Metal-insulator transition in the two-chain model of correlated fermions

The doping-induced metal-insulator transition in two-chain systems of correlated fermions is studied using a solvable limit of the $t-J$ model and the fact that various strong- and weak-coupling limits of the two-chain model are in the same phase, i.e., have the same low-energy properties. It is shown that the Luttinger-liquid parameter $K_{\rho }$ takes the universal value unity as the insulating state (half-filling) is approached, implying dominant $d$-type superconducting fluctuations, independently of the interaction strength. The crossover to insulating behavior of correlations as the transition is approached is discussed.