Inhomogeneity in the ultrafast insulator-to-metal transition dynamics in VO$_2$

TL;DR

This study uncovers inhomogeneous ultrafast dynamics in VO$_2$ micro-crystals during the insulator-to-metal transition, revealing significant local variations in response times that challenge previous homogeneous assumptions.

Contribution

It demonstrates the inhomogeneous nature of VO$_2$'s ultrafast IMT dynamics using combined femtosecond pump-probe microscopy and nano-IR imaging, highlighting local variations and their implications.

Findings

Photoinduced bandgap reorganization varies from 40 to 200 fs.

Variation in dynamics is uncorrelated with crystal size, orientation, or initial phase.

Results suggest an electronic mechanism dominates the ultrafast IMT.

Abstract

The insulator-to-metal transition (IMT) of the simple binary compound of vanadium dioxide VO$_2$ at $\sim 340$ K has been puzzling since its discovery more than five decades ago. A wide variety of photon and electron probes have been applied in search of a satisfactory microscopic mechanistic explanation. However, many of the conclusions drawn have implicitly assumed a {\em homogeneous} material response. Here, we reveal inherently {\em inhomogeneous} behavior in the study of the dynamics of individual VO$_2$ micro-crystals using a combination of femtosecond pump-probe microscopy with nano-IR imaging. The time scales of the photoinduced bandgap reorganization in the ultrafast IMT vary from $\simeq 40 \pm 8$ fs, i.e., shorter than a suggested phonon bottleneck, to $\sim 200\pm20$ fs, with an average value of $80 \pm 25$ fs, similar to results from previous studies on polycrystalline thin films. The variation is uncorrelated with crystal size, orientation, transition temperature, and initial insulating phase. This together with details of the nano-domain behavior during the thermally-induced IMT suggests a significant sensitivity to local variations in, e.g., doping, defects, and strain of the microcrystals. The combination of results points to an electronic mechanism dominating the photoinduced IMT in VO$_2$, but also highlights the difficulty of deducing mechanistic information where the intrinsic response in correlated matter may not yet have been reached.