Transport Phenomena in Perovskite Vanadate Thin Films

The metal-insulator transition (MIT) in transition metal oxides (TMOs) has been a topic of long-standing interest in condensed matter physics. Recent advances in thin film technology have greatly motivated the scientific community to tailor the MIT for nano-electronic device applications. This dissertation is focused on exploring the transport phenomena in the perovskite vanadate thin films. CaVO3 (CVO) and SrVO3 (SVO) are chosen because they are typical strongly correlated oxides on the metallic side of a MIT, but their intrinsic physical properties have not yet been studied comprehensively due to the great challenges presented by the synthesis of these materials. In this dissertation, high quality epitaxial vanadate thin films synthesized by a novel pulsed electron-beam deposition (PED) technique enable a careful study on the control of the MIT. Methods to systematically control the MIT in these materials have been demonstrated, including dimensional confinement, chemical doping, and superlattice structures. Techniques such as soft x-ray spectroscopy and magneto-transport measurements are employed to elucidate the underlying mechanisms for the induced MITs.

The transport properties of the vanadate films were studied as a function of the film thickness. A temperature-driven MIT was observed in the SVO films with thicknesses below 6.5 nm, and CVO films below 4 nm, whereas thicker films were metallic with the resistivity following the T2 law corresponding to a Fermi liquid system. This work represents the first transport study to show the MIT in the vanadates induced by the dimensional crossover from a three-dimensional metal to a two-dimensional Mott insulator, as the resulting reduction in the effective bandwidth opens a bandgap at the Fermi level. The magneto-transport measurements also confirmed the MIT is due to the electron-electron interactions other than disorder-induced localization.

The B-site doping in the SVO films with Ti4+ ions was investigated for the first time to explore the chemical doping effects. The transport study in the full composition range from SVO to SrTiO3 (STO) revealed a temperature-driven MIT in the x = 0.67 film at 95 K. The films with higher vanadium concentration were metallic, whereas the ones with lower vanadium concentration were semiconducting following Mott’s variable range hopping mechanism. The mechanisms behind the observed MIT are complicated due to competing effects among electron correlation, disorder, and percolation. Percolation likely plays a major role in the system, for which the transport properties are determined by the topology of the coexisting metallic and insulating regions.

The transport properties of SrVO3/SrTiO3 (SVO/STO) superlattices were comprehensively investigated by varying the number of repetitions of the SVO/STO bilayers and the thickness of individual SVO and STO layers. The SVO layer embedded in the superlattices showed a great enhancement in the conductivity, which is a further indication of electronic phase separation in the vanadate ultrathin layers. The transport behaviors in the superlattices can also be described as percolation phenomena. The coupling between SVO layers creates more conduction pathways with increasing number of repetitions, resulting in a crossover from insulating to metallic behavior. Also, an increase in the SVO layer thickness along with a reduction in the STO layer thickness may cause an increase in the interlayer electron hopping, leading to the crossover from insulating to a metallic behavior.