Thermodynamics of the ultrafast phase transition of vanadium dioxide

TL;DR

This paper develops a thermodynamic framework using ultrafast pump-probe measurements to identify the driving mechanism of the phase transition in vanadium dioxide, revealing the importance of thermal phonon populations.

Contribution

It introduces a simple thermodynamic approach to determine the nature of photoinduced phase transitions without complex experiments.

Findings

Population of the full thermal phonon spectrum stabilizes the metallic phase.

High-frequency oxygen modes are crucial in the transition.

The approach can be applied to other photoinduced phase transitions.

Abstract

Ultrafast photoexcitation is an emerging route to selective control of phase transitions. However, it is difficult to determine which modes govern the transformation and how effectively they are targeted by photoexcitation. This is exemplified in vanadium dioxide, which transitions from a monoclinic insulator to a rutile metal upon heating or photoexcitation. There is a long-standing debate about whether this transition is electronically or structurally driven and whether the structural component is coherent, driven by a single structural mode or thermal in nature. In this work, we develop a simple thermodynamic framework based on temperature-dependent ultrafast pump-probe measurements and contrast it to microscopic-detail-free modelling to identify the driving mechanism of the transition, revealing that population of the full thermal phonon spectrum, especially high-frequency oxygen modes, is necessary to stabilize the metallic phase. Our approach can straightforwardly be applied to determine the nature of other photoinduced phase transitions without the need for complex multi-messenger experiments and can guide new control strategies, even for incoherent transitions.