Ultrafast far-infrared studies of Vanadates - multiple routes for an insulator to metal transition
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The metal insulator transition in vanadates has been studied for decades and yet new discoveries still spring up revealing new physics, especially among two of the most studied members: Vanadium sesquioxide (V203) and Vanadium dioxide (V02). Although subtleties abound, both of the materials have first order insulator to metal phase transitions that are considered to be related to strong electron-electron (e-e) correlation. Further, ultrafast spectroscopy of strongly correlated materials has generated great interest in the field given the potential to dynamically distinguish the difference between electronic (spin) response versus lattice responses due to the associated characteristic energy and time scales. In this thesis, I mainly focus on utilizing ultrafast optical and THz spectroscopy to study phase transition dynamics in high quality V203 and V02 thin films epitaxially grown on different substrates. The main findings of the thesis are: (1) Despite the fact that the insulator to metal transition (IMT) in V203 is electron-correlation driven, lattice distortion plays an important role. Coherent oscillations in the far-infrared conductivity are observed resulting from coherent acoustic phonon modulation of the bandwidth W. The same order of lattice distortion induces less of an effect on the electron transport in V02 in comparison to V203. This is directly related to the difference in latent heat of the phase transitions in V02 and V203. (2) It is possible for the IMT to occur with very little structural change in epitaxial strained V02 films, like in the case of Cr doped or strained V203. However, in V02, this necessitates a large strain which is only possible by clamping to a substrate with larger c axis parameter through epitaxial growth. This is demonstrated for V02 films on Ti02 substrates. (3) Initiating an ultrafast photo-induced insulator-to-metal transition (IMT) is not only possible with above bandgap excitation, but also possible with high-field far-infrared excitation. With the help of the field enhancement in metamaterial split ring resonator gaps, we obtain picosecond THz electric field transients of several MVIem which is sufficient to drive the insulator to metal transition in V02.
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