title: Exact and numerical results for a dimerized coupled spin- 1/2 chain
abstract: We establish exact results for coupled spin-1/2 chains for special values of the four-spin interaction V and dimerization parameter delta. The first exact result is at delta = 1/2 and V = -2. Because we find a very small but finite gap in this dimerized chain, this can serve as a very strong test case for numerical and approximate analytical techniques. The second result is for the homogeneous chain with V = -4 and gives evidence that the system has a spontaneously dimerized ground state. Numerical diagonalization and bosonization techniques indicate that the interplay between dimerization and interaction could result in gapless phases in the regime 0</=V<-2.
authors: School of Natural Sciences, Institute for Advanced Study, Olden Lane, Princeton, New Jersey 08540 and Departamento de Fisica, Universidade Federal de Sao Carlos, Caixa Postal 676, 13565-905, Sao Carlos, Brazil.
authors: MJ
authors: Martins
authors: B
authors: Nienhuis