PAIRING OF CHARGE-CARRIERS IN THE 2-DIMENSIONAL MOLECULAR-CRYSTAL MODEL

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 (/a,/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, 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 SP in this region. © 1990 The American Physical Society.