Ground state properties of an S=1/2 distorted diamond chain

We investigate the ground state properties of an S=1/2 distorted diamond ch ain described by the Hamiltonian H = J(1) Sigma(l){((S) over right arrow(3l -1) . (S) over right arrow(3l) + (S) over right arrow(3l) . (S) over right arrow(3l+1)) + J(2)(S) over right arrow(3l-2) . (S) over right arrow(3l--1) + J(3) ((S) over right arrow(3l-2) . (S) over right arrow(3l) + (S) over r ight arrow(3l) . (S) over right arrow(3l+2)) - HSlz} (J1, J2, J3 greater th an or equal to 0), which models well a trimerized S=1/2 spin chain system C u3Cl3(H2O)(2). 2H(3)C(4)SO(2). Using an exact diagonalization method by mea ns of the Lanczos technique, we determine the ground state phase diagram in the H=0 case, composed of the dimerized, spin fluid, and ferrimagnetic pha ses. Performing a degenerate perturbation calculation, we analyze the phase boundary line between the latter two phases in the J(2), J(3) much less th an J(1) limit, the result of which is in good agreement with the numerical result. We calculate, by use of the density matrix renormalization group me thod, the ground state magnetization curve for the case (a) where J(1) = 1. 0, J(2) = 0.8, and J(3) = 0.5, and the case (b) where J(1) = 1.0, J(2) = 0. 8, and J(3) = 0.3. We find that in the case (b) the 2/3-plateau appears in addition to the 1/3-plateau which also appears in the case (a). The transla tional symmetry of the Hamiltonian H. is spontaneously broken in the 2/3-pl ateau state as well as in the dimerized state.