QUANTUM MONTE-CARLO STUDY OF TUNNELING DIFFUSION IN A DISSIPATIVE MULTISTATE SYSTEM

Real-time quantum Monte Carlo (QMC) methods have been used to study diffusion on a one-dimensional tight-binding lattice in a dissipative environment with Ohmic friction. For this system, the inherent sign problem of real-time QMC methods can be substantially reduced by employing a partial summation scheme, allowing direct calculations of long-time transport properties. At very low temperatures, the system undergoes a transition from a coherent transport mechanism to an incoherent mechanism as the Kondo parameter K goes through 1. For 0 < K < 1, perturbation theory which predicts a T2K-1 power-law temperature dependence for the diffusion coefficient D is grossly incorrect for low temperatures. Instead, D(T --> 0) = 0 for 0 < K < 1/2, and with increasing temperature D goes through a pronounced maximum. This maximum disappears for K > 1/2 and D increases monotonically with increasing temperature. For all K < 1, perturbation theory becomes exact at sufficiently high temperatures, indicating that the transport mechanism eventually becomes incoherent. For K > 1, perturbation theory is exact for all temperatures, and transport is always incoherent.