Moment formalisms applied to a solvable model with a quantum phase transition - (I). Exponential moment methods

We examine the Ising chain in a transverse field at zero temperature from t he point of view of a family of moment formalisms based upon the cumulant g enerating function, where we find exact solutions for the generating functi ons and cumulants at arbitrary couplings and hence for both the ordered and disordered phases of the model. In a t-expansion analysis, the exact Horn- Weinstein function E(t) has cuts along an infinite set of curves in the com plex Jt-plane which are confined to the left-hand half-plane ImJt < -1/4 fo r the phase containing the trial state (disordered), but are not so for the other phase (ordered), For finite couplings the expansion has a finite rad ius of convergence. Asymptotic forms for this function exhibit a crossover at the critical point, giving the excited state gap in the ground state sec tor for the disordered phase, and the first excited state gap in the ordere d phase. Convergence of the t-expansion with respect to truncation order is found in the disordered phase right up to the critical point, for both the ground state energy and the excited state gap. However, convergence is fou nd in only one of the connected moments expansions (CMX), the CMX-LT, and t he ground state energy shows convergence right to the critical point again, although to a limited accuracy. (C) 1999 Elsevier Science B.V. All rights reserved.