Article

# Functionalized Germanene as a Prototype of Large-Gap Two-Dimensional Topological Insulators

Physical Review B (Impact Factor: 3.74). 01/2014; 89(11). DOI: 10.1103/PhysRevB.89.115429

Source: arXiv

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**ABSTRACT:**We present a minimal four-band model for the 2D topological insulators based on the $p_x$ and $p_y$ orbital bands in the honeycomb lattice. Different from the $p_z$-band system such as graphene, the multi-orbital structure allows the atomic spin-orbit coupling, which lifts the degeneracy between two sets of onsite Kramers doublets $j_z=\pm\frac{3}{2}$ and $j_z=\pm\frac{1}{2}$ and generates non-trivial band topology. The topological gap is equal to the atomic spin-orbit coupling strength in the absence of sublattice asymmetry, and thus can reach large values. The energy spectra and eigen-wavefunctions are solved analytically based on Clifford algebra. The competition between spin-orbit coupling and lattice asymmetry results in band crossings at $\Gamma$, $K (K^\prime)$ points and topological band structure transitions. Flat bands also naturally arise which allow a local construction of eigenstates. In the limit of large spin-orbit coupling strength, the system is reduced to two sets of Kane-Mele models lying symmetrically with respect to zero energy. The above mechanism is related to several classes of solid state materials that have been recently proposed in literature.Physical Review B 03/2014; 90(7). DOI:10.1103/PhysRevB.90.075114 · 3.74 Impact Factor - [Show abstract] [Hide abstract]

**ABSTRACT:**Recently, this long-sought quantum anomalous Hall effect was realized in the magnetic topological insulator. However, the requirement of an extremely low temperature (approximately 30~mK) hinders realistic applications. Based on \textit{ab-initio} band structure calculations, we propose a quantum anomalous Hall platform with a large energy gap of 0.34 and 0.06 ~eV on honeycomb lattices comprised of Sn and Ge, respectively. The ferromagnetic order forms in one sublattice of the honeycomb structure by controlling the surface functionalization rather than dilute magnetic doping. Strong coupling between the inherent QSH state and ferromagnetism results in considerable exchange splitting and consequently an FM insulator with a large energy gap. The estimated mean-field Curie temperature is 243 and 509 K for Sn and Ge lattices, respectively. The large energy gap and high Curie temperature indicate the feasibility of the QAH effect in the near-room-temperature and even room-temperature regions.Physical Review Letters 05/2014; 113(25). DOI:10.1103/PhysRevLett.113.256401 · 7.51 Impact Factor - [Show abstract] [Hide abstract]

**ABSTRACT:**The adsorption of common gas molecules (N2, CO, CO2, H2O, NH3, NO, NO2, and O2) on germanene is studied with density functional theory. The results show that N2, CO, CO2, and H2O are physisorbed on germanene via van der Waals interactions, while NH3, NO, NO2, and O2 are chemisorbed on germanene via strong covalent (Ge-N or Ge-O) bonds. The chemisorption of gas molecules on germanene opens a band gap at the Dirac point of germanene. NO2 chemisorption on germanene shows strong hole doping in germanene. O2 is easily dissociated on germanene at room temperature. Different adsorption behaviors of common gas molecules on germanene provide a feasible way to exploit chemically modified germanene.Physical Chemistry Chemical Physics 08/2014; 16(41):22495-22498. DOI:10.1039/C4CP03292F · 4.49 Impact Factor