Quantum Phase Transitions and Finite Size Effects in Frustrated Two-Leg Spin Ladders

We studied the ground state properties of frustrated two-leg ladders in the framework of Heisenberg model. We considered the models of these ladders with all site spins s = 1/2 and the models with different values of spins on neighbor rungs. We found a quantum phase transition mediated by the value of the interactions between the neighbor rungs. For finite-size fragments, we also found a strong dependence of the ground state spin ordering on the size of these fragments. The region of model parameters which is characterized by such strong dependence was established numerically.