Spin-orbit coupling effects in LaTiO3/SrTiO3 heterostructures : from giant rashba-type coupling to near room-temperature quantum spin hall effect

The manipulation of the spin degrees of freedom in a solid has been of fundamental and technological interest recently for developing high-speed, low-power computation devices. There has been much work focused on developing materials which can manipulate spins when incorporated into so-called spintronic devices. In particular, complex oxides have shown to be a promising class of materials for study because of the ease of interfacing materials with vastly different properties. I have been studying the interface between the Mott insulator LaTiO3 and the band insulator SrTiO3 for such applications. To date, this system has been exceptionally understudied compared to other oxide-based systems which show emergent metallicity. In this dissertation, I focus on the measurement of spin-orbit coupling effects in this system. For ultrathin (3-4 unit cells) LaTiO3 thin films on SrTiO3, I observe a variety of effects which make it the ideal system for future study and implementation in spintronics devices. Specifically, I observe a giant Rashba-type spin splitting and the first-ever observation of the quantum spin Hall effect (QSHE) in an oxide system. There is a promising strategy to achieve spin splitting using spatial symmetry breaking without the use of a magnetic field, known as Rashba-type splitting, which can be used to electrically manipulate spins. Here I report evidence for a giant Rashba-type splitting at the interface of LaTiO3 and SrTiO3. Analysis of the magnetotransport reveals anisotropic magnetoresistance, weak anti-localization and quantum oscillation behavior consistent with a large Rashba-type splitting. In addition, the angular dependence of the quantum oscillations indicates that the Fermi surface is three-dimensional. It is surprising to find a large Rashba-type splitting in 3d transition metal oxide-based systems such as the LaTiO3/SrTiO3 interface but is promising for the manipulation of spin in oxide-based spintronics. The most remarkable result I present here is the observation of the QSHE at a Mott insulator/band insulator oxide interface where dissipation-less transport occurs in helical edge states. The QSHE is a manifestation of a two-dimensional topological insulator state which has previously been limited to low temperatures and submicron length scales. Here I show that the first observation of the QSHE upon electrically gating a full oxide system that holds up to exceptionally long length scales and nearly room-temperature. This includes quantized resistance that is dimension independent, nonlocal transport, and a large magnetoresistance. These results lay the foundation for using the QSHE in practical oxide-based spintronics devices. Finally, a proof-of-concept experiment is done to show that this system can indeed be interfaced with a ferromagnet for spintronics applications. The ferromagnet is used to pump spins into the LaTiO3/SrTiO3 heterostructure, and it is shown that spin scattering is greater at the interface between LaTiO3 and SrTiO3 compared to the bulk of the LaTiO3. This demonstrates that spins can be efficiently pumped into the interface and spin-orbit coupling is enhanced at the interface allowing for the manipulation of the spins. Altogether, my results show that the LaTiO3/SrTiO3 heterostructure is an exciting avenue for future study. The unexpectedly large spin-orbit coupling in this system provides new avenues for manipulating spintronics devices. The appearance of the near room temperature QSHE alone makes it possible to integrate this phase in practical devices for the first time.

Keywords: Applied Physics.
Advisor: Suzuki Yuri (Applied physicist) 
Goldhaber-Gordon David
Hwang Harold Yoonsung