DYNAMICAL PROPERTIES OF AN ANTIFERROMAGNET NEAR THE QUANTUM CRITICAL-POINT - APPLICATION TO LACUO2.5

For a system of two-chain spin ladders, the ground state for weak inte rladder coupling is the spin-liquid state of the isolated ladder, but is an ordered antiferromagnet (AF) for sufficiently large interactions . We generalize the bond-operator mean-field theory to describe both r egimes, and to focus on the transition between them. In the AF phase n ear the quantum critical point (QCP) we find both spin waves and a low -lying but massive amplitude mode which is absent in a conventional AF . The static susceptibility has the form chi(T) = chi(0) + aT(2), with chi(0) small for a system near criticality. We consider the dynamical properties to examine features arising from the presence of the ampli tude mode, and compute the dynamic structure factor. LaCuO2.5 is thoug ht to be such an unconventional AF, whose ordered phase is located ver y close to the QCP of the transition to the spin liquid. From the Neel temperature we deduce the interladder coupling, the small ordered mom ent, and the gap in the amplitude mode. The dynamical properties uniqu e to near-critical AF's are expected to be observable in LaCuO2.5.