Pairing of charge carriers in the two-dimensional molecular crystal model

Abstract

An investigation of the two-dimensional molecular crystal model within the adiabatic approximation demonstrates the pairing tendency of charge carriers over a wide range of doping concentrations. Close to half-filling, there exists a (\ensuremath{\pi}/a,\ensuremath{\pi}/a) charge-density wave (CDW), and the pair binding energy measured is due to the formation of immobile bipolarons. As the system is doped further away from half-filling, the binding energy remains roughly constant although the charge-density background develops domains of alternate order parameter. The binding energy of the pairs is greatly reduced when the CDW is doped away. A fermion Monte Carlo study shows that the pairing susceptibility, ${\mathrm{\ensuremath{\chi}}}_{\mathrm{SP}}$, is dramatically enhanced as the system approaches the metal-insulator transition from the metallic side. These results suggest that CDW fluctuations are relevant in the enhancement of ${\mathrm{\ensuremath{\chi}}}_{\mathrm{SP}}$ in this region.