35 research outputs found
Bloch-Zener Oscillations in Graphene and Topological Insulators
We show that superlattices based on zero-gap semiconductors such as graphene
and mercury telluride exhibit characteristic Bloch--Zener oscillations that
emerge from the coherent superposition of Bloch oscillations and multiple Zener
tunneling between the electron and hole branch. We demonstrate this mechanism
by means of wave packet dynamics in various spatially periodically modulated
nanoribbons subject to an external bias field. The associated Bloch frequencies
exhibit a peculiar periodic bias dependence which we explain within a two-band
model. Supported by extensive numerical transport calculations, we show that
this effect gives rise to distinct current oscillations observable in the I-V
characteristics of graphene and mercury telluride superlattices
The visibility of IQHE at sharp edges: Experimental proposals based on interactions and edge electrostatics
The influence of the incompressible strips on the integer quantized Hall
effect (IQHE) is investigated, considering a cleaved-edge overgrown (CEO)
sample as an experimentally realizable sharp edge system. We propose a set of
experiments to clarify the distinction between the large-sample limit when bulk
disorder defines the IQHE plateau width and the small-sample limit smaller than
the disorder correlation length, when self-consistent edge electrostatics
define the IQHE plateau width. The large-sample or bulk QH regime is described
by the usual localization picture, whereas the small-sample or edge regime is
discussed within the compressible/incompressible strips picture, known as the
screening theory of QH edges. Utilizing the unusually sharp edge profiles of
the CEO samples, a Hall bar design is proposed to manipulate the edge potential
profile from smooth to extremely sharp. By making use of a side-gate
perpendicular to the two dimensional electron system, it is shown that the
plateau widths can be changed or even eliminated altogether. Hence, the
visibility of IQHE is strongly influenced when adjusting the edge potential
profile and/or changing the dc current direction under high currents in the
non-linear transport regime. As a second investigation, we consider two
different types of ohmic contacts, namely highly transmitting (ideal) and
highly reflecting (non-ideal) contacts. We show that if the injection contacts
are non-ideal, however still ohmic, it is possible to measure directly the
non-quantized transport taking place at the bulk of the CEO samples. The
results of the experiments we propose will clarify the influence of the edge
potential profile and the quality of the contacts, under quantized Hall
conditions.Comment: Substantially revised version of manuscript arXiv:0906.3796v1,
including new figures et
Expression Analysis of PAC1-R and PACAP Genes in Zebrafish Embryos
This study describes the expression of the pituitary adenylate cyclase-activating polypeptide (PACAP1 and PACAP2) and PAC1 receptor genes (PAC1a-R and PAC1b-R) in the brain of zebrafish (Danio rerio) during development. In situ hybridization of the 24- and 48-hpf embryos revealed that PACAP genes were expressed in the telencephalon, the diencephalon, the rhombencephalon, and the neurons in the dorsal part of the spinal cord. PACAP2 mRNA appears to be the most abundant form during brain development. The two PAC1-R subtypes showed a similar expression pattern: mRNAs were detected in the forebrain, the thalamus, and the rhombencephalon. However, in the tectum, only PAC1b-R gene was detected. These results suggest that, in fish, PACAP may play a role in brain development
Probing the Band Topology of Mercury Telluride through Weak Localization and Antilocalization
We analyze the effect of weak localization (WL) and weak antilocalization
(WAL) in the electronic transport through HgTe/CdTe quantum wells. We show that
for increasing Fermi energy the magnetoconductance of a diffusive system with
inverted band ordering features a transition from WL to WAL and back, if
spin-orbit interactions from bulk and structure inversion asymmetry can be
neglected. This, and an additional splitting in the magnetoconductance profile,
is a signature of the Berry phase arising for inverted band ordering and not
present in heterostructures with conventional ordering. In presence of
spin-orbit interaction both band topologies exhibit WAL, which is distinctly
energy dependent solely for quantum wells with inverted band ordering. This can
be explained by an energy-dependent decomposition of the Hamiltonian into two
blocks.Comment: 7 pages, 6 figures, added reference