484 research outputs found
How to Distinguish between Specular and Retroconfigurations for Andreev Reflection in Graphene Rings
We numerically investigate Andreev reflection in a graphene ring with one
normal conducting and one superconducting lead by solving the Bogoliubov--de
Gennes equation within the Landauer-B\"uttiker formalism. By tuning chemical
potential and bias voltage, it is possible to switch between regimes where
electron and hole originate from the same band (retroconfiguration) or from
different bands (specular configuration) of the graphene dispersion,
respectively. We find that the dominant contributions to the Aharonov-Bohm
conductance oscillations in the subgap transport are of period in
retroconfiguration and of period in specular configuration, confirming
the predictions obtained from a qualitative analysis of interfering scattering
paths. Because of the robustness against disorder and moderate changes to the
system, this provides a clear signature to distinguish both types of Andreev
reflection processes in graphene.Comment: 5 pages, 5 figures. arXiv admin note: substantial text overlap with
arXiv:1201.620
Signatures of topology in ballistic bulk transport of HgTe quantum wells
We calculate bulk transport properties of two-dimensional topological
insulators based on HgTe quantum wells in the ballistic regime. Interestingly,
we find that the conductance and the shot noise are distinctively different for
the so-called normal regime (the topologically trivial case) and the so-called
inverted regime (the topologically non-trivial case). Thus, it is possible to
verify the topological order of a two-dimensional topological insulator not
only via observable edge properties but also via observable bulk properties.
This is important because we show that under certain conditions the bulk
contribution can dominate the edge contribution which makes it essential to
fully understand the former for the interpretation of future experiments in
clean samples.Comment: 5 pages, 4 figure
Tunable quantum spin Hall effect in double quantum wells
The field of topological insulators (TIs) is rapidly growing. Concerning
possible applications, the search for materials with an easily controllable TI
phase is a key issue. The quantum spin Hall effect, characterized by a single
pair of helical edge modes protected by time-reversal symmetry, has been
demonstrated in HgTe-based quantum wells (QWs) with an inverted bandgap. We
analyze the topological properties of a generically coupled HgTe-based double
QW (DQW) and show how in such a system a TI phase can be driven by an
inter-layer bias voltage, even when the individual layers are non-inverted. We
argue, that this system allows for similar (layer-)pseudospin based physics as
in bilayer graphene but with the crucial absence of a valley degeneracy.Comment: 9 pages, 8 figures, extended version (accepted Phys. Rev. B
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