We show that the singularity of the free energy of Ising models in the absence of a magnetic field on the triangular, square, and honeycomb lattices is related to zeros of the pseudopartition function on an elementary cycle. Using the Griffiths' smoothness postulate, we extend these results to the case in a magnetic field and derive a formula of the critical line of an Ising antiferromagnet, which is in good agreement with the numerical results.

The Yang-Lee circle theorem is extended to an ideal pseudospin-1/2 Bose gas in an external magnetic field. It is found that the zeros of the canonical partition function are located on the unit circle in the complex activity plane if the temperature is above the critical temperature of ideal Bose-Einstein condensation. No zeros exist if the temperature is below the critical temperature.

Using the Huang-Yang-Luttinger and Lee-Yang virial expansions of a dilute Bose gas as well as our theory of phase transition, we show that in the dilute limit, the critical temperature of Bose-Einstein condensation of a Bose gas is given by [Tc-T(0)c ] = T(0)c = γn1 = 3a, = 4 [ζ(3/2)]5/3/[3 ζ(5/2)] = 4. 92506..., which is in good agreement with the experimental result of Reppy et al.,γ= 5.1 ± 0.9.

Based on Meakin' s simulation results [Phys. Rev. A 34, 710 (1986); 35, 2234 (1987)], we use the bino-mial distribution to discuss the growth probability for the screened-growth model and develop a formula of the generalized dimensions. The results from this model are in good agreement with those from the simulations.

Using the approach that we developed recently, we find the critical line of an anisotropic Ising antiferromagnet on two-dimensional square and honeycomb lattices. We extend our previous lemma and conjecture to be useful in the antiferromagnetic system. We find two interesting behaviors: (1) An antiferromagnet is not necessarily most inert at the absolute zero. (2) The field-driven antiferromagnetic phase transition is possible in the honeycomb lattice.