First-order metal-insulator transition and structural phase transition: analysis of coherent phonons observed by femtosecond pulse laser in VO$_{2}$

It has been well-known that VO$_{2}$ undergoes both a structural phase transition (SPT) (electron-phonon interaction) from monoclinic (insulator phase) to tetragonal (metal phase) and of a discontinuous first-order metal-insulator transition (MIT) (Jump) (electron-electron interaction) near 68$^{o}$C. Peierls transiton and Mott transition in VO$_{2}$ remain controversial. We have investigated a relation of the MIT and the SPT in VO$_{2}$ by observing coherent phonons using a laser with a femtosecond pulse width (10$\sim $20 ft). A coherent phonon indicating a metal phase is measured after MIT. This indicates that the SPT does not affect the MIT. This is confirmed by a micro-Raman scattering experiment and XRD. The speed of the first-order MIT is interpreted as about 100 femtosecond. This is different from a well-known analysis in which the SPT and the MIT simultaneously occur. (References on the MIT: New J. Phys. 6 (1994) 52 (http://www.njp.org), Appl. Phys. Lett. 86 (2005) 242101, Physica B 369 (2005) 76; cond-mat/0607577; cond-mat/0608085; cond-mat/0609033)