Direct Detection of V-V Atom Dimerization and Rotation Dynamic Pathways upon Ultrafast Photoexcitation in VO_{2}

Abstract

Photoinduced ultrafast phase transitions can generate quasiequilibrium states with novel emergent properties modulated by the interplay of electronic and lattice degrees of freedoms. Therefore, accurately probing transient atomic structures and their dynamics is crucial to understand and control the interaction of electrons and lattice but remains a central challenge of ultrafast science. Using MeV ultrafast electron diffraction on single crystals, we quantitatively reveal the photoinduced lattice distortion of the monoclinic M1 phase of VO_{2}. Our results resolve previous controversies concerning decoupled distortion components, as well as a proposed M2 intermediate phase. Further, we emphasize the importance of quantifying the transformed phase fraction into the metallic rutile phase, beyond previously reported analyses, and we also clarify the importance of thermal heating in assisting the insulator-metal transition. Our complementary ab initio molecular dynamics calculations support the experimental findings and identify the primary A_{g} phonon mode coupling to photoexcitation. Our study provides the critical and previously missing precise 3D and time-resolved structural evolution of the crystal structure of photoexcited M1 VO_{2}. These results provide the basis upon which to rationalize this archetypal photoinduced transition.