Three-dimensional ordering in weakly coupled antiferromagnetic ladders and chains

Abstract

A theoretical description is presented for low-temperature magnetic-field\ninduced three-dimensional (3D) ordering transitions in strongly anisotropic\nquantum antiferromagnets, consisting of weakly coupled antiferromagnetic\nspin-1/2 chains and ladders. First, effective continuum field theories are\nderived for the one-dimensional subsystems. Then the Luttinger parameters,\nwhich determine the low-temperature susceptibilities of the chains and ladders,\nare calculated from the Bethe ansatz solution for these effective models. The\n3D ordering transition line is obtained using a random phase approximation for\nthe weak inter-chain (inter-ladder) coupling. Finally, considering a Ginzburg\ncriterion, the fluctuation corrections to this approach are shown to be small.\nThe nature of the 3D ordered phase resembles a Bose condensate of integer-spin\nmagnons. It is proposed that for systems with higher spin degrees of freedom,\ne.g. N-leg spin-1/2 ladders, multi-component condensates can occur at high\nmagnetic fields.\n