Anisotropic infrared response of vanadium dioxide microcrystals

Vanadium dioxide (VO${}_{2}$) undergoes a phase transition at a temperature of 340 K between an insulating monoclinic ${M}_{1}$ phase and a conducting rutile phase. Accurate measurements of possible anisotropy of the electronic properties and phonon features of VO${}_{2}$ in the insulating monoclinic ${M}_{1}$ and metallic rutile phases are a prerequisite for understanding the phase transition in this correlated system. Recently, it has become possible to grow single domain untwinned VO${}_{2}$ microcrystals, which makes it possible to investigate the true anisotropy of VO${}_{2}$. We performed polarized transmission infrared micro-spectroscopy on these untwinned microcrystals in the spectral range between 200 cm${}^{\ensuremath{-}1}$ and 6000 cm${}^{\ensuremath{-}1}$ and have obtained the anisotropic phonon parameters and low frequency electronic properties in the insulating monoclinic ${M}_{1}$ and metallic rutile phases. We have also performed ab initio GGA+$U$ total energy calculations of phonon frequencies for both phases. We find our measurements and calculations to be in good agreement.