Tests of magnetic Hamiltonians for CaV_4O_9

TL;DR

This paper evaluates different magnetic Hamiltonian models for CaV_4O_9, comparing their predictions with experimental data, and suggests specific experiments to distinguish between models based on their excitation spectra.

Contribution

It critically assesses existing models for CaV_4O_9's magnetic properties and proposes a refined model consistent with experimental observations.

Findings

The weakly coupled dimer model is inconsistent with susceptibility data.

Pickett's model can explain the observed spin gap and susceptibility with proper parameters.

Neutron or Raman scattering can distinguish between models based on excitation features.

Abstract

We study the uniform magnetic susceptibility and spin-gap for recently proposed Heisenberg model Hamiltonians for CaV_4O_9 based on the orbital ordering scenario of Marini and Khomskii and the LDA calculations of Pickett. We argue that the experimentally observed uniform susceptibility data is inconsistent with the weakly coupled dimer picture of Marini and Khomskii. The model proposed by Pickett can, with appropriate choice of parameters, lead to an explanation for the observed gap and uniform susceptibility. The resulting agreement with experiments is of a similar quality to previously studied models. We argue that this new model is best distinguished from previous ones by neutron or Raman scattering experiments, via the location of the excitation minimum in the Brillouin zone and by the possible existence or non-existence of sharply defined singlet excitations.