QUANTUM MONTE-CARLO IN THE INTERACTION REPRESENTATION - APPLICATION TO A SPIN-PEIERLS MODEL

A quantum Monte Carlo algorithm is constructed starting from the stand ard perturbation expansion in the interaction representation, The resu lting configuration space is strongly related to that of the stochasti c series expansion (SSE) method, which is based on a direct power-seri es expansion of exp(-beta H). Sampling procedures previously developed for the SSE method can therefore be used also in the interaction repr esentation formulation, The method is tested on the S=1/2 Heisenberg c hain. Then, as an application to a model of great current interest, a Heisenberg chain including phonon degrees of freedom is studied. Einst ein phonons are coupled to the spins via a linear modulation of the ne arest-neighbor exchange. The simulation algorithm is implemented in th e phonon occupation-number basis, without Hilbert space truncations, a nd is exact. Results are presented for the magnetic properties of the system in a wide temperature regime, including the T-->0 limit where: the drain undergoes a spin-Peierls transition. Some aspects of the pho non dynamics are also discussed. The results suggest that the effects of dynamic phonons in spin-Peierls compounds such as GeCuO3; and alpha '-NaV2O5 must be included In order to obtain a correct quantitative de scription of their magnetic properties,both above and below the dimeri zation temperature.