The origin of the abrupt metal-insulator transition (MIT) in VO2 has been a subject of debate for several decades and remains an important problem for condensed matter physics. The change from high temperature metallic rutile phase to low temperature insulating monoclinic occurs abruptly at 360 K for bulk VO2. The origin of the MIT, whether structural (i.e. Peierls-like instability due to V-V dimerizing and tilting along the cR axis) or electronic (i.e. Mott-Hubbard transition due to strong electron correlation effects) or some combination of the two still remains a matter of debate. Recent advances in the growth of VO2 compounds have provided an opportunity to really examine this system. For instance, nano-scale thin films (<40 nm) VO2 can tailor the cR lattice constant and severely alter temperature of the MIT. Meanwhile, Cr doping (0 < x < 0.2) can open other stable low-temperature phases; including an electrically-insulating but room-temperature ferromagnetic regime for x>0.1. Any description of the MIT must be able to include these phenomena.
Here, I will present recent soft X-ray synchrotron spectroscopy measurements of both strained VO2 films and heavily Cr-doped VO2.[3,4] Using element- and orbital-selective techniques to probe both the occupied and unoccupied partial density of states, a clear correspondence between the electronic character (i.e. insulating or metallic) and degree of p-d hybridization in both systems is observed for the first time. I will discuss these results in terms of other studies of both the parent material and W- and Mo-doped systems. These results question the metal-metal pairing as the major source of the MIT transition and provide experimental limitations to the existing theoretical models. films.
 Y. Muraoka and Z. Hiroi, Appl. Phys. Lett. 80, 583 (2002).
 K. G. West, J. Lu, L. He, D. Kirkwood, W. Chen, T. P. Adl, M. S. Osofsky, S. B. Qadri, R. Hull, and S. A. Wolf, J. Supercond. Novel Magn. 21, 87 (2008).
 L. F. J. Piper, A. DeMasi, S. W. Cho, A. R. H. Preston, J. Laverock, K. E. Smith, K. G. West, J. W. Lu, and S. A. Wolf, Phys. Rev. B 82, 235103 (2010)
 J. Laverock, L. F. J. Piper, A. R. H. Preston, B. Chen, J. McNulty, K. E. Smith, S. Kittiwatanakul, J. Lu, S. A. Wolf, P.-A. Glans, and J.-H. Guo, to be submitted (2011)