Phase-Field Study of Metal-Insulator Transition in Strongly Correlated Vanadium Dioxide
Open Access
- Author:
- Shi, Yin
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- May 07, 2020
- Committee Members:
- Long-Qing Chen, Dissertation Advisor/Co-Advisor
Long-Qing Chen, Committee Chair/Co-Chair
Venkatraman Gopalan, Committee Member
Ismaila Dabo, Committee Member
Raymond Edward Schaak, Outside Member
John Mauro, Program Head/Chair - Keywords:
- Phase-field method
Metal-insulator transition
Strongly correlated material - Abstract:
- Vanadium dioxide (VO2) is a strongly correlated system which exhibits an intriguing metal-insulator transition (MIT) accompanied by a structural transition at a temperature slightly above the room temperature. It offers potential novel device applications such as sensors, Mott field-effect transistors, and memristors, which desire guidance from mesoscopic theoretical modeling. Based on symmetry consideration, we formulate a mesoscopic phase-field model of the MIT explicitly incorporating both structural and electronic instabilities as well as free electrons and holes. We employ this model to investigate the MIT in mesoscale VO2 subject to various stimuli such as heat, stress/strain, electric field, doping, electric current, and light. First, the temperature-stress/strain phase diagrams of VO2 nanobeams and thin films under different mechanical boundary conditions are calculated consistently, which show good agreement with existing experimental observations. We also calculate the temperature-radius phase diagrams of VO2 nanoparticles and nanofibers. Second, in a VO2 slab under an electric field in an open-circuit configuration, an abrupt universal resistive transition is shown to occur inside the supercooling region, in sharp contrast to the conventional Landau-Zener smooth electric breakdown. Third, the temperature-dopant-concentration phase diagrams of VO2 doped with various metal ions are calculated consistent with the experiments. Furthermore, hole doping in VO2 may induce a metastable metallic monoclinic phase, which could be stabilized through geometrical confinement and the size effect in VO2-VO_{2-delta} bilayers leading to the decoupling of the electronic and structural phase transitions. Fourth, we demonstrate that the electric current may drive the MIT isothermally via the current-induced electron correlation weakening, inducing a few-nanosecond ultrafast resistive switching consistent with experimental measurements. The isothermal temperature-current phase diagram is further calculated and the current is also found able to drive domain walls to move. Fifth, dynamic processes of the MIT in VO2 illuminated by femtosecond laser pulses are simulated, showing the emergence of the transient metallic monoclinic phase and the bias-induced shrinkage of the photoinduced metallic phase. We also prove that during a generic metal-insulator transition, a nonequilibrium homogeneous state may be unstable against charge density modulations with certain wavelengths, and thus evolves to the equilibrium phase through transient electronic phase separation. This transient electronic phase separation is shown to take place in VO2 upon photoexcitation.