A Microscopic Approach to the Dimerization in the Frustrated Spin- 1 2 Antiferromagnetic Chains

The spontaneous dimerization of the frustrated spin-1 2 antiferromagnetic chains is studied by 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 by a Dyson-Maléev-like boson transformation. The ground-state properties and the triplet excitation spectra are obtained as functions of the coupling parameter. Secondly, based on the pseudo-spin operators, a microscopic treatment 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.