1,276 research outputs found
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
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
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