Effect of an interchain magnetic interaction on the dimerized state of a two-XY-quantum-spin-chain ladder

The model that we study is a two-XY-quantum-spin-chain system with Ising interchain magnetic interaction. This model, referred to as the spin-ladder model, is used as a first step to model the thermodynamic properties of low-dimensional spin system. Similarities between the spin-ladder model and the one-dimensional Hubbard model are used in our analysis. As a consequence we can qualitatively understand the heat capacity and spin susceptibilty and show that both measurements are necessary to analyze a spin-ladder system. We also show that the strong-interchain-coupling limit of the spin-ladder model is equivalent to a one-chain spin Heisenberg Hamiltonian. Furthermore, the spin-Peierls transition is investigated for small (I/2J1) and large (I/2J1) interchain coupling, and we show its occurrence at relatively small spin-phonon coupling (0.6). From the results obtained for the small- and large-interchain-interaction limits, we propose the conjecture that the in-phase dimerization persists for all values of the interaction parameter at zero temperature. © 1991 The American Physical Society.