Quantum phase transition in coupled spin ladders

The ground state of an array of coupled, spin-$\frac{1}{2},$ 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 Néel ordered and a nonmagnetic phase at a finite value of the interladder coupling $({\alpha }_c\simeq 0.3).$ This transition is marked by remarkable changes in the structure of the excitation spectrum.