Decoupling the metal insulator transition and crystal field effects of VO₂

VO₂ 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_c can be engineered using artificial parameters. We examined the electrical and local structural properties of Cr- and Co-ion implanted VO₂ (Cr-VO₂ and Co-VO₂) 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-VO₂ and Co-VO₂ films showed sharp MIT features. The T_c values of the Cr-VO₂ and Co-VO₂ films first decreased and then increased relative to that of pristine VO₂ as the ion flux was increased. The pre-edge peak of the V K edge from the Cr-VO₂ films with a Cr ion flux ≥ 10¹³ ions/cm² showed no temperature-dependent behavior, implying no changes in the local density of states of V 3d t_2g and e_g 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 VO₂ experiences a sharp MIT when the distance of V-V pairs undergoes an SPT without any transitions in either the VO₆ octahedrons or the V 3d t_2g and e_g states. This indicates that the MIT of VO₂ occurs with no changes of the crystal fields.