On high-temperature expansions of the Hubbard model

We discuss the application of the high-temperature expansion method to the Hubbard model. We recalculate the expansion series of the susceptibility up to the sixth order in the transfer matrix element,t, in the strong correlation limit, and up to the fourth order int in case that the repulsive potential,U, is finite, butt/U 1. It is seen that the convergence of the series is very poor.     

Operator product expansions in the massless Schwinger model

The known operator solution of the massless Schwinger model is used to calculate exactly, in three operator product expansions, the coefficient functions of the first few operators of low dimension which contribute when vacuum matrix elements are to be taken. A comparison of the results provides a test of the procedure used by M. A. Shifman, A. I. Vainshtein, and V. I. Zakharov [Nucl. Phys. B147 (1979), 385–447] in their study of QCD. It is found that the shift in vacua does not affect the calculation of coefficient functions. The vacuum insertion approximation yields somewhat misleading estimates of vacuum expectation values of some composite operators; however, the standard method used to estimate the errors of vacuum insertion indicates that the approximation is unreliable in this model.     

Gross-Neveu model through convergent perturbation expansions

We construct a continuum limit for the effective low energy Lagrangians of the Gross-Neveu model in two euclidean dimensions by showing that they are related to each other through convergent perturbation expansions. This provides a rigorous control of the ultraviolet problem in a renormalizable quantum field theory.Supported in part by the National Science Foundation under Grant MCS-81-20833Supported in part by the National Science Foundation under Grant PHY-82-03669     

Light-cone expansions,the parton model and virtual γγ scattering

It is shown that it is sufficient to use the light-cone algebra of currents and the algebra of bilocal operators to find the asymptotic behaviour of the γγ scattering amplitude when one (or two) of the photon masses q_1,2² is large, and for an arbitrary value of the energy squared s = (q₁+q₂)². A general form of this asymptotic behaviour is obtained. The box-diagram is dominant over the wide region in $\text{s(μ}{}^2\text{« s « q}{}_1{}^2\text{q}{}_2{}^2\text{/μ}{}^2\text{,μ ～ 1}\text{GeV}\text{)}$ and so the asymptotic amplitude is known completely. It is shown that the parton model of the type of ref.[8] gives the same predictions for the asymptotic behaviour of the γγ amplitude.     

Perturbative expansions for the n-vector field model of phase transition

Correlation corrections to the phenomenological expressions for the order parameter and specific heat are calculated for the n-vector field model. At n=3 the numerical coefficients for the correction terms obtained are found to coincide with those for the Heisenberg model with a large interaction range.     

Orthogonal polynomial expansions for effective interactions in the modified Lipkin model

The effective interaction in a one-dimensional model is calculated in terms of an orthogonal polynomial (OP) expansion derived from spectral distribution considerations. Configuration densities are assumed to be gaussian. The results are compared with those of a Brillouin-Wigner (BW) perturbation expansion for the same model. While for a strong perturbation the BW series diverges, the OP expansion is shown to converge smoothly.     

Nonlinear gyrokinetic simulations of ion-temperature-gradient turbulence for the optimized Wendelstein 7-X stellarator

Ion-temperature-gradient turbulence constitutes a possibly dominant transport mechanism for optimized stellarators, in view of the effective suppression of neoclassical losses characterizing these devices. Nonlinear gyrokinetic simulation results for the Wendelstein 7-X stellarator [G. Grieger, in (IAEA, Vienna, 1991) Vol. 3, p. 525]-assuming an adiabatic electron response-are presented. Several fundamental features are discussed, including the role of zonal flows for turbulence saturation, the resulting flux-gradient relationship, and the coexistence of ion-temperature-gradient modes with trapped ion modes in the saturated state.     

Form factor expansions in the 2D Ising model and Painlevé VI

We derive a Toda-type recurrence relation, in both high- and low-temperature regimes, for the λ  -extended diagonal correlation functions C(N,N;λ)$C(N,N;\lambda )$ of the two-dimensional Ising model, using an earlier connection between diagonal form factor expansions and tau-functions within Painlevé VI (PVI) theory, originally discovered by Jimbo and Miwa. This greatly simplifies the calculation of the diagonal correlation functions, particularly their λ-extended counterparts.     

The optimized expansion technique in the three-dimensional Gross-Neveu model

An alternative approach to the study of the three-dimensional Gross-Neveu model using optimized expansion (OE) is proposed. The generalized principle of optimization used to obtainV _eff with an accuracy up to second-order OE is formulated. This method predicts the same phase structure as does the 1/N method.     

Studies of the exactly dimerized bilayer Heisenberg model via strong coupling expansions

The one-triplet excitation spectra and thermodynamic properties for the dimerized phase of the frustrated bilayer Heisenberg model are studied using strong-coupling expansion theory. The model has an exact dimerized ground state as well as exact one-triplet excitations in a special case that the frustration J₂ is equal to the in-plane coupling J₁. We demonstrate that the models with and without frustrations have distinct excitation spectra, so their thermodynamic properties exhibit quite different behaviors. Especially, the low-temperature behaviors of the frustrated model with J ₁=J ₂ are independent of the inter-dimer couplings, due to the exact one-triplet excitations. Received 16 March 2000 and Received in final form 2 July 2000     

A soluble model for the study of the leptodermous expansions in finite nuclei

In order to study the validity of the leptodermous expansions in nuclei we introduce a density profile which makes possible exact analytic calculations of the nuclear binding energy, in a three dimensional geometry. It is shown thus that the leptodermous expansion of the nuclear energy possesses the correct largeA limit and due to the presence of exponential terms the correct smallA limit as well.     

Nonlinear spinor theory and Weinberg's model

It is shown that the formalism for eliminating vacuum degeneracy can be used to reduce the Heisenberg-Ivanenko nonlinear spinor Lagrangian to a Weinberg-type model Lagrangian of the weak and electromagnetic interactions.     

Eigenstate expansions in a soluble model of identical particle scattering

Solutions to the Schrödinger equation and the inhomogeneous equation for the case of two identical particles interacting with a center of force are studied. Eigenstate expansions for solving each equation are explicitly introduced and their properties discussed. The case when the interparticle interaction v₁₂ is zero is then examined; this is a completely soluble problem. The eigenstate expansion solutions for the Schrödinger and inhomogeneous equations are used to explore the means by which the correct solution is obtained. Finally, approximate solutions, obtained by truncating the eigenfunction expansions, are introduced. It is seen that both methods lead to the correct amplitude when τ₁₂ = 0, even though the approximate solution to the inhomogeneous equation does not lead, in the end, to an antisymmetric solution.