bluuebunny/crossref_metadata_2025 huggingface dataset:
A New Approach to the Dynamics of Holes and Spins
Authors: Ichinose Ikuo , Matsui Tetsuo
Published: November 2004

We briefly review our recent approach towards the physics of the Hubbard t-J model. It is characterized by (i) path-integral quantization of electrons in the slave boson and fermion representation, (ii) integration over spin degrees of freedom on the odd lattice sites around the short-range antiferromagnetic order, and (iii) introduction of auxiliary collective field describing the hole pairs on nearest-neighbour links as an order parameter of superconductivity. The results obtained so far include (A) derivation of the CP1+ Dirac-fermion theory as the low-energy effective field theory in the superconducting state, (B) derivation of the U(1) lattice gauge theory with Higgs coupling as the Ginzburg-Landau theory of hole pairs, and (C) some discussions on the phase structure based on the nonlinear sigma model renormalized by holes and the solvable one-pair model.
PARQUET EQUATIONS FOR NUMERICAL SELF-CONSISTENT-FIELD THEORY
Authors: Bickers N.E.
Published: November 2004

In recent years increases in computational power have provided new motivation for the study of self-consistent-field theories for interacting electrons. In this set of notes, the so-called parquet equations for electron systems are derived pedagogically. The principal advantages of the parquet approach are outlined, and its relationship to simpler self-consistent-field methods, including the Baym-Kadanoff technique, is discussed in detail.
PHASE SEPARATION AND SUPERCONDUCTIVITY IN THE U=∞ LIMIT OF THE EXTENDED MULTIBAND HUBBARD MODEL
Authors: Grilli M. , Raimondi R. , Castellani C. , Di Castro C. , Kotliar G.
Published: November 2004

The three band Hubbard model with nearest neighbour repulsion is studied in the U=∞ limit using the large N expansion technique to order [Formula: see text]. A charge transfer (CT) instability is found like in weak coupling theory. However at small doping a major role is played by the Brinkman-Rice point. The CT instability is always associated with a diverging compressibility leading to a phase separation. The evaluation to order [Formula: see text] of the effective scattering amplitude in the Cooper channel shows the presence of superconducting instabilities in the s and d wave channel near the phase separation.
SUPERCONDUCTIVITY IN HIGH AND VERY HIGH MAGNETIC FIELDS
Authors: Tešanović Zlatko
Published: November 2004

An external magnetic field can actually enhance superconductivity, pro vided it is strong enough. If only few lowest Landau levels are occupied, the quasi 1D nature of the electronic motion leads to an enhancement of transition temperature. The orbital frustration of the order parameter is much simplified in this limit. This limit of superconductivity, of great interest in itself, can also be used as a starting point to study orbital frustration in familiar, “low-field”, strongly type II superconductors. I briefly review some of the characteristics of superconductivity in high magnetic fields.
Raman Scattering in Mott-Hubbard Systems
Authors: Shastry B. Sriram , Shraiman Boris I.
Published: November 2004

We present a theory of Raman Scattering in the Hubbard model. The scattering of light has resonant and non-resonant contributions. The resonant term gives rise to scattering by spin degrees of freedom in the insulating case, for which we derive a general form of the effective scattering Hamiltonian. The later involves four terms with distinct symmetry properties, one of them being the “chiral” spin operator ∑ Si · (Sj×Sk). The corresponding four distinct correlation function can be measured directly in the experiments with different scattering geometries. The nonresonant term contributes to the scattering in the doped case and is shown to probe the fluctuations of the “stress tensor”. This quantity is not conserved and hence its fluctuations at small q inherent in optical experiments, need not be small, in striking contrast with the density fluctuation in usual metals.
THE STABILITY OF LOW INDEX METAL SURFACES TO TOPOLOGICAL DEFECTS
Authors: CONRAD EDWARD H.
Published: November 2004

The study of defect formation at metal surfaces is a fundamental problem in surface physics. An understanding of defect formation is pertinent to growth and diffusion mechanisms. In addition, surface roughening, faceting, and surface melting are all defect mediated phase transitions involving the formation of different topological defects. While the importance of defects at surfaces is well recognized, the study of surface defects has been hampered by the lack of sufficiently accurate experimental techniques. In fact, it is only in the past 6 years that experiments on the thermal generation of defects on metal surfaces have been performed. This review attempts to outline both the theoretical and experimental work on surface defect formation on metal systems.
ADIABATIC TRANSPORT PROPERTIES AND BERRY’S PHASE IN HEISENBERG-ISING RING
Authors: KOREPIN V.E. , WU A.C.T.
Published: November 2004

In a recent paper, B. Sutherland and B.S. Shastry have constructed an adiabatic process for the Heisenberg spin chain (spin ½) with respect to a change of boundary conditions. In this paper we calculate Berry’s phase for this process. We also evaluate the dependence of energy levels on boundary conditions which permits us to calculate the effective charge-carrying mass.
NON-ABELIAN FLUX, LATTICE HOPPING, SPIN SYSTEMS, AND THE ELECTRIC DIPOLE MOMENT
Authors: ZEE A.
Published: November 2004

It has been suggested that chiral spin state may serve as a starting point for a theory of superconductivity. Here we propose non-Abelian generalizations of the flux and chiral spin states in both two and three dimensions. In three dimensions, T violation may be manifest through the appearance of an electric dipole moment. We may naturally speculate that the T violation observed in particle physics may ultimately be connected to the opening and widening of mass gaps. In two dimensions, the effective long distance theory may include a non-Abelian Chern-Simons term.
FREEDERICKSZ-LIKE TRANSITIONS IN COMPENSATED NEMATIC LIQUID CRYSTALS
Authors: BARBERO G. , EVANGELISTA L.R. , GABBASOVA Z.
Published: November 2004

The occurrence of the Freedericksz transitions at the inversion point of the main dielectric anisotropy in nematic liquid crystals is discussed. It is shown that, if this kind of order-disorder phase transition can take place, they are always of the first order. The critical fields are evaluated, and the relative phase diagram is discussed.
NUMERICAL STUDIES ON THE VIBRATIONAL SPECTRUM OF FIBONACCI CHAIN: A MULTIFRACTAL ANALYSIS
Authors: SALEJDA WLODZIMIERZ
Published: November 2004

A harmonic Hamiltonian modelling the lattice dynamics of the one-dimensional Fibonacci-type quasicrystal is studied numerically. The multifractal analysis of vibrational spectrum is performed. It is found that the integrated normalized density of states [Formula: see text], where x denotes the square of the eigenenergy of the dynamic matrix, exhibits a finite range of scaling indices α (i.e. α min ≤α≤ α max ) describing the local scaling laws of [Formula: see text]. The α-f spectra and the Renyi dimensions [Formula: see text] are calculated in a wide range of model parameters taking into account the next-nearest-neighbour (NNN) interactions of atoms. In particular, we have observed that: (1) The α-f spectra are smooth in the interval [Formula: see text]; (2) If the so-called parameter of quasi-periodicity Q increases, then αmin and the fractal dimension of vibrational spectra [Formula: see text] decrease; (3) If the strength of NNN interactions h grows then α min decreases but D increases.
ON CHARGED FERMIONS IN TWO DIMENSIONS
Authors: RANDJBAR-DAEMI S. , SALAM ABDUS , STRATHDEE J.
Published: November 2004

The integer quantum Hall effect and associated magnetic phenomena are reconsidered in a two-dimensional system with a flat boundary. The electromagnetic properties of this system are governed by an effective Lagrangian which includes an induced Chern-Simons term. The effective Lagrangian is relevant for the description of fields which are slowly varying about a uniform magnetic background associated with a fermionic ground state in which a whole number of Landau levels is filled. It is singular for field values that correspond to partially filled levels. The underlying assumption of translation invariance of the fermionic ground state fails in the vicinity of boundaries where the effective field theory is essentially nonlocal. The width of the boundary layer and the current flowing in it are estimated.
CHARGE AND SPIN FLUCTUATIONS IN THE HUBBARD MODEL: FERMI LIQUID PROPERTIES AT LOW TEMPERATURES
Authors: LAVAGNA M.
Published: November 2004

We review the slave-boson representations of the Hubbard model. When both spin and charge fermion degrees of freedom are represented by Bose fields, the approach is equivalent at the "self-consistent" saddle point level to the Gutzwiller approximation (GA). We show how the determination of the Lagrange multipliers, introduced to enforce the constraints, involves a Mott-Hubbard gap near half-filling for U > U c (localization edge). The quantum fluctuations are then considered within a renormalized basis of boson fields (restoring the spin-rotation invariance), which brings out two distinct channels, symmetric and antisymmetric. It is remarkable that both spin and charge fluctuations are obtained at the same level of the gaussian fluctuations, as oppossed to the standard 1/N expansion. This provides the microscopic basis for a Fermi liquid theory. Both dynamic effects and extension to finite temperature which were beyond the scope of the variational procedure can now be described by this approach. The physical implications of the model (dynamic correlation functions, T3Ln T term of the specific heat, superfluid instability) are finally considered, all of which reflect the Fermi liquid nature of the system.
THE HUBBARD-STRATONOVICH TRANSFORMATION AND THE HUBBARD MODEL
Authors: SORELLA SANDRO
Published: November 2004

The Hubbard-Stratonovich transformation allows one to formulate the problem of calculating the ground state properties of a many-body theory as one of sampling a distribution. This distribution is constructed by propagating a trial wave function under the influence of a one-body time-dependent external field. However, a straightforward application of the Hubbard-Stratonovich transformation gives distributions which are not always positive definite for a generic trial wave function. In this work it is rigorously shown that, for the Hubbard model, in many cases the non-positiveness of this distribution is not important for reaching the infinite imaginary time limit, i.e., the ground state properties.
THE HUBBARD MODEL: FROM SMALL TO LARGE U
Authors: BAERISWYL DIONYS , VON DER LINDEN WOLFGANG
Published: November 2004

The Hubbard model is investigated starting from both the small and large U limits. This allows one to derive an interpolation formula for the double occupancy at half-filling for dimensionalities d = 1, 2, 3. It shows a smooth behavior as a function of U and tends to zero only for U → ∞. A quantity that probes more sensitively the nature of the ground state is the momentum distribution function n(k). At half filling n(k) is smooth at k F both for d = 1 and d = 2, at least for not too small values of U. In one dimension for all other band fillings the slope of n(k) has a power-law singularity at k F with an exponent α increasing steadily from zero at U = 0 to 1/8 for U → ∞; the system is a "marginal Fermi liquid". A similar behavior may occur close to half-filling for d = 2, but for small densities one expects the usual step function of a normal Fermi liquid.
NOVEL MANY-BODY EFFECTS IN PHOTOEMISSION AND INVERSE PHOTOEMISSION SPECTRA OF ULTRATHIN TRANSITION-METAL FILMS
Authors: CHEN CHANGFENG
Published: November 2004

We present an overview of some recent developments in the theoretical modeling of transition-metal systems, particularly the ultrathin-film structures, focusing on the effects of electron-electron interactions. We describe the progress in the understanding of how to model realistic strongly correlated electron systems using and going beyond the local-density-approximation single-particle electronic structures. Results of exact many-body calculations of photoemission and inverse photoemission spectra of ultrathin nickel, iron and cobalt films are shown to illustrate the application of our approach. Interesting new features induced by many-body effects are found and discussed. Comparison with available experimental results is presented and further work, both experimental and theoretical, is suggested.
STATES OF ANYON MATTER
Authors: WILCZEK FRANK
Published: November 2004

The basic definition and elementary properties of anyons are briefly reviewed. Average field theory is introduced as a method to roughly assess the qualitative properties of aggregates of anyons, and is argued to be reliable in certain limits. The basic mechanism of anyon superconductivity is discussed in simple physical terms. The mechanism is argued to be fundamentally different from spontaneous symmetry breaking, and specifically from BCS pairing. The formal principle involved, previously discussed under the rubric "spontaneous fact violation", is given a new more precise mathematical formulation: it is the spontaneous projectivization of a linear symmetry. Tight conceptual connections are made between the fractional quantized Hall effect and anyon superconductivity, basically by inverting the average field procedure. Values of the quantum statistical parameter are identified at which anyon superconducting states are likely to occur. Values of the filling fractions are identified, as a function of the quantum statistics parameter, at which anyon quantized Hall states are likely to occur. The existence of a new state of matter, the anyon metal, is suggested, and some of its properties are roughly assessed. Possible generalizations of the ideas to cases involving inhomogeneous, three-dimensional or finite systems are discussed in a preliminary but reasonably definite fashion.
LYAPUNOV EXPONENTS IN CHAOTIC SYSTEMS: THEIR IMPORTANCE AND THEIR EVALUATION USING OBSERVED DATA
Authors: ABARBANEL HENRY D. I. , BROWN REGGIE , KENNEL M. B.
Published: November 2004

We review the idea of Lyapunov exponents for chaotic systems and discuss their evaluation from observed data alone. These exponents govern the growth or decrease of small perturbations to orbits of a dynamical system. They are critical to the predictability of models made from observations as well as known analytic models. The Lyapunov exponents are invariants of the dynamical system and are connected with the dimension of the system attractor and to the idea of information generation by the system dynamics. Lyapunov exponents are among the many ways we can classify observed nonlinear systems, and their appeal to physicists remains their clear interpretation in terms of system stability and predictability.

We discuss the familiar global Lyapunov exponents which govern the evolution of perturbations for long times and local Lyapunov exponents which determine the predictability over a finite number of time steps.
ENTROPY OF A TWO LEVEL ATOM IN CONTACT WITH A SQUEEZED RESERVOIR
Authors: TUCCI ROBERT R.
Published: November 2004

We study the entropy of a two level atom in contact with a non-minimum uncertainty squeezed reservoir and a coherent drive. Let δ be the phase of the coherent drive, and ΩR/γ the Rabi frequency divided by the atomic decay rate. For large, near minimum uncertainty squeezing, we find that the atomic quantum state is approximately a pure state, or, equivalently, a minimum entropy state, if ΩR/γ and δ satisfy (ΩR/γ)| cos δ| = 1/2. (Previous workers have only noted the occurrence of a pure state at δ = 0 or 180°.) We find that the atom saturates separately in each quadrature of the coherent drive. We also find that the thermodynamic order of the atom (given by the magnitude of the Bloch vector or by the negative of the atomic entropy) may increase as the background fluctuations or disorder increase. This behavior does not occur with non-squeezed reservoirs; it may be interpreted as a negative temperature effect, if atomic temperatures are defined in terms of the derivatives of the atomic entropy with respect to the variances of the two reservoir quadratures. We give an extensive collection of plots of the atomic entropy and temperatures. We find that a harmonic oscillator always cools down to lower temperatures than a 2 level atom if both bodies are in contact with the same squeezed reservoir.
DENSITY MATRIX FORMALISM, DOUBLE-SPACE AND THERMO FIELD DYNAMICS IN NON-EQUILIBRIUM DISSIPATIVE SYSTEMS
Authors: SUZUKI MASUO
Published: November 2004

General relationship among the density matrix formalism, the double-space formulation and thermo field dynamics is discussed in non-equilibrium dissipative quantum systems. The concept of weakly equivalent operators in the double space formulation is introduced to review many well-known results and it is shown to be useful in mapping between the density matrix formalism and the double space formulation. A new non-equilibrium thermo field dynamics is formulated to discuss dissipative quantum systems. A simple example of damped harmonic oscillators is discussed in the present representation.
THE BOUND ON THE CRITICAL POINT IN THE NEAREST-NEIGHBOR MAJORITY VOTE MODEL
Authors: LUAN CHANGFU
Published: November 2004

In this paper, the relation of the nearest-neighbor majority vote model to the contact process by coupling is studied. Using this method, we obtain that if the parameter of the nearest-neighbor d-dimensional majority vote model is δ ∈ ((2d − 1)/4d, (2d + 1)/4d), then there is no phase transition for this model. It is an improvement of the result δ ∈ (d/(2d + 1), (2d + 1)/4d) in Ref. 1.
CONTINUOUS MODEL OF CRYSTAL MELTING AND DESTRUCTION
Authors: KORNEEV A. A. , TAPINSKAYA O. V. , TRONIN V. N.
Published: November 2004

In this paper we show that the existence of microscopic defects (both point and linear) in the crystal lattice changes the elastic moduli of the solid. Owing to the change of the shear modulus induced by equilibrium defects, the relative shear modulus becomes temperature dependent and this dependence is close to the empirical value. For a wide class of metals, the values of the melting temperature obtained from the requirement that the shear modulus turn to zero are in good agreement with experiment.
INTERACTION OF AN ATOM WITH A CAVITY RADIATION FIELD
Authors: KIEN FAM LE , SHUMOVSKY A.S.
Published: November 2004

We review the dynamical statistical theory of the interaction of an atom with a cavity radiation field in the framework of exactly solvable Jaynes-Cummings type models.
QUANTUM THEORIES FOR IDENTICAL PARTICLES
Authors: LEINAAS JON MAGNE , MYRHEIM JAN
Published: November 2004

We recall some of the arguments which led us to the concept of fractional statistics in one and two dimensions. This concept has been much studied and is well understood in terms of Schrödinger quantization. Heisenberg quantization is an equally natural approach to the same problem, it is not equivalent, and it leads to many interesting open questions.
DISCRETE GAUGE THEORIES
Authors: ALFORD MARK G. , MARCH-RUSSELL JOHN
Published: November 2004

In this review we discuss the formulation and distinguishing characteristics of discrete gauge theories, and describe several important applications of the concept. For the abelian (ℤN) discrete gauge theories, we consider the construction of the discrete charge operator F(Σ*) and the associated gauge-invariant order parameter that distinguishes different Higgs phases of a spontaneously broken U(1) gauge theory. We sketch some of the important thermodynamic consequences of the resultant discrete quantum hair on black holes. We further show that, as a consequence of unbroken discrete gauge symmetries, Grand Unified cosmic strings generically exhibit a Callan-Rubakov effect. For non-abelian discrete gauge theories we discuss in some detail the charge measurement process, and in the context of a lattice formulation we construct the non-abelian generalization of F(Σ*). This enables us to build the order parameter that distinguishes the different Higgs phases of a non-abelian discrete lattice gauge theory with matter. We also describe some of the fascinating phenomena associated with non-abelian gauge vortices. For example, we argue that a loop of Alice string, or any non-abelian string, is super-conducting by virtue of charged zero modes whose charge cannot be localized anywhere on or around the string (“Cheshire charge”). Finally, we discuss the relationship between discrete gauge theories and the existence of excitations possessing exotic spin and statistics (and more generally excitations whose interactions are purely “topological”).
ANYON SUPERCONDUCTIVITY AND CHARGE-VORTEX DUALITY
Authors: LEE DUNG-HAI , FISHER MATTHEW P.A.
Published: November 2004

We review recent work on a bosonic formulation of both anyon superconductivity and the fractional quantum Hall effect (FQHE). Central to this approach is the concept of charge-vortex duality in two-dimensional (2d) boson systems. A formal duality transformation which takes one from a particle to a vortex representation of a 2d boson model is described in detail. The duality transformation is employed to obtain detailed properties of both the FQHE hierarchy and a hierarchy of anyon superconducting phases.
A LATTICE MODEL OF FRACTIONAL STATISTICS
Authors: KANTOR RONALD , SUSSKIND LEONARD
Published: November 2004

We present a new formulation of Chern-Simons theory on a three-dimensional lattice, with either the linear gauge group R or the finite cyclic gauge group Z N. By coupling extended objects called dumb-bells to the lattice Chern-Simons field, we obtain a model exhibiting fractional statistics in the continuum limit. Internal dumb-bell angular momenta take values consistent with their statistics. The Z N-model has a gauge-decoupled condensate. Either model can admit a Maxwell term.
FLUID DYNAMICS, CHERN-SIMONS THEORY, AND THE QUANTUM HALL EFFECT
Authors: BAHCALL SAFI , SUSSKIND LEONARD
Published: November 2004

In this paper we show that classical fluid dynamics in a plane is a gauge theory useful for studying aspects of the quantum Hall system. When the fluid is charged and placed in a magnetic field, Chern-Simons fields appear naturally and the fractional statistics of vortex excitations can be understood qualitatively. Applying the fluid picture to a gas of anyons shows that it superconducts.
KINETICS OF IRREVERSIBLE MONOLAYER AND MULTILAYER ADSORPTION
Authors: BARTELT M.C. , PRIVMAN V.
Published: November 2004

We review recent theoretical developments of microscopic lattice and continuum models of the kinetics of irreversible monolayer and multilayer surface adsorption. Such models have been used to describe adhesion and reaction processes of colloidal particles and proteins at solid surfaces. Theoretical results surveyed here include the void-filling rate equation approach, exact results for low dimensionalities, the mean-field theory, and large-time kinetics arguments. Numerical simulations serving to test and supplement the analytical theories, are reviewed as well. We also elucidate the crossover from the discrete to continuum behavior, analyzed via scaling arguments in the large-time limit of the deposition kinetics.
GRAIN BOUNDARIES: PHASE TRANSITIONS AND CRITICAL PHENOMENA
Authors: RABKIN E.I. , SHVINDLERMAN L.S. , STRAUMAL B.B.
Published: November 2004

Recent theories of grain boundary structure have been reviewed briefly. The possibility of existence of the same variety of phase transitions on grain boundaries as that on the crystal external surface has been demonstrated. Recent experimental data and theoretical models concerning grain boundary phase transitions are critically analysed. Grain boundary phase transitions connected with the formation of thin disordered layers on the boundary (prewetting, premelting) are particularly distinguished. Results of recent indirect experiments, which may be treated in terms of prewetting and premelting, have been reviewed. Experimentally observed critical phenomena in the vicinity of the prewetting transition on the tin-germanium interphase boundary have been discussed in terms of the critical exponents theory. Some ideas regarding directions of further research are presented.
MIXING IN REACTION-DIFFUSION PROBLEMS
Authors: SOKOLOV I.M. , BLUMEN A.
Published: November 2004

Mixing of two viscous liquids often produces arrays of alternating striations. The geometrical structure of these arrays controls the course of the reactions between the liquids, both when the reaction occurs after mixing and also when mixing and reaction take place simultaneously. In the present review we consider some aspects of the problem for the A+B→0 irreversible reaction under stoichiometric conditions. We focus on the influence of the initial distribution of striation thicknesses, on the time evolution of this distribution, on the structure of the reaction layers and on the role of mixing.
METHOD OF THERMODYNAMIC QUASIAVERAGES
Authors: YUKALOV V.I.
Published: November 2004

A new method is developed to define pure states for many-body systems with spontaneous symmetry breaking. The advantage of the method is in the use of solely the standard thermodynamic limit, as compared to the Bogolubov method of infinitesimal external sources which invokes two limiting procedures: the standard thermodynamic limit and the elimination of external sources. The general conditions for obtaining pure states are formulated. When these conditions do not hold mixed states appear. The method is illustrated by calculations for two simple models.
Screening of a Positive Muon by a Semion Gas
Authors: McMullen T. , Jena P. , Khanna S. N.
Published: November 2004

μSR is one of the experimental techniques used to search for manifestations of the broken [Formula: see text] and [Formula: see text] symmetries predicted by some theories of highly correlated electron systems. When [Formula: see text] and [Formula: see text] are broken, the screening of the positively charged muon by the electron system generates a magnetic field in addition to any intrinsic magnetic field that may be present. We estimate the magnetic field that is induced at a muon site when the muon is screened by the fractional statistics gas. The value depends on the distance between the muon and the anyon plane, and a simple theoretical approach to the calculation of the minimum energy muon sites is outlined and some results for Bi 2 Sr 2 CaCu 2 O 8 and YBa 2 Cu 3 O 7 are presented.
BRAID GROUPS, ANYONS AND GAUGE INVARIANCE
Authors: Wu Yong-Shi
Published: November 2004

The structure of braid groups on topologically nontrivial surfaces is reviewed. The physical meaning of the braid relations and their implications on quantum mechanical properties of anyonic quasiparticles are discussed. These include not only the exotic statistics of anyons but also the Aharonov-Bohm effect for the anyons on a surface with holes. Several results on the novel properties of anyons or their states, which were previously derived by microscopic considerations, are reproduced by this seemingly kinematic and topology-dependent braid group analysis. It is suggested that in the thermodynamic limit, the global excitations in a system on a surface of nontrivial topology do not interfere with properties of local anyonic quasiparticles.
Anyon Superconductivity and the Fractional Quantum-Hall Effect
Authors: Lee Dung-Hai
Published: November 2004

I review the concept of statistics transmutation in two dimensions and apply it to the understanding of Fractional quantum-Hall effect and anyon superconductivity. A contrast of the electromagnetic properties of an ordinary 2D superconductor, a quantum-Hall liquid and an anyon superconductor is presented. The relevance of the anyon model to copper-oxide superconductors is also discussed.
"MUTUAL FRACTIONAL STATISTICS" AND "RELATIVE ANYONS"
Authors: Hu Chia-Ren
Published: November 2004

A topological argument similar to that of Leinaas and Myrheim implies that a non-trivial statistical phase factor can also arise from exchanging twice a pair of distinguishable particles in two dimensions. Some general properties of this phase factor are deduced. Wilczek's model for anyons and the Laughlin theory for the quasiparticles in the fractional quantum Hall ground states are examined in light of these properties, and the former is generalized for systems containing many species of anyons. The statistical properties of holons and spinons relative to each other are briefly discussed as an example.
Variational Many Body States for the U=∞ Hubbard Model
Authors: Hellberg C. Stephen , Mele E. J.
Published: November 2004

We study a general class of variational wavefunctions for strongly correlated lattice fermions. The wavefunctions considered here automatically satisfy local constraints of no double occupancy and include correlations between opposite spin particles in a very physical way. We calculate the energy and correlation functions for the one dimensional U=∞ Hubbard model, where a comparison with exact results is made. We briefly report on preliminary results for the t-J model.
A POSSIBLE OPTICAL PHONON DECAY OF LOW-LYING ELECTRONIC CRYSTAL-FIELD STATES OF TmPO4 CRYSTAL
Authors: XIA SHANGDA
Published: November 2004

We present in this paper an optical phonon [Formula: see text] decay mechanism to explain the disappearance of two electronic lines at 248 cm −1 (Γ1) and 254 cm −1 (Γ4) in experimental electronic Raman scattering spectrum of TmPO 4 crystal. The [Formula: see text] phonon model of primitive cell is described in detail. The derivation and calculation of electrical-phonon coupling crystal-field Hamiltonian [Formula: see text] and the calculation of coupling matrix elements [Formula: see text] between the initial and final electron-phonon states are outlined and demonstrated. The various channels of this decay are discussed and estimated.
SUPERCONDUCTIVITY IN THE HUBBARD MODEL WITH STRONG COULOMB REPULSION
Authors: IZYUMOV YU. A. , LETFULOV B. M.
Published: November 2004

Within the Hubbard model with strong Coulomb interaction U ≫ t (the t-J model), the possibility is studied for the superconducting state to exist due to charge and spin fluctuations in a system. Within the generalized random phase approximation (GRPA) used earlier by the authors for the calculation of magnetic susceptibility, the effective interaction between electrons in the Cooper channel was calculated. The calculations were performed by the diagram technique for the Hubbard operators. In GRPA, the effective interaction takes into account all diagram sequences with fermion loops and spin Green functions. The effective interaction is expressed via dynamical magnetic and dielectric susceptibilities; therefore it is considerably enhanced near the boundaries of the paramagnetic phase's instability towards the appearance of magnetic order, or of a state with charge density wave.

Equations for the strong coupling in a superconductor are derived for the normal Z and anomalous φ parts of the electron Green function, linearized in the order parameter φ. The superconducting transition temperature T c is calculated with various sets of parameters in the (t/U, n) plane, n being the electron concentration. It is shown that in the itinerant magnetism regime (n < nc), where there is no localized magnetic moments in the system, spin fluctuations in the vicinity of antiferromagnetic phase transition lead to electron coupling with d-type symmetry, in analogy to the case of weak Coulomb repulsion. In the localized magnetic moment regime (n > nc), a quasistatistical term appears in the magnetic susceptibility. This terms leads to the pair-breaking mechanism, acting analogously to magnetic impurities. The dynamical part of the susceptibility may result in pairing interaction; however, high values of T c can hardly be expected in this case because of the pair-breaking interaction with localized magnetic moments. In the "Hubbard 1" approximation, the critical concentration nc for band with symmetric density of states equals 2/3. For n > 0.88, CDW instability appears in the system. Near its boundary, charge fluctuations may lead to coupling interaction with trivial symmetry of the order parameter. However, the critical line of the CDW instability lies within the region of ferromagnetic phase, and superconductivity becomes suppressed due to spin splitting of the electron spectrum.
SOME FEATURES OF SQUEEZED NEGATIVE BINOMIAL STATE
Authors: JOSHI AMITABH , LAWANDE S. V.
Published: November 2004

Properties of electromagnetic field in the squeezed negative binomial state are investigated in terms of photon number distribution and Wigner function. The relationship of the density matrix of the squeezed negative binomial state to the density matrix of the squeezed thermal state is shown explicitly. The possibility of generation of the negative binomial state is also discussed.
LINEAR VERSUS NONLINEAR AMPLIFICATION: A GENERALIZATION OF THE NOVIKOV-MONTROLL-SHLESINGER-WEST CASCADE
Authors: VLAD MARCEL OVIDIU
Published: November 2004

The framework of this paper is the theory of statistical fractals. A general approach for the scale-invariant stochastic amplification is derived. The model is based on the assumption that the amplification coefficients as well as the probability of an amplification event are random variables selected from a certain probability law. Both linear and nonlinear cases are considered. For linear processes a general solution is available. Like in the case of constant amplification factors the probability density of an amplified variable has a long tail. However, the analytical form of the tail is different: it can be expressed as a sum of inverse power laws modulated by oscillatory functions of the logarithm of the amplified variable, having different characteristic periods. In the nonlinear case no general analytic solutions are available. For a mapping with saturation and a single stable fixed point the probability density of the amplified varibale may have a singularity at a fixed point. A situation with amplification and diminution is also considered. In this case the solution has the features of a double statistical fractal. A generalized version of the linear model is applied to the problem of overdispersed molecular clocks.
Determinant Monte Carlo for the Hubbard Model with Arbitrarily Gauged Auxiliary Fields
Authors: CHEN LIANG , TREMBLAY A.-M.S.
Published: November 2004

Monte Carlo methods for the Hubbard model rely on a Hubbard-Stratonovich (HS) decomposition (auxiliary field method) to perform importance sampling on classical variables. Freedom in the choice of the local HS fields can be formally seen as a gauge choice. While the choice of gauge does not influence observable quantities, it may influence intermediate quantities in the calculation, such as the famous “fermion sign”, and it may also influence the efficiency with which the algorithm explores phase space. The effect of arbitrary gauge choices on both aspects of the algorithm are investigated. It is found that in the single spin-flip determinantal approach, certain gauges lead to a better exploration of phase space. This improvement is demonstrated, in the intermediate coupling regime, by histograms which for the first time show the behavior expected from grand canonical simulations. It is also found that the improved phase space exploration can in practice offset the apparent disadvantage of a smaller fermion sign.
FILLING LANDAU LEVELS: FERMI SEA GROUND STATE ENERGY, COMPETING INTERACTIONS AND MARGINAL DISPERSIONS IN GENERALIZED FLUX PHASES
Authors: Douçot Benoit , Nori Franco , Rammal R.
Published: November 2004

ABSTRACT: We review recent studies on the energetics of fermions confined to a two dimensional square lattice, and the relations of these results to mean-field approaches to the t−J model. Our goal has been to compute the kinetic energy of the Fermi sea of the spinless fermions for any value of the (1) fermion concentration, (2) magnetic flux, and (3) frustration. For the unfrustrated case, we confirm that the ground state energy, χ(Φ), is a minimum for Φ=π(1−δ), which corresponds to one flux quantum per spinless fermion. We then proceed to do a systematic study of frustration effects, coming from longer range couplings, which modify the picture obtained for the unfrustrated case. The frustrating influence of the kinetic energy of the holes (e.g., by breaking magnetic bonds and suppressing the long-range order present in the undoped systems) is the main focus of this work. We find that, in general, E(Φ) always exhibits cusp-like minima which position moves linearly as a function of the fermion density x. Frustration can induce a competition between different local minima. By first considering the local minima for one particle only, we can understand most of the qualitative features of E(Φ). These local minima occur at simple rational fractions of Φ0, and when the flux slightly deviates from these values a one-particle Landau level structure develops. It is precisely such a spectrum that generates a family of cusps that “move away” from the original flux value as x is increased. Every cusp corresponds to an integer number of filled Landau levels, and the minimum energy cusp corresponds to the one level case. Furthermore, we use perturbation theory, valid for low fermion density x, in order to analyze quantitatively the behavior of the cusp-like energy minima; which originate from the Landau level structure when the flux is close to a rational value. If the flux is slightly away from a given rational value [Formula: see text] each of the q subbands generates a secondary Landau level structure. We have derived a t2−t3 phase diagram indicating regions of similar behavior (i.e., adiabatic continuations can be performed with each region, preserving the E(Φ) structure) and the boundaries between them. We have studied several points belonging to those boundaries and found that anomalous behavior, (e.g., cancelation of the k2 term in the dispersion relation) induced by frustration, can occur.
Unified Slave Boson Representation of Spin and Charge Degrees of Freedom for Strongly Correlated Fermi Systems
Authors: Frésard R. , Wölfle P.
Published: November 2004

We consider a slave boson representation of lattice electrons in terms of bosons for the empty, singly and doubly occupied site. The approach generalizes an earlier formulation by Kotliar and Ruckenstein. As examples the Hubbard model and the t-J model are considered. The emphasis is on a detailed derivation and discussion of the formal aspects. Simple meanfield solutions are discussed for illustrative purposes.
EXACTLY SOLVABLE MODELS AND FINITE SIZE CORRECTIONS
Authors: SUZUKI JUNJI , NAGAO TARO , WADATI MIKI
Published: November 2004

Recent developments in the theory of exactly solvable models are reviewed. Particular attention is paid to the finite size corrections to the Bethe ansatz equations. Baxter’s formula which relates a 2-dimensional statistical model with a 1-dimensional spin model is extended into the finite temperature case. A combination of this extension and the theory of finite size corrections gives a systematic method to evaluate low temperature expansions of physical quantities. Applications of the method to 1-dimensional quantum spin models are discussed. Throughout this paper, the usefulness of the soliton theory should be observed.
FLUCTUATION EFFECTS NEAR THE UPPER CRITICAL MAGNETIC FIELD IN HIGH-TcSUPERCONDUCTORS
Authors: KTITOROV S.A. , PETROV YU. V. , SHALAEV B.N. , SHERSTINOV V.S.
Published: November 2004

The effect of fluctuations on the thermodynamics of high-Tc superconductors near the upper critical magnetic field is studied. Lattice effects and, in particular, Harper broadening and splitting of the Landau levels are shown to reduce the problem in consideration to some renormalizable field theory with q-component, a complex order parameter. The number q is determined by the relation Φ/Φ0=p/q, where Φ is the magnetic flux through the plaquette, p and q are integral numbers, Φ0=hc/e is the Onsager flux quantum. A critical behaviour of the model for small values q=1, 2, 3, 4 is analysed and the phase transition is shown to be of the second order. A crossover from one- to three-dimensional fluctuation regime is analysed. The case of q→∞ is qualitatively discussed too.
NUMBER-PHASE SQUEEZED STATES AND PHOTON FRACTIONING
Authors: D’ARIANO G.M.
Published: November 2004

The number-phase squeezing mechanisms related to the multiphoton and photonfractioning procedures are analyzed. The main ingredients in these approaches are nonlinear multimode realizations of group-theoretical states and nonunitary subdynamics of the field modes. The states which are squeezed in the number of photons are suited to optical communications in local area networks; the states squeezed in the phase of the field optimize high-sensitive interferometry.

Corresponding to the different kinds of squeezing and to the pertaining grouptheoretical states one has different types of amplifying devices with the same dynamical group of the states. In particular, two recently proposed devices – the photon-number amplifier (PNA) and the photon-number duplicator (PND) – attain both the multiphoton and photon-fractioning transformations. The PNA amplifies the field without degrading the direct-detection signal-to-noise ratio and thus can be used to produce number-squeezed states; the PND provides two copies of the same input number-state and, therefore, can connect two-mode states with one-mode states. Both the PNA and PND allow the realization of virtually lossless optical taps, which are essential in designing highly transparent local area networks. The PND can also be used in series with a conventional phase-insensitive amplifier to produce phase-squeezed states.
HIGH-TEMPERATURE SUPERCONDUCTIVITY FROM ELECTRON-LATTICE COUPLING
Authors: APOSTOL M. , VASILIU-DOLOC L.
Published: November 2004

Within the polaron theory of superconductivity for La 2−x Sr x CuO 4 and La 2−x Ba x CuO 4 the coupling is considered between the charge carriers and both the stretching mode of the apical oxygens and the sheet-bending mode of the basal-plane oxygens, the latter via the tilting angle of the Cu-oxygen octahedra. The same tilting angle enters the coupling strengths of these lattice vibrations, leading to renormalized frequencies and their oxygen-isotope shifts. It is suggested that the anomalies occurring in the superconducting critical temperature, its isotope shift and the isotope shifts of the lattice frequencies around x=0.12 are related to the orthorhombic-tetragonal transition of these compounds.
THE EDWARDS MODEL: LOCALISATION VIA THE EDWARDS MODEL
Authors: EDWARDS S.F.
Published: November 2004

The model arose through the study of electron, i.e. Anderson localisation.1 It can be usefully applied to the Baumgartner-Muthukumar localisation of polymers, and this offers a new way of looking at the Anderson problem.
ANYONS IN A MAGNETIC FIELD
Authors: KARLHEDE ANDERS , WESTERBERG ERIK
Published: November 2004

We consider N anyons, with statistical parameter ν, in a homogeneous magnetic fieldand discuss exact and perturbative solutions. We show that the energy, E(ν), obeys[Formula: see text] for any bose or fermi state and that the extremal cases correspondto the states where the relative angular momenta of all pairs of particles are negativeand non-negative, respectively. The anyonic continuations of these extremal states aregiven, their energy is linear in ν. We conjecture that the above bounds on [Formula: see text] hold forany ν. This implies that the energy of any many-anyon state decreases monotonicallywith the statistical parameter ν (ν taking any real value). For small enough ν, E=const. [Formula: see text] and the wave function is known. Increasing ν, the wave functionbecomes singular at some bose point. In the following intermediate region (which maybe absent), E(ν) decreases more slowly; the exact form of E(ν) and the wave functionare unknown. Increasing ν further, above some bose point the energy becomes constantand the wave function is again known.
MODULAR CATEGORIES AND 3-MANIFOLD INVARIANTS
Authors: TURAEV VLADIMIR G.
Published: November 2004

The aim of this paper is to give a concise introduction to the theory of knot invariants and 3-manifold invariants which generalize the Jones polynomial and which may be considered as a mathematical version of the Witten invariants. Such a theory was introduced by N. Reshetikhin and the author on the ground of the theory of quantum groups. Here we use more general algebraic objects, specifically, ribbon and modular categories. Such categories in particular arise as the categories of representations of quantum groups. The notion of modular category, interesting in itself, is closely related to the notion of modular tensor category in the sense of G. Moore and N. Seiberg. For simplicity we restrict ourselves in this paper to the case of closed 3-manifolds.
INTEGRABLE DEFORMATIONS OF CONFORMAL THEORIES AND BOOTSTRAP TREES
Authors: MUSSARDO GIUSEPPE
Published: November 2004

Recent results in the study of massive integrable quantum field theories in (1+1) dimensions considered as perturbed conformal minimal models are presented. The on mass-shell properties of such theories, with a particular emphasis on the bootstrap principle, are investigated.
ANYON WAVE FUNCTIONS FROM ANYON FIELD OPERATORS
Authors: MARCHETTI P.A.
Published: November 2004

We consider a Quantum Field Theory in two space dimensions exhibiting anyon particle excitations and we show how one can derive the structure of wave functions for anyons from the commutation properties of the anyon field operators.
TRICRITICAL POINT IN ORDER-DISORDER PHASE TRANSITIONS FOR NEMATICS
Authors: BARBERO G. , EVANGELISTA L. R. , KREKHOV A. P.
Published: November 2004

A complete analysis of the order-disorder phase transition in nematic liquid crystals induced by an external field is presented. The case of nearly compensated nematics in weak anchoring situation is discussed. The investigation shows that the parameter controlling the order of the phase transition is a decreasing function of the anchoring strength. The important limiting cases of very weak and very strong anchoring are considered too.
LINEAR/NONLINEAR MECHANICAL PROPERTIES OF ELECTRORHEOLOGICAL MATERIALS
Authors: GAMOTA DANIEL R. , FILISKO FRANK E.
Published: November 2004

The stress response of an electrorheological (ER) material is modified by the application of an electric field. Various studies have shown that the ER material can behave as a linear viscous, linear viscoelastic, nonlinear viscoelastic, plastic, or viscoelastic-plastic body. Furthermore, several different experimental techniques are conducted to observe the ER material's behavior as a function of strain, strain frequency, field strength, and ER material concentration. Small amplitude dynamic studies are used to observe the ER material's linear viscoelastic properties, while moderate and large amplitude studies are used to observe the material's fundamental nonlinear dynamic properties. Finally, constant shear rate experiments are performed to observe the apparent viscosity of the ER material during flow conditions.