Emergence of metallic monoclinic states of VO${}_2$ films induced by K deposition

In order to study the origin of metallization of VO${}_2$ induced by electron injection, we deposit K atoms onto the surface of VO${}_2$ films grown on TiO${}_2$ (001) substrates, and we investigate the change in the electronic and crystal structures using $insitu$ photoemission spectroscopy and x-ray absorption spectroscopy (XAS). The deposition of K atoms onto a surface of insulating monoclinic VO${}_2$ leads to a phase transition from insulator to metal. In this metallization state, the V-V dimerization characteristic to the monoclinic phase of VO${}_2$ still exists, as revealed by the polarization dependence of the XAS spectra. Furthermore, the monoclinic metal undergoes a transition to a monoclinic insulator with decrease in temperature, and to a rutile metal with increase in temperature. These results indicate the existence of a metallic monoclinic phase around the boundary between the insulating monoclinic and metallic rutile phases in the case of electron-doped VO${}_2$.