Topological insulators (TIs), a newly discovered quantum state of matter, exhibit novel spin-momentum locked surface Dirac states protected by time reversal symmetry, while the bulk insulates. The aim of this project is to understand the epitaxial growth and doping of TI heterostructures that will enable exploration of novel quantum phenomena such as Majorana Fermions and dissipationless spin and charge transport.
The prototype three-dimensional TI Bi2Se3 is comprised of five (quintuple) bismuth and selenium atomic layers stacked on top of one another, with covalent in-plane bonds and weak (van der Waals) interactions between the quintuple layers. We showed earlier that as a result, spiral formation is inherent to the molecular beam epitaxy (MBE) growth of the Bi2Se3 (PRL108, 115501 (2012)), which invariably leads to a high density (~109/cm2) of grain boundaries (GBs) upon spiral coalescence.
We have since systematically studied the atomic structures of these GBs and their impact on the Dirac states in MBE-grown Bi2Se3(0001) films. Using a combination of atomic resolution scanning tunneling microscopy (STM) and scanning transmission electron microscopy (STEM) imaging, we have determined that the low-angle (0 < θ < 15°) tilt GBs in Bi2Se3 consist of alternating edge dislocation pairs. The strain introduced by these dislocations is quantitatively analyzed using the Geometrical Phase Analysis (GPA) method. The results show that in-plane tensile and compressive strains lead to periodic depressions and expansions in the  direction along the boundary.
We further carried out investigations of the impact of the strain on the Dirac states using density functional theory calculations, and spatially resolved di/dV tunneling spectroscopy that probes the local density of states (LDOS) at the position of the STM tip. We find that Dirac states are enhanced under tensile strain, and destroyed under compressive strain, as evident from the opening of a gap in the LDOS.
In addition, in collaboration with Dr. Berend Jonker at the Naval Research Laboratory, we have demonstrated the first direct electrical detection of spin-momentum locking in Bi2Se3, one of the most striking properties of TIs. This is accomplished by using a ferromagnetic metal/tunnel barrier contact, which is inherently sensitive to surface and interface spin, to probe the spin polarization created by an unpolarized current via spin-momentum locking. This is the first direct electrical access to the spin-momentum locked surface states in TIs.
- “Tuning Dirac states by strain in the topological insulator Bi2Se3”, Y. Liu, Y. Y. Li, S. Rajput, D. Gilks, L. Lari, P. L. Galindo, M. Weinert, V. K. Lazarov, and L. Li , Nat. Physics 10, 294 (2014).
- “Electrical detection of charge-current-induced spin polarization due to spin-momentum locking in Bi2Se3”, C. H. Li, O. M. J. van ‘t Erve, J. T. Robinson, Y. Liu, L. Li, and B. T. Jonker, Nat. Nanotechnology 9, 218(2014).
- “Charging Dirac states at antiphase domain boundaries in the three-dimensional topological insulator Bi2Se3”,Y. Liu, Y. Y. Li, D. Gilks, V. K. Lazarov, M. Weinert, and L. Li, Phys. Rev. Lett. 110, 186804 (2013).
- “Spiral growth without dislocations: Molecular beam epitaxy of the topological insulator Bi2Se3 on epitaxial graphene/SiC(0001)”, Y. Liu, M. Weinert, and L. Li, Phys. Rev. Lett. 108, 115501 (2012).