Photovoltaic And Spin-Orbit Coupling Effects Of SrTiO₃-based Oxide Interfaces

In the last two decades, ABO3 type perovskite oxides have attract extensive studies, due to their complex but intriguing phase diagram under the variety of temperature, magnetic field, dopant and so on. With the developments of thin-film growth technology, especially pulsed laser deposition (PLD) and the equipped reflection high energy electron diffraction (RHEED), researchers can epitaxially construct oxide interface with high quality, within which the emergent properties were found. In today’s material science, much effort is devoted to exploring complex oxide interface. My doctoral work aimed at the band structure of SrTiO3 based polar/nonpolar interface, exploring possible application, and structure of Ti 3d’s muti-subbands. I will introduce my work with the following three sections.With the proper procedure of treatments on the surface of SrTiO3 (001), we have achieved the single TiO2-terminated surface with unit-cell steps. LaAlO3 and LaVO3 films can be deposited on such surface layer by layer under optimum conduction in our PLD-RHEED system, giving us ability to control the growth velocity and thickness. The in-situ RHEED diffraction oscillation, and the XRD, TEM, CTR measurements after growth indicate the high-quality crystalline structure of thin films. These oxide interfaces offer us the opportunity to study interfacial properties and physics.In the second part, I show the direct observation of residual polar potentials in the LaAlO3 overlayers with a thickness down to a few uc by exploiting the photovoltaic (PV) effect as a more effective and reliable probe, with the interfacial q2-DEG serving as a natural electrode. The use of the PV effect is essential in enabling a more definitive characterization of the intrinsic polar potential. The lower bound of the measured polar potential across the uc-scale LaAlO3 overlayers is 1.0 V. This finding not only provides further insight into the underlying physical origin of the interfacial q2-DEG, but also points to new opportunities in exploring PV materials with atomic layer control for a wide range of applications.In the third part, I demonstrate a definitive verification of an unconventional spin-orbit coupling effect related to the delicate Ti 3d subbands via weak antilocalization (WAL) and Hall characterizations, using the LaAlO3/SrTiO3 and LaVO3/SrTiO3, as two prototype systems. In response to the sweeping back gate voltage Vg, the strength of the dominant k-cubic spin-orbit coupling mainly contributed from the dxz/yz states first increases and then decreases to form a dome feature, accompanied by a apparent single-to two-carrier transition. These commonality observations suggest that the spin-orbit coupling behavior can be mainly attributed to the Ti 3d subband ordering irrespective of the detailed boundary conditions defined by the overlayer materials, and the spin splitting maximum emerges at the dxy-dxz/yz crossing region, where the spin-orbit coupling is enhanced due to the orbit mixing.