Two-dimensional S=1/2 antiferromagnet on a plaquette lattice

TL;DR

This paper investigates a simplified 2D spin-1/2 antiferromagnetic model on a plaquette lattice, analyzing phase transitions between ordered and singlet states using numerical and analytical methods, with results aligning with Quantum Monte Carlo data.

Contribution

It introduces a simplified model for the magnetic structure of CaV4O9 and analyzes phase transitions using cluster expansion and spin wave theory, providing a phase diagram consistent with Quantum Monte Carlo results.

Findings

Phase diagram matches Quantum Monte Carlo results.

First order transition between ordered and singlet states.

Competition between two singlet ground states and Neel order.

Abstract

We consider a simplified model of the magnetic structure of the two-dimensional compound $CaV_4O_9$ in terms of interacting square plaquettes of spins with two distinct antiferromagnetic exchange constants. We analyze the competition between two types of singlet ground states and the Neel ordered one in terms of respectively, numerical cluster expansion and nonlinear spin wave theory. The resulting phase diagram agrees well with known Quantum Monte Carlo results and suggests a first order transition between ordered and singlet ground states as a function of the exchange constants.