UCLA

The recently discovered time-reversal-invariant topological insulator (TI) has led to the flourishing of unique physics along with promises for innovative electronic and spintronic applications. However, the as-grown TI materials are not truly insulating but with a non-trivial bulk carrier density, which makes difficulties to the transport methods. In our work, we study the fundamental transport properties of TI and its heterostructure, in which various approaches are utilized to better reveal the surface state properties. In particular, in Chapter 2, in-situ Al surface passivation of Bi2Se3 inside MBE is investigated to inhibit the degradation process, reduce carrier density and reveal the pristine topological surface states. In contrast, we show the degradation of surface states for the unpassivated control samples, in which the 2D carrier density is increased by 39.2% due to ambient n-doping, the Shubnikov-de Hass oscillations are completely absent, and a deviation from WAL weak antilocalization is observed. In Chapter 3, through optimizing the material composition to achieve bulk insulating state, we present the ambipolar effect in 4-9 quintuple layers (Bi0.57Sb0.43)2Te3 thin films. We also demonstrate the evidence of a hybridized surface gap opening in (Bi0.57Sb0.43)2Te3 sample with thickness below six quintuple layers through transport and scanning tunneling spectroscopy measurements. By effective tuning the Fermi level via gate-voltage control, we unveil a striking competition between weak antilocalization and weak localization at low magnetic fields in nonmagnetic ultrathin films. In Chapter 4, we study the magnetic properties of Bi2Se3 surface states in the proximity of a high Tc ferrimagnetic insulator YIG. Proximity-induced magnetoresistance loops are observed by transport measurements with out-of-plane and in-plane magnetic fields applied. More importantly, a magnetic signal from the Bi2Se3 up to 130 K is clearly observed by magneto-optical Kerr effect measurements. Our results demonstrate the proximity-induced TI magnetism at higher temperatures, which is an important step toward room-temperature application of TI-based spintronic devices.

The engineering of a TI and FMI heterostructure will open up numerous opportunities to study high temperature TI-based spintronic devices, in which the TI is controlled by breaking the TRS using a FMI with perpendicular magnetization component. A YIG film with out-of-plane anisotropy at > 300 K could potentially manipulate the magnetic properties of a TI may even above room temperature.