Theoretical Exploration of the Metal Insulator Transition in Vanadium Dioxide with an Eye Towards Applications: A First Principles Approach

摘要

Vanadium oxides are very interesting compounds which exhibit exotic transport phenomena. In particular vanadium dioxide (VO2) undergoes a first- order transition from a high-temperature metallic phase to a low-temperature insulating phase at almost the room temperature (T = 340 K). The resistivity jumps by several orders of magnitude through this transition, and the crystal structure changes from rutile (R-phase) at high-temperature to monoclinic (so- called M1-phase) at low-temperature. The latter is characterized by a dimerization of the vanadium atoms into pairs, as well as a tilting of these pairs with respect to the c-axis. VO2 has also attracted a great deal of attention for its ultrafast optical response, switching between the R and the M1 phase. Despite the large number of experimental studies focusing on this material the physics driving this phase transition and the resulting optical properties is still mysterious. There are intensive reports around the world to make devices such as switches, transistors, detectors, varistors, phase change memory, exploiting the unique properties of VO2. Two physical effects, Peierls, i.e. dimerization, and the Mott mechanism due to strong Coulomb repulsion are important in the metal-insulator transition (MIT) of VO2. Understanding the detailed interplay and the relative importance of both Peierls and Mott mechanism is important for controlling this material with an eye towards applications. For example, whether the driving force of this transition is electronic (i.e. occurring on femtosecond timescales) or structural (occurring on the picosecond timescale) is important to understand the speed of the switching from the M1 to the rutile phase. The insights obtained in this study together with the computational machinery developed, will serve as a basis for rational material design of VO2 based applications.