Formation of quantum spin Hall state on Si surface and energy gap scaling with strength of spin orbit coupling

Abstract

For potential applications in spintronics and quantum computing, it is desirable to place a quantum spin Hall insulator [i.e., a 2D topological insulator (TI)] on a substrate while maintaining a large energy gap. Here, we demonstrate a unique approach to create the large-gap 2D TI state on a semiconductor surface, based on first-principles calculations and effective Hamiltonian analysis. We show that when heavy elements with strong spin orbit coupling (SOC) such as Bi and Pb atoms are deposited on a patterned H-Si(111) surface into a hexagonal lattice, they exhibit a 2D TI state with a large energy gap of ≥0.5 eV. The TI state arises from an intriguing substrate orbital filtering effect that selects a suitable orbital composition around the Fermi level, so that the system can be matched onto a four-band effective model Hamiltonian. Furthermore, it is found that within this model, the SOC gap does not increase monotonically with the increasing strength of SOC. These interesting results may shed new light in future design and fabrication of large-gap topological quantum states.

Orbital engineering of two-dimensional materials with hydrogenation: A realization of giant gap and strongly correlated topological insulators

“…Finally, we discuss the possibility of experimentally realizing these novel two dimensional materials. For materials like Pb, it has been theoretically demonstrated that substrates like SiH (111) [14][15][16] can be used as an effective tool to filter the p z orbitals to create a large gap topological insulator based on the p x,y orbitals. In addition, high quality thin films of Pb have been experimentally grown on insulating substrates using the molecular beam epitaxial technique [40], thus opens up the possibility to integrate conventional growth technology to produce large gap QSHI with p-orbitals.…”

“…Figs. 6a and 6b Pb is patterned on SiH [14][15][16]. Thus, hydrogen is added to the two W atoms on one side of the thin film to filter out the d 3z 2 -r 2 at the Fermi level.…”

“…Recent studies have highlighted the importance of orbitals selection in determining the bulk energy gap [12][13][14][15][16][17][18][19][20][21][22][23]. For instance, stanene [12] has a small bulk gap of 0.1 eV when the p z orbital dominates the effective low energy band structure.…”

“…However, through fluorination, the spin-orbit interaction can be confined on the in-plane p x and p y orbitals of stanene, thus enhancing its bulk gap to 0.3 eV. Other materials with large intrinsic spin-orbit coupling like Bi have also been demonstrated to contain above room temperature bulk gap using Si substrate as a tool of orbital filtering [14,15]. Furthermore, orbital filtering is also applicable in materials with asymmetric sublattices like III-Bi [22] and GaAs (111) [23] to generate QSHI with large Rashba effect.…”

π conjugation in the epitaxial Si(111)- 3×3 surface: Unconventional “bamboo hat” bonding geometry for Si

“…So the newly observed ( √ 3× √ 3) surface in heteroepitaxial Si(111) thin films is a big surprise, because it is fundamentally different from all the previous models, especially considering the unusual planar ring structure unexpected for Si. Clarifying the physical mechanism of such a unique surface reconstruction is thus of particular importance, which may profoundly renew our interest in Si surface and open a new route to realizing Dirac and topological bands in Si surface [30][31][32][33], as an interesting alternative to silicene.…”