Dynamics ofS=1/2 Two-Leg Spin Ladder Systems

Abstract

We calculate the dynamical structure factor of the S =1 /2 two-leg spin ladder system, using a continued fraction method based on Lanczos algorithm. Characteristics of the lowest excited states are discussed from strong to weak interchain-coupling regions. S =1 /2 two-leg spin ladder systems with antiferromagnetic interchain interactions have attracted great attention both theoretically and experimentally. When the interchain coupling (J⊥) is stronger than the intrachain coupling (J� ), the ground state consists of the product of rung singlets, and the elementary excitation is described on the basis of the rung triplet. Using several theoretical methods, it was shown that a singlet and a triplet two-magnon bound-states exist below the twomagnon continuum in addition to the elementary one-triplet excitation. 1) - 7) Twomagnon singlet bound-state was observed recently by phonon-assisted optical absorption. 8),9) From the theoretical analysis of the experimental findings, it was confirmed that the above picture of the elementary excitation holds even in J⊥/J� ∼ 1. 7),9) Furthermore, momentum dependence of the one-triplet and two-magnon spectral densities was calculated for J⊥/J � ≥ 1. 7),9) In this paper, we investigate dynamical properties of S =1 /2 two-leg spin ladder systems from strong to weak interchain-coupling regions. Using a continued fraction method based on Lanczos algorithm, 10) the dynamical structure factor (DSF) is calculated. On the basis of the finite size effects, characteristics of the lowest excited states are discussed. The S =1 /2 two-leg spin ladder is described by the following Hamiltonian,