Magnetization properties of some quantum spin ladders

The experimental realization of various spin ladder systems has prompted th eir detailed theoretical investigations. Hen we study the evolution of grou nd-state magnetization with an external magnetic field for two different an tiferromagnetic systems: a three-legged spin-1/2 ladder, and a two-legged s pin-1/2 ladder with an additional diagonal interaction. The finite system d ensity-matrix renormalization-group method is employed for numerical studie s of the three-chain system, and an effective low-energy Hamiltonian is use d in the limit of strong interchain coupling to study the two- and three-ch ain systems. The three-chain system has a magnetization plateau at one-thir d of the saturation magnetization. The two-chain system has a plateau at ze ro magnetization due to a gap above the singlet ground state. It also has a plateau at half of the saturation magnetization for a certain range of val ues of the couplings. We study the regions of transitions between plateaus numerically and analytically, and find that they are described, at first or der in a strong-coupling expansion, by an XXZ spin-1/2 chain in a magnetic field; the second-order terms give corrections to the XXZ model, We also st udy numerically some low-temperature properties of the three-chain system, such as the magnetization, magnetic susceptibility and specific heat. [S016 3-1829(99)303001-5].