Quantum Phase Transition in Coupled Spin Ladders

The ground state of an array of coupled, spin-half, antiferromagnetic ladders is studied using spin-wave theory, exact diagonalization (up to 36 sites) and quantum Monte Carlo techniques (up to 256 sites). Our results clearly indicate the occurrence of a zero-temperature phase transition between a N\'eel ordered and a non-magnetic phase at a finite value of the inter-ladder coupling ($\alpha_c\simeq0.3$). This transition is marked by remarkable changes in the structure of the excitation spectrum.