Strong charge fluctuations in the single-electron box: A quantum Monte Carlo analysis

We study strong electron tunneling in the single-electron box, a small meta llic island coupled to an electrode by a tunnel junction, by means of quant um Monte Carlo simulations. We obtain results, at arbitrary tunneling stren gth, for the free energy of this system and the average charge on the islan d as a function of an external bias voltage. in much of the parameter range an extrapolation to the ground state is possible. Our results for the effe ctive charging energy for strong tunneling are compared to earlier-in part controversial-theoretical predictions and Monte Carlo simulations. [S0163-1 829(99)03108-2].