PHASE-TRANSITIONS IN THE ONE-DIMENSIONAL PAIR-HOPPING MODEL - A RENORMALIZATION-GROUP STUDY

The phase diagram of a one-dimensional tight-binding model with a pair-hopping term (amplitude V) has been the subject of some controversy. Using two-loop renormalization-group equations and the density matrix renormalization group with lengths L less than or equal to CO, we argue that no spin-gap transition occurs at half-filling for positive V, contrary to recent claims. However, we point out that away from half-filling, a phase-separation. transition occurs at finite V. This transition and the spin-gap transition occurring at half-filling and negative V are analyzed numerically.