A photoinduced metallic phase of monoclinic vanadium dioxide
  The complex interplay between several active degrees of freedom (charge, lattice, orbtial and spin order) is thought to determine the electronic properties of many oxides, but the respective role of the various contributions is often extremely difficult to determine. Vanadium dioxide (VO$_2$) is a particularly notorious example. Here we report on combined ultrafast electron diffraction (UED) and infrared transmissivity experiments in which we directly watch and separate the lattice and charge density reorganizations that are associated with the optically-induced semiconductor-metal transition (SMT) in VO$_2$. These studies have uncovered a previously unreported photoinduced transition to a metastable state with the periodic lattice distortion (PLD) characteristic of the insulator intact, but differing by a 1D rearrangement of charge density along the octahedrally coordinated vanadium dimer chains and a transition to metal-like mid IR optical properties. The results demonstrate that UED is capable of following details of both lattice and electronic structural dynamics on the ultrafast timescale. 
Vance R. Morrison, Robert P. Chatelain, Kunal L. Tiwari, Ali Hendaoui,   Andrew Bruhacs, Mohamed Chaker, Bradley J. Siwick
cond-mat.str-el cond-mat.mtrl-sci