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 0K${\mathit{T}}^{2\mathit{K}\mathrm{-}1}$ power-law temperature dependence for the diffusion coefficient D is grossly incorrect for low temperatures. Instead, D(T→0)=0 for 0KD goes through a pronounced maximum. This maximum disappears for K>1/2 and D increases monotonically with increasing temperature. For all KK>1, perturbation theory is exact for all temperatures, and transport is always incoherent.