Monoclinic and Correlated Metal Phase in VO_2 as Evidence of the Mott   Transition: Coherent Phonon Analysis
  In femtosecond pump-probe measurements, the appearance of coherent phonon oscillations at 4.5 THz and 6.0 THz indicating the rutile metal phase of VO_2 does not occur simultaneously with the first-order metal-insulator transition (MIT) near 68^oC. The monoclinic and correlated metal(MCM) phase between the MIT and the structural phase transition (SPT) is generated by a photo-assisted hole excitation which is evidence of the Mott transition. The SPT between the MCM phase and the rutile metal phase occurs due to subsequent Joule heating. The MCM phase can be regarded as an intermediate non-equilibrium state. 
Hyun-Tak Kim, Yong-Wook Lee, Bong-Jun Kim, Byung-Gyu Chae, Sun Jin   Yun, Kwang-Yong Kang, Kang-Jeon Han, Ki-Ju Yee, and Yon...