VO2 Does Not Host a Photoinduced Long-lived Monoclinic Metallic Phase

Abstract

Femtosecond laser pulses can trigger phase transitions and sometimes even achieve transient nonequilibrium states that are not accessible in equilibrium. Vanadium dioxide (VO2) exhibits a first-order insulator-to-metal transition accompanied by a monoclinic-to-rutile structural transition near room temperature. Recently there is extensive debate over whether VO2 hosts a transient long-lived monoclinic metallic phase with femtosecond photoexcitation. We developed a new method to synthesize quasi-single-crystal freestanding VO2 films and reexamined the photoinduced phase transition with mega-electron-volt ultrafast electron diffraction. Benefitting from the well separated diffraction spots and high temporal resolution, we found no evidence for the transient monoclinic metallic phase through the accurate measurements of the photoinduced evolution of the diffraction intensity in the momentum space. Based on the laser fluence thresholds measured at various temperatures, we suggest that the phase transition threshold is thermally driven although the transition occurs within 200 femtoseconds, highlighting the interplay of the electron-electron correlation and the lattice vibrations in driving the phase transition in VO2.

Decoupled ultrafast electronic and structural phase transitions in photoexcited monoclinic VO 2

“…The similar thresholds of electronic excitation for both SPT and IMT revealed here are consistent with the experimental measurements carefully characterizing the heating effects where a single threshold is observed for both SPT and IMT (26). A recent experimental work also demonstrated only one threshold for both the superstructure and nonsuperstructure peaks via well-separated diffraction spots of quasi-single-crystal VO 2 (40). Thus, both ultrafast SPT and IMT of VO 2 can be triggered with the same laser pulses, indicating the same fluence thresholds in experiments.…”

Recent Development of Ultrafast Optical Characterizations for Quantum Materials

“…Furthermore, with newly synthesized quasi-single-crystal freestanding VO 2 films and high resolution mega-electron-volt ultrafast electron diffraction, Xu et al measured the photoinduced evolution of the diffraction intensity in the momentum space and their results indicated no evidence for the transient monoclinic metallic phase. [154] These studies suggested against the existence of a decoupled structural and electronic phase transition. On the other hand, Johnson et al measured the spacial evolution of VO 2 thin films with nano-scale resolution after excitation by using time-and spectrally-resolved coherent X-ray imaging.…”