Decoupling the metal insulator transition and crystal field effects of VO.

VO is a highly correlated electron system which has a metal-to-insulator transition (MIT) with a dramatic change of conductivity accompanied by a first-order structural phase transition (SPT) near room temperature. The origin of the MIT is still controversial and there is ongoing debate over whether an SPT induces the MIT and whether the T can be engineered using artificial parameters. We examined the electrical and local structural properties of Cr- and Co-ion implanted VO (Cr-VO and Co-VO) films using temperature-dependent resistance and X-ray absorption fine structure (XAFS) measurements at the V K edge. The temperature-dependent electrical resistance measurements of both Cr-VO and Co-VO films showed sharp MIT features. The T values of the Cr-VO and Co-VO films first decreased and then increased relative to that of pristine VO as the ion flux was increased. The pre-edge peak of the V K edge from the Cr-VO films with a Cr ion flux ≥ 10 ions/cm showed no temperature-dependent behavior, implying no changes in the local density of states of V 3d t and e orbitals during MIT. Extended XAFS (EXAFS) revealed that implanted Cr and Co ions and their tracks caused a substantial amount of structural disorder and distortion at both vanadium and oxygen sites. The resistance and XAFS measurements revealed that VO experiences a sharp MIT when the distance of V-V pairs undergoes an SPT without any transitions in either the VO octahedrons or the V 3d t and e states. This indicates that the MIT of VO occurs with no changes of the crystal fields.