Spin locking and freezing phenomena in the antiferromagnetic Heisenberg model on the three-leg ladder

The antiferromagnetic Heisenberg model on the three-leg ladder is studied using the generalized Jordan-Wigner transformation in dimensions higher than 1, and the bond-mean-field theory. The magnetic susceptibility and other thermodynamic quantities are analyzed as a function of the rung-to-leg coupling ratio alpha and temperature T. We fit the experimental susceptibility data of the three-leg material Sr2Cu3O5 of Azuma and co-workers with good agreement. One of the main findings of this work is the proposal that close to two-thirds of the spin degrees of freedom on each of the rungs of the ladder lock at low T for small alpha, then collectively almost 2/3 of the spin degrees of freedom on all the rungs freeze completely at low T for alpha greater than a threshold value. The approach developed here can be used to study the three-leg ladder for all values of alpha, and is thus suitable for the description of the crossover regime between the weak- and strong-coupling regimes.