Numerical diagonalization study of an S=1/2 ladder model with open boundary conditions

Low-lying energy states of an S=1/2 ladder model are investigated by applying the numerical diagonalization method to finite clusters. This model has the antiferromagnetic intrachain coupling J (J>0) and the ferromagnetic interchain one -J (>0). Both the inverse correlation length -1() and an energy gap are shown to be finite at least for 0.05, the former of which is shown to approach the same value as that of the S=1 antiferromagnetic Heisenberg chain with increasing . A generation mechanism of the gap is also discussed in terms of a simple model by using the Lieb-Mattis theorem.