Quantum fluctuations in high field magnetization of 2D square lattice J1-J2 antiferromagnets

TL;DR

This study explores quantum fluctuations in the high-field magnetization of 2D J1-J2 antiferromagnets, revealing deviations from classical behavior near disordered phases and assessing the impact of interlayer coupling.

Contribution

It provides a comprehensive analysis of quantum effects on magnetization in the J1-J2 model using spin-wave theory and exact diagonalization, highlighting the breakdown of classical approximations near disordered regions.

Findings

Quantum fluctuations cause strong deviations from classical magnetization curves.

Spin-wave theory agrees with exact diagonalization in ordered phases.

Quantum corrections are significant near disordered regions and on the FM side.

Abstract

The J1-J2 square lattice Heisenberg model with spin S=1/2 has three phases with long-range magnetic order and two unconventionally ordered phases depending on the ratio of exchange constants. It describes a number of recently found layered vanadium oxide compounds. A simple means of investigating the ground state is the study of the magnetization curve and high-field susceptibility. We discuss these quantities by using the spin-wave theory and the exact diagonalization in the whole J1-J2 plane. We compare both results and find good overall agreement in the sectors of the phase diagram with magnetic order. Close to the disordered regions the magnetization curve shows strong deviations from the classical linear behaviour caused by large quantum fluctuations and spin-wave approximation breaks down. On the FM side (J1<0) where one approaches the quantum gapless spin nematic ground state this region is surprisingly large. We find that inclusion of second order spin-wave corrections does not lead to fundamental improvement. Quantum corrections to the tilting angle of the ordered moments are also calculated. They may have both signs, contrary to the always negative first order quantum corrections to the magnetization. Finally we investigate the effect of the interlayer coupling and find that the quasi-2D picture remains valid up to |J_\perp/J1| ~ 0.3.