Collective excitations in ferrimagnetic Heisenberg ladders - art. no. 144429

We study ground-state properties and the low-lying excitations of Heisenber g spin ladders composed of two ferrimagnetic chains with alternating site s pins (S-1 > S-2) by using the bosonic Dyson-Maleev formalism and Lanczos nu merical techniques. The emphasis is on properties of the ferrimagnetic phas e, which is stable for antiferromagnetic interchain couplings J(perpendicul ar to) greater than or equal to 0. There are two basic implications of the underlying lattice structure: (i) the spin-wave excitations form folded aco ustic and optical branches in the extended Brillouin zone and (ii) the grou nd-state parameters (such as the on-site magnetizations and spin-stiffness constant) show a crossover behavior in the weak-coupling region 0 < J(<perp endicular to>) < 1. The above peculiarities of the ladder ferrimagnetic sta te are studied up to second order in the quasiparticle interaction and by a numerical diagonalization of ladders containing up to N = 12 rungs. The pr esented results for the ground-state parameters and the excitation spectrum can be used in studies on the low-temperature thermodynamics of ferrimagne tic ladders.