STRONG-COUPLING EXPANSIONS AT FINITE TEMPERATURES - APPLICATION TO QUANTUM DISORDERED AND QUANTUM CRITICAL PHASES

By combining conventional finite-temperature many-body perturbation th eory with cluster expansions, we develop a systematic method to carry out high order arbitrary temperature perturbative calculations on a co mputer. The method is well suited to studying the thermodynamic proper ties of quantum disordered and quantum critical phases at finite tempe ratures. As an application, we calculate the magnetic susceptibility, internal energy. and specific heat of the bilayer Heisenberg model. It is shown that for a wide range of coupling constants these expansions show excellent convergence at all temperatures. Comparing the direct series (without extrapolations) for the bulk susceptibility to quantum Monte Carlo simulations we find an almost perfect agreement between t he two methods even at the quantum critical coupling separating the di merized and antiferromagnetic phases. The convergence fails only at ve ry low temperatures, which are also difficult to reach by quantum Mont e Carlo simulations.