Edge-decorated graphene nanoribbons are investigated with the density functional theory; they reveal three stable geometric structures. The first type is a tubular structure formed by the covalent bonds of decorating boron or nitrogen atoms. The second one consists of curved nanoribbons created by the dipole-dipole interactions between two edges when decorated with Be, Mg, or Al atoms. The final structure is a flat nanoribbon produced due to the repulsive force between two edges; most decorated structures belong to this type. Various decorating atoms, different curvature angles, and the zigzag edge structure are reflected in the electronic properties, magnetic properties, and bonding configurations. Most of the resulting structures are conductors with relatively high free carrier densities, whereas a few are semiconductors due to the zigzag-edge-induced anti-ferromagnetism.

FeTe1−x Se x is a family of iron-based superconductors with a well-known critical temperature (T c) of 14.5 K for x = 0.45. Also well-established is the presence of topological surface states of FeTe0.55Se0.45 in which topological superconductivity sets in. By using density functional calculations and the Bardeen–Cooper–Schrieffer (BCS) theory for traditional superconductors we calculated the T c of the surface layers of FeTe0.5Se0.5 with the exchange interaction as the source of attractive force to form the Cooper pairs. The estimated T c is in reasonable agreement with the experimental value and suggests that the exchange interaction is behind the topological superconductivity on the surface of FeTe0.55Se0.45.

We found that thin antimony films contacted by a single layer of a variety of atoms can form topological surface and interface states in the gap of bulk energy bands that preserve time-reversal symmetry. Using density functional calculation, we have included bismuth, graphene, boron-nitride and boron-doped graphene layers in our investigation. In most cases, Dirac cones are found in the band structures and spin textures indicate no back scattering of conduction electrons. In the case of a BC 3 layer deposited on an Sb film as thin as three bilayers, a Dirac cone consisting mostly of interface states is formed close to the Fermi level. If the composite is gated by an adjustable voltage, electron transport can switch between two modes of opposite spins, and between one or two modes of 100% spin polarization. The results should be very useful in the applications of topological conduction and spintronics.

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MoS2 nanoribbons with armchair-terminated edges are semiconductors suitable for the tuning of electronic and magnetic properties. Our first-principles density function calculations reveal that a variety of transition-metal atomic chains deposited on some of the ribbons is able to transform the semiconductors into half metals, allowing transport of 100% spin-polarized currents. Furthermore, we found that a Si atomic chain is equally capable of achieving half metallicity when adsorbed on the same nanoribbon. These results should be useful for spintronic application.

Tin telluride is a topological crystalline insulator that has gapless surface states protected by mirror symmetry. The symmetry remains intact when the insulator is reduced in thickness and becomes a thin film, according to ab initio calculations based on density functional theory. Furthermore, a SnTe thin film in contact with a heterogeneous atomic layer is capable of closing energy gap caused by quantum tunneling between the two thin film surfaces and therefore distinguishes two conducting channels through surface and interface states respectively. Our calculations of SnTe films deposited with a lead telluride layer have two Dirac cones separated in energy, while the same film in contact with strontium telluride have the cones separated in momentum. The composite with a magnetic manganese telluride layer, however, loses both the mirror and time-reversal symmetry. MethodWe use spin-polarized density-functional calculations as implemented in the Vienna ab initio simulation package [17,18] to derive all electronic bands. For exchange-correlation functional the version of Perdew, Burke

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