F. Y. Wu

Theory of resistor networks: the two-point resistance

The resistance between two arbitrary nodes in a resistor network is obtained in terms of the eigenvalues and eigenfunctions of the Laplacian matrix associated with the network. Explicit formulae for two-point resistances are deduced for regular lattices in one, two and three dimensions under various boundary conditions including that of a Mobius strip and a Klein bottle. The emphasis is on lattices of finite sizes. We also deduce summation and product identities which can be used to analyse large-size expansions in two and higher dimensions.

Knot theory and statistical mechanics

Recent development in the mathematical theory of knots using the method of statistical mechanics is examined. We show that knot invariants can be obtained by considering statistical‐mechanical models on a lattice. Particularly, we establish that the Kauffman’s bracket polynomial is the partition function of a q‐state vertex model previously considered by Perk and Wu, and that the Jones polynomial is generated by a q 2‐state Potts model partition function. The generation of further new knot and link invariants many very well rely on computed‐aided studies of solutions of certain Yang‐Baxter equations.

Equivalence of the Potts model or Whitney polynomial with an ice-type model

The partition function of the Potts model (1952) on any lattice can readily be written as a Whitney polynomial (1932). Temperley and Lieb (Proc. R. Soc., vol.A322, p.251 of 1971) have used operator methods to show that, for a square lattice, this problem is in turn equivalent to a staggered ice-type model. Here the authors rederive this equivalence by a graphical method, which they believe to be simpler, and which applies to any planar lattice. For instance, they also show that the Potts model on the triangular or honeycomb lattice is equivalent to an ice-type model on a Kagome lattice.

Spanning trees on graphs and lattices in d dimensions

The problem of enumerating spanning trees on graphs and lattices is considered. We obtain bounds on the number of spanning trees NST and establish inequalities relating the numbers of spanning trees of different graphs or lattices. A general formulation is presented for the enumeration of spanning trees on lattices in d≥2 dimensions, and is applied to the hypercubic, body-centred cubic, face-centred cubic and specific planar lattices including the kagome, diced, 4-8-8 (bathroom-tile), Union Jack and 3-12-12 lattices. This leads to closed-form expressions for NST for these lattices of finite sizes. We prove a theorem concerning the classes of graphs and lattices with the property that NST~exp (nz) as the number of vertices n→∞, where z is a finite non-zero constant. This includes the bulk limit of lattices in any spatial dimension, and also sections of lattices whose lengths in some dimensions go to infinity while others are finite. We evaluate z exactly for the lattices we consider, and discuss the dependence of z on d and the lattice coordination number. We also establish a relation connecting z to the free energy of the critical Ising model for planar lattices.

Number of spanning trees on a lattice

The number of spanning trees on a large lattice is evaluated exactly for the square, triangular and honeycomb lattices.

The one-dimensional Hubbard model: a reminiscence

In 1968 we published the solution of the ground state energy and wave function of the one-dimensional Hubbard model, and we also showed that there is no Mott transition in this model. Details of the analysis have never been published, however. As the Hubbard model has become increasingly important in condensed matter physics, relating to topics such as the theory of high-Tc superconductivity, it is appropriate to revisit the one-dimensional model and to recall here some details of the solution.

Duality transformation in a many‐component spin model

It is shown that the duality transformation relates a spin model to its dual whose Boltzmann factors are the eigenvalues of the matrix formed by the Boltzmann factors of the original spin model. The duality relation valid for finite lattices is obtained, and applications are given.

Exact results for the Potts model in two dimensions

By considering the zeros of the partition function, we establish the following results for the Potts model on the square, triangular, and honeycomb lattices: (i) We show that there exists only one phase transition; (ii) we give an exact determination of the critical point; (iii) we prove the exponential decay of the correlation functions, in one direction at least, for all temperatures above the critical point. The results are established forq ⩾ 4, whereq is the number of components.

Percolation and the Potts model

The Kasteleyn-Fortuin formulation of bond percolation as a lattice statistical model is rederived using an alternate approach. It is shown that the quantities of interest arising in the percolation problem, including the critical exponents, can be obtained from the solution of the Potts model. We also establish the Griffith inequality for critical exponents for the bond percolation problem.

DIMERS ON TWO-DIMENSIONAL LATTICES

We consider close-packed dimers, or perfect matchings, on two-dimensional regular lattices. We review known results and derive new expressions for the free energy, entropy, and the molecular freedom of dimers for a number of lattices including the simple-quartic (4^4), honeycomb (6^3), triangular (3^6), kagome (3.6.3.6), 3-12 (3.12^2) and its dual [3.12^2], and 4-8 (4.8^2) and its dual Union Jack [4.8^2] Archimedean tilings. The occurrence and nature of phase transitions are also analyzed and discussed.