Exploring Mottness and Electronic Order in Strongly Correlated Vanadate Thin Films

Mott insulators provide a window into some of the most intensely investigated phenomena in physics. Recent discoveries—including the emergence of high-temperature superconductivity in doped Mott insulators not from the cuprate family of superconductors—have inspired investigation into systems with strong electron-electron interactions. One of these systems is La1-xSrxVO3, in which one end-member, LaVO3, is a Mott insulator and the other, SrVO3, is a strongly correlated metal. Herein, we detail two studies regarding this system. In our oxidation study, we optimized the growth parameters for LaVO3, finding that the ultra-high vacuum technique of molecular beam epitaxy (MBE) was uniquely suited for the growth of this material based on observations made via reflection high-energy electron diffraction (RHEED) and atomic force microscopy (AFM). Through multiple growth experiments and observations, we arrived at the ideal condition that promoted the formation of a phase-pure perovskite LaVO3, which we summarize by comparing the result of a film grown under an oxygen partial pressure of 5*10-8 torr with that of one grown with 5*10-7 torr. X-ray diffraction (XRD) data confirmed the presence of the desired crystal structure in the former sample and an undesired second phase with a planar separation of 4.34 Å, which corresponds to V2O5, in the latter. Electronic transport measurements were performed from room temperature to 4 K that confirm an insulating ground state, which is far more pronounced in the former sample. In our Sr-doping study, we grew multiple samples from the La1-xSrxVO3 system from x=0 to x=1 both as random alloys and ordered superlattices, heterostructures with alternating layers of LaVO3 and the other end-member SrVO3. The latter was found to prefer less exposure to oxygen than LaVO3 and required turning off the oxygen beam immediately following growth and cooling down the SrVO3 sample in vacuum, with residual oxygen partial pressure in the 10-9 torr range. This also prompted us to investigate how the electronic/optical properties of LaVO3 depended on the oxygen partial pressure it was exposed to while cooling down after growth, giving us qualitative information on oxygen-vacancy doping of Mott insulating LaVO3. Electronic transport and optical measurements also allowed us to explore the effects of Sr doping on the system and how ordering changes the behavior of doped films. Optical spectroscopy measurements indicated a clear feature that corresponds to a Mott gap of ~1.8 eV in the ideal-oxygen LaVO3 film while the vacancy-doped LaVO3 film as well as the x=0.1 and x=0.2 Sr-doped random alloy films caused this gap-like feature to become weaker and less defined, consistent with what would be expected for a doped Mott insulator