Quantum Phase Transitions in Anisotropic Spin Ladders with Antiferromagnetic Rungs

Abstract

We apply perturbation theory to study quantum phase transitions in anisotropic two-leg spin ladders with antiferromagnetic rungs in the strong interchain coupling limit. The energy gap is expanded up to the fourth order in the ratio of the intrachain to interchain couplings. The transition point is determined by examing the disappearance of the energy gap. Present results for the anisotropic ferromagnetic-leg spin ladder are consistent qualitatively with the nonlinear -model analysis and in good agreement with the numerical results by the diagonalization method.