CORRELATION LENGTHS IN QUANTUM SPIN LADDERS

Analytic expressions for the temperature dependences of the correlatio n length are given for antiferromagnetic spin-1/2 Heisenberg ladders u sing a finite-size nonlinear sigma model approach. These calculations rely on identifying three successive crossover regimes as a function o f temperature. Tn each of these regimes, precise and controlled approx imations are formulated. The analytical results are found to be in exc ellent agreement with Monte Carlo simulations for the Heisenberg Hamil tonian.