Monoclinic and Correlated Metal Phase inVO2as Evidence of the Mott Transition: Coherent Phonon Analysis

Abstract

In femtosecond pump-probe measurements, the appearance of coherent phonon oscillations at 4.5 and 6.0 THz indicating the rutile metal phase of VO2 does not occur simultaneously with the first-order metal-insulator transition (MIT) near 68 degrees C. The monoclinic and correlated metal (MCM) phase between the MIT and the structural phase transition (SPT) is generated by a photoassisted 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 nonequilibrium state.

Inhomogeneity of the ultrafast insulator-to-metal transition dynamics of VO2

“…Frequencies and linewidths are consistent with corresponding incoherent Raman scattering and track the different insulating phases, of M1, M2 and triclinic with their characteristic frequencies. Previous coherent phonon measurements have disagreed in whether one or two phonon modes are observed in the 200-cm À 1 range (6 THz vicinity), and in their precise frequencies 35,39 . These findings can now be reconciled given possible different insulating phases of the crystallites in the thin films.…”

Compact silicon photonic waveguide modulator based on the vanadium dioxide metal-insulator phase transition

“…The phase transition has been reported to occur on a time scale of 10 −8 s under an applied electric field or by direct heating [5,7] and down to 10 −13 s using ultrafast optical pumping [3]. Furthermore, recent evidence suggests that the change in electronic properties can be induced independent of a structural transition [4,5], explaining the surprisingly short switching times observed by optical pumping, and suggesting that similarly short transitions might be achieved using an externally gated electric field. When switched by direct thermal stimulus, the rate of switching back to the insulating phase can be quite slow and depends on the thermal properties of a particular device; however, optically and electrically switched VO 2 films have been reported to regain their insulating-phase properties over time scales on the order of 10 −8 s [3,5].…”

“…Monophase VO 2 can be transformed from a relatively transparent insulating state with monoclinic crystal structure to a metallic rutile phase upon application of thermal [2], optical [3,4], or electrical [5,6] stimuli. The phase transition has been reported to occur on a time scale of 10 −8 s under an applied electric field or by direct heating [5,7] and down to 10 −13 s using ultrafast optical pumping [3].…”

Instantaneous Band Gap Collapse in Photoexcited MonoclinicVO2due to Photocarrier Doping

“…1, top right) have revolved around the central question [2][3][4][5] of whether the crystallographic PT is the major cause for the electronic PT or if strong electron correlations are needed to explain the insulating low-T phase. While the M 1 structure is a necessary condition for the insulating state below T C , the existence of a monoclinic metal (mM) and its relevance to the thermally driven PT is under current investigation [6][7][8][9][10][11][12]. In particular, the role of carrier doping at temperatures close to T C by charge injection from the substrate or photoexcitation has been increasingly addressed [6,8,[13][14][15][16].…”