis and the Charybdis of the failure of mean-field theories to capture adequately the effects of interactions. In the present article we focus on one technique -- the Lanczos method -- which, at least in the case of the 1-D Peierls-Hubbard model, appears to allow us to sail the narrow channel between these two hazards. In contrast to Quantum Monte Carlo methods, which circumvent the exponential growth of states by statistical techniques and importance sampling, the Lanczos approach attacks this problem head-on by diagonalizing the full Hamiltonian. Given the restrictions of present computers, this approach is thus limited to studying finite clusters of roughly 12--14 sites. Fortunately, in one dimension, such clusters are usually sufficient for extracting many of the properties of the infinite system provided that one makes full use of the ability to vary the boundary conditions. In this article we shall apply the Lanczos methodology and novel phase randomization'' techniques to study the 1-D Peierls-Hubbard model, with particular emphasis on the optical absorption properties, including the spectrum of absorptions as a function of photon energy. Despite the discreteness of the eigenstates in our finite clusters, we are able to obtain optical spectra that, in cases where independent tests can be made, agree well with the known exact results for the infinite system. Thus we feel that this combination of techniques represents an important and viable means of studying many interesting novel materials involving strongly correlated electrons. 26 refs., 6 figs.
Date: January 1, 1989
Creator: Loh, E.Y.; Campbell, D.K. & Gammel, J.T.
Partner: UNT Libraries Government Documents Department

Stable matrix-multiplication algorithms for low-temperature numerical simulations of fermions

Description: In this note, we discuss the use of matrix factorizations to stabilize the numerical matrix multiplications and inversions needed to simulate systems of interacting fermions at low temperatures. While the essence of a stable numerical algorithm is presented, we mainly emphasize the concepts of stabilization. 10 refs.
Date: January 1, 1988
Creator: Loh, E.Y. Jr.; Gubernatis, J.E.; Scalettar, R.T.; Sugar, R.L. & White, S.R.
Partner: UNT Libraries Government Documents Department

The two band model for halogen-bridged transition metal linear chain complexes

Description: Halogen-bridged transition-metal complexes have been of interest to chemists for many decades as dyes and strongly dichroic materials. However they have only recently begun to receive detailed consideration in the physics community. Their potential important arises because of: the increasing appreciation of strong, competing electron-electron and electron-phonon interactions in low-dimensional materials and the consequent need to expand many-body techniques. The MX materials offer a rapidly expanding, near single-crystal, class of quasi-1-D systems which can be ''tuned'' (by chemistry, pressure, doping, etc.) between various ground state extremes: from strong charge-disproportionation and large lattice distortion to weak charge-density-wave and small lattice distortion, to magnetic and undistorted; the opportunity to probe doping- and photo-induced local defect states (polarons, bipolarons, kinks, excitons) and their interactions in controlled environments and the same large range of ground states; and, the similarities between models and theoretical issues in these materials and the recently discovered oxide superconductors. The MX materials are also closely connected conceptually with mixed-stack charge-transfer salts. The MX class, then, is important in its own right, but also as a template for concepts and electronic structure techniques in strongly interacting (both electron-electron (e-e) and electron-phonon (e-p)), low dimensional electronic materials. This paper discusses the two band model for the isolated MX chains. 14 refs., 6 figs.
Date: January 1, 1988
Creator: Bishop, A.R.; Gammel, J.T.; Loh, E.Y. Jr.; Phillpot, S.R. & Weber-Milbrodt, S.M.
Partner: UNT Libraries Government Documents Department

Lattice gas simulations of one and two-phase fluid flows using the Connection Machine-2

Description: In this paper, we report recent lattice gas simulations for single-phase and two-phase flows for two dimensional problems using the Connection Machine-2. For the single-phase fluid problem, we use the standard 7-bit lattice gas model with the maximum collision rules. The velocity and vorticity field of the Kelvin-Helmholtz instability is studied. It is shown that the lattice gas method preserves the main properties of the flow patterns observed in other numerical simulations. Using colored particles and holes, the lattice gas method is extended to simulate immiscible fluids with adjustable surface tension, using a purely local collision scheme. The locality of this model allows us to implement a very fast and parallel algorithm on the Connection Machine-2. Because this new model correctly describes short-range particle-particle interactions between liquids and also particle-solid interactions between the fluid and the wall, cohesion and wettability can be simulated. Applications of the current model to several physical systems including spinodal decomposition, Rayleigh-Taylor flows and wettability in two-phase flows through porous media are discussed. 15 refs., 10 figs.
Date: 1990-09~
Creator: Chen, S.; Doolen, G. D.; Eggert, K.; Grunau, D. & Loh, E. Y., Jr.
Partner: UNT Libraries Government Documents Department

MX chains: 1-D analog of CuO planes

Description: We study a two-band Peierls-Hubbard model for halogen-bridged mixed-valence transition metal linear chain complexes (MX chains). We include electron-electron correlations (both Hubbard and PPP-like expressions) using several techniques including calculations in the zero-hopping limit, exact diagonalization of small systems, mean field approximation, and a Gutzwiller-like Ansatz for quantum phonons. The adiabatic optical absorption and phonon spectra for both photo-excited and doping induced defects (kinks, polarons, bipolarons, and excitons) are discussed. A long period phase which occurs even at commensurate filling for certain parameter values may be related to twinning. The effect of including the electron-phonon in addition to the electron-electron interaction on the polaron/bipolaron (pairing) competition is especially interesting when this class of compounds is viewed as a 1-D analog of high-temperature superconductors. 6 refs., 4 figs.
Date: January 1, 1989
Creator: Gammel, J.T.; Batistic, I.; Bishop, A.R.; Loh, E.Y. Jr. & Marianer, S.
Partner: UNT Libraries Government Documents Department

Competing electron-electron/electron-phonon interactions and polyacetylene

Description: Using Lanczos exact diagonalization, we investigate the effects of the competition between the electro-electron and electron-phonon interactions in the context of the 1-D tight-binding Peierls-Hubbard Hamiltonian, studying various structural, optical, and vibrational properties of strongly correlated systems. We use polyacetylene as our experimental guide, and perform a parameter space search to determine the level at which a unique set of parameters can model this prototypical conducting polymer and, more generally, the applicability of the simple'' 1-D Peierls-Hubbard Hamiltonian to these highly interesting materials. 9 refs., 3 tabs.
Date: April 8, 1991
Creator: Gammel, J.T. (Los Alamos National Lab., NM (USA) Bayreuth Univ. (Germany, F.R.). Physics Inst.); Campbell, D.K. (Los Alamos National Lab., NM (USA)) & Loh, E.Y. Jr. (Thinking Machines Corp., Cambridge, MA (USA))
Partner: UNT Libraries Government Documents Department

Hadron spectra from high energy proton proton interactions

Description: It is proposed to measure the hadron spectra resulting from high energy proton-proton collisions using a single arm focusing spectrometer. These measurements will provide elastic and inelastic P-P cross sections for incident beam energies up to 200 GeV/c and for momentum transfers |t| from 0.01 up to about 10 to 15 (BeV/c){sup 2}. In addition, they will obtain yields of pions and kaons produced in the interactions.
Date: June 1, 1970
Creator: Diebold, R.; /Argonne; Guerriero, L.; U., /Bari; Lanou, R.; U., /Brown et al.
Partner: UNT Libraries Government Documents Department

Stable simulations of many fermion systems

Description: As the inverse temperature {beta} becomes large, the diverse numerical scales present in exp({minus}{beta}H) plague simulations of many-fermion systems on finite-precision computers. Representation of matrices in factorized form stabilizes these calculations, allowing efficient, low-temperature studies of condensed-matter models.
Date: January 1, 1989
Creator: Loh, E.Y. Jr.; Gubernatis, J.E.; Scalapino, D.J.; Sugar, R.L.; White, S.R.; Scalettar, R.T. (Thinking Machines Corp., Cambridge, MA (USA) et al.
Partner: UNT Libraries Government Documents Department

Simulation of pore scale porous media flow using lattice gas methods

Description: Carbon dioxide-foam injection is an important technique for improving the recovery of oil from porous rocks. Huh, et. al. (1989) recently presented results of two-dimensional laboratory micromodel studies conducted to better understand this process for improved oil recovery. These experimental results indicate that the introduction of CO{sub 2} foam may be expected to have a substantial effect on the relative permeability curves that would be used to model improved oil recovery in a reservoir. However, in order to determine expected changes in relative permeability a computational technique is require to quantify the experimental results, and to help determine the appropriate relative permeability curves for reservoir scale calculation. Two computational methodologies are needed to utilize the experimental information to improve reservoir simulations. First, a method is needed for quantitatively describing the phenomena observed in the micromodel geometry. Second, a method is needed to extend these effects to the different scales of heterogeneity that may be expected to exist between the laboratory and the field. This paper is focused on the first of these methods, and although it does not yet fully solve the problem of representing these fluid systems in the laboratory, it presents a promising approach to this problem. The paper briefly discusses the application of the lattice gas method for solution of the nondimensional Navier-Stokes equations for flow of fluids through the complex microscopic geometry of porous media. In particular, the approach presented herein allows the simulation not only of single fluids through the media, but of systems of two or more fluids ranging from fully miscible to completely immiscible. 11 refs., 4 figs.
Date: January 1, 1991
Creator: Eggert, K.; Chen, Shiyi; Travis, B.; Grunau, D. (Los Alamos National Lab., NM (USA)); Loh, E. (Thinking Machines Corp., Cambridge, MA (USA)) & Kovarik, F. (Houston Univ., TX (USA). Inst. for Improved Oil Recovery Research)
Partner: UNT Libraries Government Documents Department

Measurement of Pressure Dependent Fluorescence Yield of Air: Calibration Factor for UHECR Detectors

Description: In a test experiment at the Final Focus Test Beam of the Stanford Linear Accelerator Center, the fluorescence yield of 28.5 GeV electrons in air and nitrogen was measured. The measured photon yields between 300 and 400 nm at 1 atm and 29 C are Y(760 Torr){sup air} = 4.42 {+-} 0.73 and Y(760 Torr){sup N{sub 2}} = 29.2 {+-} 4.8 photons per electron per meter. Assuming that the fluorescence yield is proportional to the energy deposition of a charged particle traveling through air, good agreement with measurements at lower particle energies is observed.
Date: July 6, 2005
Creator: Belz, J.W.; Burt, G.W.; Cao, Z.; Chang, F.Y.; Chen, C.C.; Chen, C.W. et al.
Partner: UNT Libraries Government Documents Department