Instantaneous band gap collapse in photoexcited monoclinic VO$_2$ due to photocarrier doping

Abstract: Using femtosecond time-resolved photoelectron spectroscopy we demonstrate that photoexcitation transforms monoclinic VO$_2$ quasi-instantaneously into a metal. Thereby, we exclude an 80 femtosecond structural bottleneck for the photoinduced electronic phase transition of VO$_2$. First-principles many-body perturbation theory calculations reveal a high sensitivity of the VO$_2$ bandgap to variations of the dynamically screened Coulomb interaction, supporting a fully electronically driven isostructral insulator-to-metal transition. We thus conclude that the ultrafast band structure renormalization is caused by photoexcitation of carriers from localized V 3d valence states, strongly changing the screening \emph{before} significant hot-carrier relaxation or ionic motion has occurred.

Instantaneous Bandgap Collapse in Photoexcited Monoclinic VO$_2$ Due to Photocarrier Doping

The paper "Instantaneous bandgap collapse in photoexcited monoclinic VO$_2$ due to photocarrier doping," explores the rapid electronic phase transition in vanadium dioxide (VO$_2$), a material known for its metal-insulator transition properties. This study deploys femtosecond time-resolved photoelectron spectroscopy (TR-PES), coupled with first-principles many-body perturbation theory, to ascertain the dynamics and origins of the photoinduced insulator-to-metal transition (IMT).

The core finding presented in the study is the observation of a quasi-instantaneous collapse of the bandgap in VO$_2$ upon photoexcitation, dispelling the notion of a structural bottleneck previously posited to be around 80 femtoseconds. This research accentuates the electronic origin of this phase transition, detached from any immediate lattice transformation, standing in contrast to historical discussions which suggested the necessity of structural changes coinciding with electronic transitions.

In the methodological approach, TR-PES was utilized to capture real-time changes in the density of states (DOS) near the Fermi energy. The results demonstrated that the bandgap closure occurs within the duration of the excitation laser pulse, estimated to be under 60 femtoseconds, which indicates a purely electronic-driven transition. Notably, photoexcitation was shown to significantly alter the screening of the V $3d$ valence electrons, instigating this phase shift before any substantial ionic movement could ensue.

Theoretical support for these observations comes from many-body perturbation theory calculations, highlighting the sensitivity of the VO$_2$ bandgap to changes in the dynamically screened Coulomb interaction. The introduction of photocarriers alters the occupation of localized $3d$ states in a manner that modulates this interaction, leading to an ultrafast insulator-to-metal transition. Further analysis indicated that photoinduced holes were especially effective in driving this transition by disrupting the bandgap without the intervention of structural changes, proposing a "hole-driven insulator-to-metal transition."

This breakthrough has broad implications for the understanding and manipulation of phase transitions in correlated materials like VO$_2$. The findings provide a clearer insight into the intrinsic electronic factors governing phase transitions, enabling potential advancements in the design of optoelectronic devices and applications where rapid switching is paramount. Furthermore, the elucidation of the intrinsic transition pathway paves the way for future research focused on the coupling between electronic and lattice dynamics in complex materials.

The study concludes by underscoring the absence of a structural bottleneck in the photoinduced insulator-metal transition in VO$_2$, with the electronic transition preceding lattice adjustments. This finding corroborates recent experimental observations of an excited mM-like monoclinic phase and posits new questions regarding the interplay of thermal and athermal pathways in correlated electron systems. The approach and insights from this research establish a paradigm for probing ultrafast phase dynamics, opening vistas for theoretical and experimental advancements in the field of condensed matter physics.