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## Comportamento Crítico e Transições de Fases Dinâmicas em Autômatos Celulares Probabilísticos; Critical behavior and phase transitions in dynamic probabilistic cellular automata.

Fonte: Biblioteca Digitais de Teses e Dissertações da USP Publicador: Biblioteca Digitais de Teses e Dissertações da USP
Tipo: Tese de Doutorado Formato: application/pdf
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Estudamos o comportamento crítico e transições de fase em modelos estocásticos irreversíveis, através de simulações numéricas, análise de campo médio e séries perturbativas. Na primeira parte do trabalho, analisamos o comportamento crítico de autômatos celulares irreversíveis, cujas regras dinâmicas são invariantes sob as operações de simetria do grupo C3v. Estudamos as transições de fase dinâmicas que ocorrem nos modelos e obtemos, através de simulações de Monte Carlo, expoentes críticos estáticos e dinâmicos. Nossos resultados indicam que os modelos pertencem a mesma classe de universalidade do modelo de Potts de três estados. Essa conjectura também foi desenvolvida considerando expansões análogas àquelas utilizadas na teoria de Landau de transições de fase. Na segunda parte do trabalho utilizamos o formalismo de operadores como uma forma ele abordar problemas de sistemas ele não-equilíbrio. Aplicamos o formalismo para construir séries perturbativas para modelos irreversíveis ele dois estados.; We study the critical behavior and phase transitions that take place in irreversible stochastic models through numerical simulations, mean field analysis and perturbative series. In the first part of this work we analyze the critical behavior of irreversible cellular automata whose dynamic rules are invariant under the symmetry operations of the point group C3v. We study the dynamical phase transitions that occur in the models and we obtain the static and dynamic critical exponents by the use of Monte Carlo simulations. Our results indicate that these models are in the same universality class as the three-state Potts model. This conjecture is also developed by considering expansions that are similar to those used in the Landau theory of phase transitions. In the second part of this work we use the operator fonnalism as a way to approach non- equilibrium systems. We apply this fonnalism in order to build perturbative series for two-state irreversible models.

## Exciton effects in soliton and bipolaron lattice states of doped electron-phonon Peierls systems

Harigaya, Kikuo; Shimoi, Yukihiro; Abe, Shuji
Tipo: Artigo de Revista Científica
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Exciton effects on soliton and bipolaron lattice states are investigated using an electron-lattice Peierls model with long-range Coulomb interactions. The Hartree-Fock (HF) approximation and the single-excitation configuration- interaction (single-CI) method are used to obtain optical absorption spectra. We discuss the following properties: (1) The attraction between the excited electron and the remaining hole makes the excitation energy smaller when the correlations are taken into account by the single-CI. The oscillator strengths of the lower excited states become relatively larger than in the HF calculations. (2) We look at variations of relative oscillator strengths of two or three kinds of excitons described by the single-CI. While the excess-electron concentration is small, the ratio of the oscillator strengths of the exciton with the lowest energy increases almost linearly. The oscillator strengths accumulate at this exciton as the concentration increases.; Comment: Printed figures should be requested from the author K.Harigaya (harigaya@etl.go.jp)

## Two-Dimensional t-J Model at Low Electron Density

Hellberg, C. Stephen; Manousakis, Efstratios
Tipo: Artigo de Revista Científica
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The phase diagram of the two-dimensional t-J model is determined accurately at low electron density by a combination of analytic and numerical techniques. The ground state exhibits three phases in the limit of zero electron density: an unpaired state at small J/t, a gas of s-wave pairs for 2 < J/t < 3.4367, and a phase separated state at larger interaction strengths. Bound states of larger clusters are never realized, and the instabilities present at small densities are discussed.; Comment: Revised version discusses p-wave and d-wave pairing at small finite densities for J/t < 2. 4 pages, two-column Revtex, uuencoded figures

## Many-body and Covalence Effects in the Polarization of Ferroelectric Perovskites

Resta, R.; Sorella, S.
Tipo: Artigo de Revista Científica
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The ferroelectric polarization of perovskite oxides is much larger than implied by displacement of static ionic charges. We use an explicitly correlated scheme to investigate the phenomenon; charge transport is evaluated as a geometric quantum phase. Both covalence and electron-electron interaction enhance polarization in the weakly correlated regime. At higher values of the electron-electron interaction, the system undergoes a transition from a band insulator to a Mott insulator: the static ionic charge is continuous across the transition, whereas the polarization is discontinuous. Above the transition, oxygen transports a positive charge.; Comment: REVTeX 10 pages + 3 PostScript figures appended to the file

## Superconductivity in an exactly solvable Hubbard model with bond-charge interaction

Tipo: Artigo de Revista Científica
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The Hubbard model with an additional bond-charge interaction $X$ is solved exactly in one dimension for the case $t=X$ where $t$ is the hopping amplitude. In this case the number of doubly occupied sites is conserved. In the sector with no double occupations the model reduces to the $U=\infty$ Hubbard model. In arbitrary dimensions the qualitative form of the phase diagram is obtained. It is shown that for moderate Hubbard interactions $U$ the model has superconducting ground states.; Comment: Revtex, 14 pages, 1 figure (uuencoded compressed tar-file)

## Symmetries and Mean-Field Phases of the Extended Hubbard Model

Eriksson, A. B.; Einarsson, T.; Ostlund, S.
Tipo: Artigo de Revista Científica
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The two-dimensional extended Hubbard model that includes a nearest- neighbor Heisenberg interaction is studied using a mean-field theory where quasiparticles are defined by an U(8) group of canonical transformations. The theory is a generalization of the ordinary BCS theory, and Balian and Werthamer's theory of He-3 that permits both broken gauge, spin and sublattice symmetry. This allows us to investigate superconductivity, antiferromagnetic order, charge density waves and, by twisting the spin quantization axis, spiral antiferromagnetic order in the same theory. Our results for positive Hubbard U and Heisenberg exchange J suggest that antiferromagnetic ordering dominates close to half filling, while spiral states and d-wave superconductivity compete when doping is introduced. For moderate values of J, we find a phase diagram where a phase transition occurs from an antiferromagnet to a d-wave superconductor as doping is increased. A narrow region of (s+id)-wave superconductor is found for some values of J and U.; Comment: Revtex, 27 pages, 8 figures uuencoded with uufiles. Replaced because (s+id)-wave superconductivity is found (not (s+d)-wave)

## Dynamic Scaling of Ion-Sputtered Surfaces

Cuerno, Rodolfo; Barabasi, Albert-Laszlo
Tipo: Artigo de Revista Científica
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We derive a stochastic nonlinear equation to describe the evolution and scaling properties of surfaces eroded by ion bombardment. The coefficients appearing in the equation can be calculated explicitly in terms of the physical parameters characterizing the sputtering process. We find that transitions may take place between various scaling behaviors when experimental parameters such as the angle of incidence of the incoming ions or their average penetration depth, are varied.; Comment: 13 pages, Revtex, 2 figures

## Gamma phonons and microscopic structure of orthorhombic KNbO3 from first-principles calculations

Postnikov, A. V.; Borstel, G.
Tipo: Artigo de Revista Científica
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{}From a series of total energy calculations by the full-potential linear muffin-tin orbital method, the total energy hypersurface as function of atomic displacements from equilibrium positions has been fitted for different Gamma phonon modes in orthorhombic KNbO3. Frequencies and eigenvectors of all TO Gamma phonons have been calculated in the harmonic approximation, and in the quantum oscillator scheme -- for A2 and B2 modes. The microscopic structure of the orthorhombic phase has been analyzed in a series of supercell calculations for different patterns of Nb displacements, providing indications in favour of the chain structure, with oppositely directed neighboring chains.; Comment: 10 pages, including 3 LaTeX figures

## Universality Classes, Statistical Exclusion Principle and Properties of Interacting Fermions

Byczuk, Krzysztof; Spalek, Jozef
Tipo: Artigo de Revista Científica
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We point to the possibility of existence of the statistical-spin-liquid state as the state which differs from either Fermi or Luttinger liquid states. In the statistical spin liquid the double occupancies are excluded from the physical space. Each of the above three cases (Fermi, Luttinger and spin liquids) represents an universality class for the interacting many-particle fermion systems. The properties of the spin liquid such as the chemical potential, the entropy, and the magnetization curve, as well as the quasiparticle structure are briefly discussed.; Comment: 11 pages, RevTex, 3 figures available upon request

## Phenomenological Models for the Gap Anisotropy of Bi-2212 as Measured by ARPES

Norman, M. R.; Randeria, M.; Ding, H.; Campuzano, J. C.
Tipo: Artigo de Revista Científica
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Recently, high resolution angle-resolved photoemission spectroscopy has been used to determine the detailed momentum dependence of the superconducting gap in the high temperature superconductor Bi-2212. In this paper, we first describe tight binding fits to the normal state dispersion and superlattice modulation effects. We then discuss various theoretical models in light of the gap measurements. We find that the simplest model which fits the data is the anisotropic s-wave gap $\cos(k_x)\cos(k_y)$, which within a one-band BCS frame- work suggests the importance of next near neighbor Cu-Cu interactions. Various alternative interpretations of the observed gap are also discussed, along with the implications for microscopic theories of high temperature superconductors.; Comment: 14 pages, revtex, 9 uuencoded postscript figures

## Theory of Electronic Raman Scattering in Disordered $d-$wave Superconductors

Devereaux, T. P.
Tipo: Artigo de Revista Científica
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A theory for the effect of impurity scattering on electronic Raman scattering in unconventional superconductors is presented. The impurity dependence of the the spectra can be used to to distinguish between conventional (anisotropic $s-$wave) or unconventional ($d$-wave) energy gaps, and excellent agreement with the data on three cuprate superconductors is shown for $d_{x^{2}-y^{2}}$ pairing. The full frequency dependence of the spectra can be reproduced by including inelastic scattering due to spin fluctuations and/or Coulomb scattering on a partially nested Fermi surface.; Comment: 11 pages, Revtex, 3 figures

## Phenomenological BCS theory of the high-$T_c$ cuprates

Fehrenbacher, R.; Norman, M. R.
Tipo: Artigo de Revista Científica
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A BCS model characterized by a phenomenological pair potential with on-site ($V_0$), nearest ($V_1$), and next nearest ($V_2$) neighbour coupling constants, and an empirical quasiparticle dispersion taken from angle-resolved photoemission spectra is considered. The model can consistently explain the experimental data concerning the pair state of the hole doped cuprates. Three ingredients are required to make the interpretation possible: the existence of flat bands, a very small effective on-site repulsion, and a slightly dominating effective nnn attraction $V_2$ of the order of 60-80meV with a ratio $V_2/V_1 \approx 1.5$.; Comment: 13 pages, uuencoded Postscript

## Magnetization switching in nanoscale ferromagnetic grains: description by a kinetic Ising model

Richards, Howard L.; Sides, Scott W.; Novotny, M. A.; Rikvold, Per Arne
Tipo: Artigo de Revista Científica
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The magnetic relaxation of ferromagnetic powders has been studied for many years, largely due to its importance to recording technologies. However, only recently have experiments been performed that resolve the magnetic state of individual sub-micron particles. Motivated by these experimental developments, we use droplet theory and Monte Carlo simulations to study the time and field dependence of some quantities that can be observed by magnetic force microscopy. Particular emphasis is placed on the effects of finite particle size. The qualitative agreement between experiments on switching and our simulations in individual single-domain ferromagnets suggests that the switching mechanism in such particles may involve local nucleation and subsequent growth of droplets of the stable phase.; Comment: 17 pages, RevTeX 3.0, 15 tar-compressed and uu-encoded figures. Submitted to J. Magn. Magn. Mater

## Gapless Time-Reversal Symmetry Breaking Superconductivity

Tikofsky, A. M.; Bailey, D. B.
Tipo: Artigo de Revista Científica
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We consider a layered superconductor with a complex order parameter whose phase switches sign from one layer to the next. This system is shown to exhibit gapless superconductivity for sufficiently large interlayer pairing or interlayer hopping. In addition, this description is consistent with experiments finding signals of time-reversal symmetry breaking in high-temperature superconductors only at the surface and not in the sample bulk.; Comment: 3 pages, REVTeX 3.0, no figures.

## Linear Response of Thin Superconductors in Perpendicular Magnetic Fields: An Asymptotic Analysis

Dorsey, Alan T.
Tipo: Artigo de Revista Científica
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The linear response of a thin superconducting strip subjected to an applied perpendicular time-dependent magnetic field is treated analytically using the method of matched asymptotic expansions. The calculation of the induced current density is divided into two parts: an outer'' problem, in the middle of the strip, which can be solved using conformal mapping; and an inner'' problem near each of the two edges, which can be solved using the Wiener-Hopf method. The inner and outer solutions are matched together to produce a solution which is uniformly valid across the entire strip, in the limit that the effective screening length $\lambda_{\rm eff}$ is small compared to the strip width $2a$. From the current density it is shown that the perpendicular component of the magnetic field inside the strip has a weak logarithmic singularity at the edges of the strip. The linear Ohmic response, which would be realized in a type-II superconductor in the flux-flow regime, is calculated for both a sudden jump in the magnetic field and for an ac magnetic field. After a jump in the field the current propagates in from the edges at a constant velocity $v=0.772 D/d$ (with $D$ the diffusion constant and $d$ the film thickness), rather than diffusively...

## Raman spectrum and charge fluctuations in the copper-oxide superconductors

Niksic, H.; Tutis, E.; Barisic, S.
Tipo: Artigo de Revista Científica
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The effect of the charge fluctuations on the electronic spectrum and the Raman spectrum of high temperature superconductors is examined within the slave boson approach. Instead of using the saddle point approximation for slave bosons, we confine ourselves to the non-crossing approximation (NCA) in summing the diagrams for the Green functions, thus obtaining the renormalized hole spectrum and its lifetime on equal footing. The electronic Raman spectrum is calculated, showing the characteristic featureless behaviour up to the frequency of the order of renormalized $\Delta_{pd}$ parameter. The dependence on the polarization of the incident and the scattered light agrees with experiments.; Comment: 4 pages plus 3 compressed & uuencoded eps figures, LATEX 2.09, published in Physica C 235-240 (1994) 2265-2266, proceedings from the M2S-HTSC IV conference

## The effect of large $U_d$ on the Raman spectrum in the copper-oxide superconductors

Niksic, H.; Tutis, E.; Barisic, S.
Tipo: Artigo de Revista Científica
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The effect of the charge fluctuations on the electronic and Raman spectrum of high temperature superconductors is examined, using the slave boson approach to the large Coulomb repulsion $U_d$ on the copper site. Instead of the saddle point approximation $\srv{b}\ne 0$ for the slave boson, characteristic for various $N\ide\infty$ approaches, we confine ourselves to the non-crossing approximation (NCA) diagrams for the Green functions. In this way the effects of charge fluctuations and of the constraint of no double occupancy on the copper site on the shape of the electronic spectrum are studied primarily, while the slave boson diagrams responsible for copper-copper spin correlations are intentionally not included. The novel feature of the charge fluctuation dynamics shows up in the slave boson spectrum as the plateau extending over the range of 1eV below $\omega=0$. This is further reflected in the electronic Raman spectrum that we calculate for various combinations of the polarization of the incoming and scattered light. The Raman spectrum shows the characteristic featureless behaviour up to frequencies of the order of 1eV while the polarization dependence is also in qualitative agreement with experiment.; Comment: 20 pages plus 9 compressed & uuencoded ps figures...

## Neutron scattering and superconducting order parameter in YBa2Cu3O7

Mazin, I. I.; Yakovenko, Victor M.
Tipo: Artigo de Revista Científica
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We discuss the origin of the neutron scattering peak at 41 meV observed in YBa$_2$Cu$_3$O$_7$ below $T_c$. The peak may occur due to spin-flip electron excitations across the superconducting gap which are enhanced by the antiferromagnetic interaction between Cu spins. In this picture, the experiment is most naturally explained if the superconducting order parameter has $s$-wave symmetry and opposite signs in the bonding and antibonding electron bands formed within a Cu$_2$O$_4$ bilayer.; Comment: In this version, only few minor corrections and the update of references were done in order to make perfect correspondence with the published version. RevTeX, psfig, 5 pages, and 3 figures

## PHASE TRANSITION OF N-COMPONENT SUPERCONDUCTORS

Bergerhoff, B.; Litim, D.; Lola, S.; Wetterich, C.
Tipo: Artigo de Revista Científica