Nearly critical ground state ofLaCuO2.5

Using a combination of analytical techniques and quantum Monte Carlo simulations we investigate the coupled spin-ladder system LaCuO{sub 2.5}. At a critical ratio of the interladder to intraladder coupling (J{sup {prime}}/J){sub 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{sup 2} with a universal prefactor. At intermediate temperatures the system crosses over to a {open_quotes}decoupled ladders regime{close_quotes} with pseudogap-type behavior, similar to uncoupled ladders. This can explain the gaplike experimental data for the magnetic susceptibility of LaCuO{sub 2.5} despite the presence of long-range Neel order. {copyright} {ital 1997} {ital The American Physical Society}