Nearly critical ground state of LaCuO2.5

Using a combination of analytical techniques and Quantum Monte Carlo simulations we investigate the coupled spin ladder system LaCuO2.5. At a critical ratio of the interladder to intraladder coupling (J'/J)_c \approx 0.11 we find a quantum phase transition between a Neel ordered and a disordered state. At criticality the uniform susceptibility behaves as \chi(T)=aT^2 with a universal prefactor. At intermediate temperatures the system crosses over to a ``decoupled ladders regime'' with pseudo-gap type behavior, similar to uncoupled ladders. This can explain the gap-like experimental data for the magnetic susceptibility of LaCuO2.5 despite the presence of the long range Neel order.