Ground state of an S=1/2 distorted diamond chain in a finite magnetic field

We investigate the ground state magnetization curve of an S = 1/2 distorted diamond chain. It is found that, in a certain region of the exchange interaction constants, a (2/3)-plateau appears in addition to a (1/3)-plateau. which appears in a much wider region. In the (2/3)-plateau state the translational symmetry of the Hamiltonian is spontaneously broken. We also discuss the phase diagram on an interaction constant plane that shows the region of the appearance of the (2/3)-plateau. (C) 2000 Elsevier Science Ltd. All rights reserved.