Dynamical Properties of an Antiferromagnet near the Quantum Critical Point: Application to LaCuO_2.5

TL;DR

This paper studies the dynamical properties of an antiferromagnet near a quantum critical point, using a generalized bond-operator mean-field theory to explore the transition from a spin-liquid to an ordered state, with applications to LaCuO2.5.

Contribution

It introduces a generalized mean-field approach to describe both phases and the transition, highlighting the presence of a massive amplitude mode near the quantum critical point.

Findings

Identification of a low-lying amplitude mode in the AF phase near QCP

Static susceptibility follows a $ ext{chi}(T) = ext{chi}_0 + a T^2$ form near criticality

Application to LaCuO2.5 suggests observable signatures of near-critical behavior

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 $\chi(T) = \chi_0 + a T^2$, with $\chi_0$ small for a system near criticality. We consider the dynamical properties to examine novel features due to the presence of the amplitude mode, and compute the dynamic structure factor. 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 AFs are expected to be observable in LaCuO$_{2.5}$.