Observation of First-Order Metal-Insulator Transition without Structural   Phase Transition in VO_2
  An abrupt first-order metal-insulator transition (MIT) without structural phase transition is first observed by current-voltage measurements and micro-Raman scattering experiments, when a DC electric field is applied to a Mott insulator VO_2 based two-terminal device. An abrupt current jump is measured at a critical electric field. The Raman-shift frequency and the bandwidth of the most predominant Raman-active A_g mode, excited by the electric field, do not change through the abrupt MIT, while, they, excited by temperature, pronouncedly soften and damp (structural MIT), respectively. This structural MIT is found to occur secondarily. 
Yong-Sik Lim, Hyun-Tak Kim, B. G. Chae, D. H. Youn, K. O. Kim, K. Y.   Kang, S. J. Lee, K. Kim