A MICROSCOPIC APPROACH TO THE DIMERIZATION IN FRUSTRATED SPIN-1/2 ANTIFERROMAGNETIC CHAINS

The spontaneous dimerization of frustrated spin-1/2 antiferromagnetic chains is studied via a microscopic approach based on a proper set of composite operators (i.e., pseudo-spin operators). Two approximation schemes are developed. Firstly, a spin-wave approximation is made via a Dyson-Maleev-like boson transformation. The ground-state properties and the triplet excitation spectra are obtained as functions of the coupling parameter. Secondly, a microscopic treatment, based on pseudo-spin operators, is formulated within the framework of the powerful, systematic coupled-cluster method (ccm). Comparison between various approximations is made. The advantage of the CCM for the purposes of systematic improvement is emphasized.