Perfect transmission and Aharanov-Bohm oscillations in topological insulator nanowires with nonuniform cross section

Topological insulator nanowires with uniform cross section, combined with a magnetic flux, can host both a perfectly transmitted mode and Majorana zero modes. Here we consider nanowires with rippled surfaces---specifically, wires with a circular cross section with a radius varying along its axis---and calculate their transport properties. At zero doping, chiral symmetry places the clean wires (no impurities) in the AIII symmetry class, which results in a $\mathbb{Z}$ topological classification. A magnetic flux threading the wire tunes between the topologically distinct insulating phases, with perfect transmission obtained at the phase transition. We derive an analytical expression for the exact flux value at the transition. Both doping and disorder breaks the chiral symmetry and the perfect transmission. At finite doping, the interplay of surface ripples and disorder with the magnetic flux modifies quantum interference such that the amplitude of Aharonov-Bohm oscillations reduces with increasing flux, in contrast to wires with uniform surfaces where it is flux-independent.Comment: 12 pages, 6 figures. v2. 2 new figures and a new appendi

Epitaxial graphene on SiC(0001): More than just honeycombs

The potential of graphene to impact the development of the next generation of electronics has renewed interest in its growth and structure. The graphitization of hexagonal SiC surfaces provides a viable alternative for the synthesis of graphene, with wafer-size epitaxial graphene on SiC(0001) now possible. Despite this recent progress, the exact nature of the graphene-SiC interface and whether the graphene even has a semiconducting gap remain controversial. Using scanning tunneling microscopy with functionalized tips and density functional theory calculations, here we show that the interface is a warped carbon sheet consisting of three-fold hexagon-pentagon-heptagon complexes periodically inserted into the honeycomb lattice. These defects relieve the strain between the graphene layer and the SiC substrate, while still retaining the three-fold coordination for each carbon atom. Moreover, these defects break the six-fold symmetry of the honeycomb, thereby naturally inducing a gap: the calculated band structure of the interface is semiconducting and there are two localized states near K below the Fermi level, explaining the photoemission and carbon core-level data. Nonlinear dispersion and a 33 meV gap are found at the Dirac point for the next layer of graphene, providing insights into the debate over the origin of the gap in epitaxial graphene on SiC(0001). These results indicate that the interface of the epitaxial graphene on SiC(0001) is more than a dead buffer layer, but actively impacts the physical and electronic properties of the subsequent graphene layers

Nodal-line semimetals from Weyl superlattices

The existence and topological classification of lower-dimensional Fermi surfaces is often tied to the crystal symmetries of the underlying lattice systems. Artificially engineered lattices, such as heterostructures and other superlattices, provide promising avenues to realize desired crystal symmetries that protect lower-dimensional Fermi surface, such as nodal lines. In this work, we investigate a Weyl semimetal subjected to spatially periodic onsite potential, giving rise to several phases, including a nodal-line semimetal phase. In contrast to proposals that purely focus on lattice symmetries, the emergence of the nodal line in this setup does not require small spin-orbit coupling, but rather relies on its presence. We show that the stability of the nodal line is understood from reflection symmetry and a combination of a fractional lattice translation and charge-conjugation symmetry. Depending on the choice of parameters, this model exhibits drumhead surface states that are exponentially localized at the surface, or weakly localized surface states that decay into the bulk at all energies.Comment: 11 pages, 8 figures, Editors' Suggestio