Study of the 3-leg antiferromagnetic ladder using the bond-mean-field theory.

摘要

In this thesis, we study the spin-1/2 antiferromagnetic 3-leg Heisenberg ladder using the generalized Jordan-Wigner (JW) transformation and the bond-mean-field approach of Azzouz. By expressing the spin operators of the Heisenberg Hamiltonian using the JW fermions, the resulting interacting fermion system is decoupled within the bond-mean-field theory, and the energy spectrum is obtained. It is found by analyzing the mean-field parameter, that in the large coupling limit, the 3-leg ladder behaves as a single chain with an effective coupling Jeff. Using the Green's function formalism, we derive the spin correlation functions, and the uniform and static susceptibility chi(T).;Analysis of chi(T) as function of temperature T for various rung-to-leg ladder exchange coupling ratio ( J⊥J ) indicates a strong dependence on J⊥J , and a finite susceptibility as T → 0. Comparison of our results with the experimental data of the 3-leg material Sr2Cu3O 5 reported by Azuma et al., shows good agreement. Finally using our free energy expression, we investigate the temperature dependence of the entropy, specific heat, and the normalized ratio of the specific heat to the spin susceptibility (the Wilson-Sommerfeld ratio). Our results provide an insight on the effect of the exchange coupling J⊥J on the thermodynamics of the 3-leg ladder.