Dynamical properties of an antiferromagnet near the quantum critical point: Application toLaCuO2.5

Abstract

For a system of two-chain spin ladders, the ground state for weak interladder 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 regimes, and to focus on the transition between them. In the AF phase near 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 $\ensuremath{\chi}(T)={\ensuremath{\chi}}_{0}{+aT}^{2},$ with ${\ensuremath{\chi}}_{0}$ small for a system near criticality. We consider the dynamical properties to examine features arising from the presence of the amplitude mode, and compute the dynamic structure factor. ${\mathrm{LaCuO}}_{2.5}$ is thought to be such an unconventional AF, whose ordered phase is located very close to the QCP of the transition to the spin liquid. From the N\'eel temperature we deduce the interladder coupling, the small ordered moment, and the gap in the amplitude mode. The dynamical properties unique to near-critical AF's are expected to be observable in ${\mathrm{LaCuO}}_{2.5}.$