Nearly critical ground state ofLaCuO2.5

Using a combination of analytical techniques and quantum Monte Carlo simulations we investigate the coupled spin-ladder system ${\mathrm{LaCuO}}_{2.5}$. At a critical ratio of the interladder to intraladder coupling (${\mathrm{J}}^{|\mathrm{I}\mathrm{H}}$/J${)}_{\mathrm{c}}$≈0.11 we find a quantum phase transition between a Néel ordered and a disordered state. At criticality the uniform susceptibility behaves as χ(T)=${\mathrm{aT}}^2$ with a universal prefactor. At intermediate temperatures the system crosses over to a ``decoupled ladders regime'' with pseudogap-type behavior, similar to uncoupled ladders. This can explain the gaplike experimental data for the magnetic susceptibility of ${\mathrm{LaCuO}}_{2.5}$ despite the presence of long-range Néel order.