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

Physical Review B (Impact Factor: 3.66). 01/2014; 89(11). DOI: 10.1103/PhysRevB.89.115429
Source: arXiv

ABSTRACT We propose new two-dimensional (2D) topological insulators (TIs) in
functionalized germanenes (GeX, X=H, F, Cl, Br or I) using first-principles
calculations. We find GeI is a 2D TI with a bulk gap of about 0.3 eV, while
GeH, GeF, GeCl and GeBr can be transformed into TIs with sizeable gaps under
achievable tensile strains. A unique mechanism is revealed to be responsible
for large topologically-nontrivial gap obtained: owing to the
functionalization, the $\sigma$ orbitals with stronger spin-orbit coupling
(SOC) dominate the states around the Fermi level, instead of original $\pi$
orbitals with weaker SOC; thereinto, the coupling of the $p_{xy}$ orbitals of
Ge and heavy halogens in forming the $\sigma$ orbitals also plays a key role in
the further enlargement of the gaps in halogenated germanenes. Our results
suggest a realistic possibility for the utilization of topological effects at
room temperature.

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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). · 3.66 Impact Factor
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    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).

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