The M2 phase of vanadium dioxide: a view from infrared and optical spectroscopy

Bulk single crystalline vanadium dioxide (VO$_{2}$) undergoes a metal-insulator transition (MIT) at 340K. This thermally-driven MIT is accompanied by a structural phase transition that results in pairing of all vanadium ions in the insulating, monoclinic M$_{1}$ phase. However, there also exists an insulating monoclinic M$_{2}$ phase, usually only accessible via external strain or chemical doping, in which only half of the vanadium chains exhibit pairing. The M$_{2}$ phase of VO$_{2}$ is vital for understanding the roles of electronic correlations and vanadium pairing to the MIT. Recent x-ray diffraction studies show that small pure VO$_{2}$ crystals can exhibit an M$_{2}$ phase below 318K, likely due to internal strain.\footnote{ B.S. Mun et al. Physica Status Solidi (RRL) - Rapid Research Letters 5, 107 (2011).} These crystals undergo phase transitions from M$_{2}$ to M$_{1}$ and from M$_{1}$ to rutile metal upon heating. We have performed reflectance micro-spectroscopy with polarized light and generalized spectroscopic micro-ellipsometry between 12 meV and 5.5 eV on these VO$_{2}$ crystals as a function of temperature, uncomplicated by external strain or chemical doping. We report infrared and optical data on the M$_{1}$, M$_{2}$ and rutile phases and compare electronic and phonon properties of M$_{1}$ and M$_{2}$ phases.