Dynamical structure factors ofS=12two-leg spin-ladder systems

Abstract

We investigate dynamical properties of S = 1/2 two-leg spin ladder systems. In a strong coupling region, an isolated mode appears in the lowest excited states, while in a weak coupling region, an isolated mode is reduced and the lowest excited states become a lower bound of the excitation continuum. We find in the system with equal intrachain and interchain couplings that due to a cyclic four-spin interaction, the distribution of the weights for the dynamical structure factor and characteristics of the lowest excited states are strongly influenced. The dynamical properties of two systems proposed for SrCu2O3 are also discussed.PACS numbers: 75.40. Gb, 75.40.Mg, 78.70.Nx, 75.10.Jm S = 1/2 two-leg spin ladder systems with antiferromagnetic interactions have attracted great attention both theoretically and experimentally [1]. Fascinating aspects of elementary excitation as well as thermodynamic properties have been revealed. Using several theoretical methods, it was shown that S = 0 and S = 1 two-triplet bound-states exist below the two-triplet continuum in addition to the one-triplet excitation [2,3,4,5,6,7,8]. Such an S = 0 two-triplet bound-state was identified in the optical conductivity measured for an S = 1/2 two-leg spin ladder material (Ca, La) 14 Cu 24 O 41 [9]. In this experiment, the strength of the interchain coupling (J ⊥ ) and the intrachain coupling (J ) was estimated as 9,10]. In addition to the interchain and intrachain couplings, a cyclic four-spin interaction, which acts among four S = 1/2 spins forming a plaquette, has been introduced to explain experimental findings for cuprate two-leg spin ladder systems. The analysis of the one-triplet mode observed by inelastic neutron-scattering experiments for La 6 Ca 8 Cu 24 O 41 revealed that a cyclic four-spin interaction is necessary to reproduce the observed dispersion relation [11,12]. For a S = 1/2 two-leg spin ladder material SrCu 2 O 3 , two sets of coupling constants,[14] with J cyc and J diag being the coupling constants for a cyclic four-spin interaction and a diagonal interaction, have been proposed to reproduce temperature dependence of the susceptibility. It seems thus difficult to decide the proper model only from temperature dependence of the susceptibility. Detailed information on dynamical properties and low-lying excitations is desirable to discuss characteristics of S = 1/2 two-leg spin ladder systems.In this paper, we calculate the dynamical structure factor (DSF) of S = 1/2 two-leg spin ladder systems using continued fraction method based on Lanczos algorithm. Dynamical properties of S = 1/2 pure two-leg spin ladder systems have already been studied in Refs. 15, 3, 16, 17, and 4. We perform systematic calculation for DSF and characteristics of the lowest excited states, laying stress on the effects of a cyclic four-spin interaction.Let us first consider the S = 1/2 two-leg spin ladder systems described by the following Hamiltonian,(1) where S l,i denotes the S = 1/2 spin operator in the i th rung of the l = 1, 2 chain, and N is the t...

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Magnetic excitation spectra of strongly correlated quasi-one-dimensional systems: Heisenberg versus Hubbard-like behavior

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