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${}^{-1}$ and 6000 cm${}^{-1}$ 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.

Figure 1

(a) Optical image of the VO${}_2$ microcrystal studied. The double-ended arrow indicates the size of the crystal. (b) Schematic cross-sectional view of the microcrystal and substrate. The substrate is oxidized silicon. The thicknesses of the various layers are given in parentheses. (c) Diagram of the polarizations used with respect to the crystallographic axes in (i) the monoclinic ($M$${}_1$) phase and (ii) the rutile phase.

Figure 2

The experimentally derived imaginary part of the complex dielectric function for monoclinic ($M_1$) VO${}_2$ at $T$ $=$ 295 K for (a) $\stackrel{\rightharpoonup }{E}\parallel b_m$ ($A_u$) and (b) $\stackrel{\rightharpoonup }{E}\parallel a_m$ ($B_u$) in the phonon spectral region.

Figure 3

Comparison of VO${}_2$ monoclinic ($M_1$) center frequencies of $A_u$ and $B_u$ phonon modes from our experiment and Ref. 23.

Figure 4

(a) The $a_r$ and $c_r$ axes infrared conductivity (σ${}_1$) of rutile VO${}_2$ at $T$ $=$ 400 K. The plots are consistent with the $c_r$ axis dc conductivity constraint explained in Supplemental Material.[29] The known dc conductivity along $c_r$ is shown by the red circle in (a).[26] Lifting this constraint produces the error bars shown in (b) and (c). These error bars arise from systematic uncertainties explained in Supplemental Material;[29] the systematic uncertainties affect the conductivity (σ${}_1$) of both axes in a similar manner.