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Nanoribbons are model systems for studying nanoscale effects in graphene. For ribbons with zigzag edges, tunable bandgaps have been predicted due to coupling of spin-polarized edge states, which have yet to be systematically demonstrated experimentally. Here we synthesize zigzag nanoribbons using Fe nanoparticle-assisted hydrogen etching of epitaxial graphene/ SiC(0001) in ultrahigh vacuum. We observe two gaps in their local density of states by scanning tunnelling spectroscopy. For ribbons wider than 3 nm, gaps up to 0.39 eV are found independent of width, consistent with standard density functional theory calculations. Ribbons narrower than 3 nm, however, exhibit much larger gaps that scale inversely with width, supporting quasiparticle corrections to the calculated gap. These results provide direct experimental confirmation of electron-electron interactions in gap opening in zigzag nanoribbons, and reveal a critical width of 3 nm for its onset. Our findings demonstrate that practical tunable bandgaps can be realized experimentally in zigzag nanoribbons.

The surface states of 3D topological insulators (TIs) exhibit a helical spin texture with spin locked at right angles with momentum. The chirality of this spin texture is expected to invert crossing the Dirac point, a property that has been experimentally observed by optical probes. Here, we directly determine the chirality below the Dirac point by electrically detecting spin-momentum locking in surface states of a p-type TI, Sb2Te3. A current flowing in the Sb2Te3 surface states generates a net spin polarization due to spin-momentum locking, which is electrically detected as a voltage on an Fe/Al2O3 tunnel barrier detector. Measurements of this voltage as a function of current direction and detector magnetization indicate that hole spin-momentum locking follows the right-hand rule, opposite that of electron, providing direct confirmation that the chirality is indeed inverted below Dirac point. The spin signal is linear with current, and exhibits a temperature dependence consistent with the semiconducting nature of the TI film and freeze-out of bulk conduction below 100 K. Our results demonstrate that the chirality of the helical spin texture of TI surface states can be determined electrically, an enabling step in the electrical manipulation of spins in next generation TI-based quantum devices.

Modifications of the electronic bands of thin FeSe films due to oxygen vacancies in the supporting SrTiO3(001) substrate -and the interplay with spin-orbit coupling, magnetism, and epitaxy -are investigated by first-principles supercell calculations. Unfolded (k-projected) bands show that the oxygen vacancies both provide electron doping to the interface FeSe layer and also have notable effects on the details of the bands around the Fermi level, including renormalizing the width of the Fe-3d band near the Fermi level by a factor of about 0.6, and causing a splitting of ∼40 meV at the M point for the checkerboard antiferromagnetic configuration. For an FeSe bilayer, the modifications to the bands are mainly limited to the interface FeSe layer. While spin-orbit-coupling induced band splittings of ∼30 meV at M for the ideal FeSe/SrTiO3(001) interfaces are comparable to the splitting due to oxygen vacancies, the effects are not simply additive. Calculations and comparison to our scanning tunneling microscopy images of MBE-grown FeSe films on SrTiO3(001) suggest that a common defect may be Se bound to an oxygen vacancy at the interface, 73.22.Pr

Using scanning tunneling microscopy and transmission electron microscopy, we demonstrate the existence of antiphase boundaries between neighboring grains shifted by a fraction of a quintuple layer in epitaxial (0001) films of the three-dimensional topological insulator Bi(2)Se(3). Scanning tunneling spectroscopy and first-principles calculations reveal that these antiphase boundaries provide electrostatic fields on the order of 10(8) V/m that locally charge the Dirac states, modulating the carrier density, and shift the Dirac point by up to 120 meV. This intrinsic electric field effect, demonstrated here near interfaces between Bi(2)Se(3) grains, provides direct experimental evidence at the atomic scale that the Dirac states are indeed robust against extended structural defects and tunable by electric field. These results also shed light on the recent observation of coexistence of Dirac states and two-dimensional electron gas on Bi(2)Se(3) (0001) after adsorption of metal atoms and gas molecules.