1,007 research outputs found
Pressure-induced commensurate stacking of graphene on boron nitride
Combining atomically-thin van der Waals materials into heterostructures
provides a powerful path towards the creation of designer electronic devices.
The interaction strength between neighboring layers, most easily controlled
through their interlayer separation, can have significant influence on the
electronic properties of these composite materials. Here, we demonstrate
unprecedented control over interlayer interactions by locally modifying the
interlayer separation between graphene and boron nitride, which we achieve by
applying pressure with a scanning tunneling microscopy tip. For the special
case of aligned or nearly-aligned graphene on boron nitride, the graphene
lattice can stretch and compress locally to compensate for the slight lattice
mismatch between the two materials. We find that modifying the interlayer
separation directly tunes the lattice strain and induces commensurate stacking
underneath the tip. Our results motivate future studies tailoring the
electronic properties of van der Waals heterostructures by controlling the
interlayer separation of the entire device using hydrostatic pressure.Comment: 17 pages, 4 figures and supplementary information. Updated to
published versio
Diffuse MeV Gamma-rays and Galactic 511 keV Line from Decaying WIMP Dark Matter
The origin of both the diffuse high-latitude MeV gamma-ray emission and the
511 keV line flux from the Galactic bulge are uncertain. Previous studies have
invoked dark matter physics to independently explain these observations, though
as yet none has been able to explain both of these emissions within the
well-motivated framework of Weakly-Interacting Massive Particles (WIMPs). Here
we use an unstable WIMP dark matter model to show that it is in fact possible
to simultaneously reconcile both of these observations, and in the process show
a remarkable coincidence: decaying dark matter with MeV mass splittings can
explain both observations if positrons and photons are produced with similar
branching fractions. We illustrate this idea with an unstable branon, which is
a standard WIMP dark matter candidate appearing in brane world models with
large extra dimensions. We show that because branons decay via three-body final
states, they are additionally unconstrained by searches for Galactic MeV
gamma-ray lines. As a result, such unstable long-lifetime dark matter particles
provide novel and distinct signatures that can be tested by future observations
of MeV gamma-rays.Comment: 19 pages, 4 figure
VLBI Monitoring Observations of Water Masers Around the Semi-Regular Variable Star R Crateris
We monitored water-vapor masers around the semi-regular variable star R
Crateris with the Japanese VLBI Network (J-Net) at the 22 GHz band during four
epochs with intervals of one month. The relative proper motions and
Doppler-velocity drifts of twelve maser features were measured. Most of them
existed for longer than 80 days. The 3-D kinematics of the features indicates a
bipolar expanding flow. The major axis of the asymmetric flow was estimated to
be at P.A. = 136 degrees. The existence of a bipolar outflow suggests that a
Mira variable star had already formed a bipolar outflow. The water masers are
in a region of apparent minimum radii of 1.3 x 10^12 m and maximum radii of 2.6
x 10^12 m, between which the expansion velocity ranges from 4.3 to 7.4 km/s.
These values suggest that the water masers are radially accelerated, but still
gravitationally bound, in the water-maser region. The most positive and
negative velocity-drifting features were found relatively close to the systemic
velocity of the star. We found that the blue-shifted features are apparently
accelerated and the red-shifted apparently decelerated. The acceleration of
only the blue-shifted features seems to be consistent with that of the
expanding flow from the star.Comment: 15 pages, 5 figures, Accepted for publication in PASJ (2001),
preprint can be obtained via WWW on
http://www.nro.nao.ac.jp/library/report/list.htm
Interactions between Trypanosoma cruzi Secreted Proteins and Host Cell Signaling Pathways
Chagas disease is one of the prevalent neglected tropical diseases, affecting at least 6-7 million individuals in Latin America. It is caused by the protozoan parasite Trypanosoma cruzi, which is transmitted to vertebrate hosts by blood-sucking insects. After infection, the parasite invades and multiplies in the myocardium, leading to acute myocarditis that kills around 5% of untreated individuals. T. cruzi secretes proteins that manipulate multiple host cell signaling pathways to promote host cell invasion. The primary secreted lysosomal peptidase in T. cruzi is cruzipain, which has been shown to modulate the host immune response. Cruzipain hinders macrophage activation during the early stages of infection by interrupting the NF-kB P65 mediated signaling pathway. This allows the parasite to survive and replicate, and may contribute to the spread of infection in acute Chagas disease. Another secreted protein P21, which is expressed in all of the developmental stages of T. cruzi, has been shown to modulate host phagocytosis signaling pathways. The parasite also secretes soluble factors that exert effects on host extracellular matrix, such as proteolytic degradation of collagens. Finally, secreted phospholipase A from T. cruzi contributes to lipid modifications on host cells and concomitantly activates the PKC signaling pathway. Here, we present a brief review of the interaction between secreted proteins from T cruzi and the host cells, emphasizing the manipulation of host signaling pathways during invasion.Fundação de Amparo a Pesquisa do Estado de Sao Paulo (FAPESP)Coordenacao de Aperfeicoamento de Pessoal de Nivel Superior (CAPES)CNNUniv Fed Sao Paulo, Escola Paulista Med, Dept Microbiol Imunol & Parasitol, Sao Paulo, BrazilUniv Fed Minas Gerais, Inst Ciencias Biol, Dept Biol Geral, Belo Horizonte, MG, BrazilUniv Fed Sao Paulo, Escola Paulista Med, Dept Microbiol Imunol & Parasitol, Sao Paulo, BrazilFAPESP: 07/50551-2FAPESP: 11/51475-3Web of Scienc
Magnetoresistance in Co-hBN-NiFe tunnel junctions enhanced by resonant tunneling through single defects in ultrathin hBN barriers
Hexagonal boron nitride (hBN) is a prototypical high-quality two-dimensional
insulator and an ideal material to study tunneling phenomena, as it can be
easily integrated in vertical van der Waals devices. For spintronic devices,
its potential has been demonstrated both for efficient spin injection in
lateral spin valves and as a barrier in magnetic tunnel junctions (MTJs). Here
we reveal the effect of point defects inevitably present in mechanically
exfoliated hBN on the tunnel magnetoresistance of Co-hBN-NiFe MTJs. We observe
a clear enhancement of both the conductance and magnetoresistance of the
junction at well-defined bias voltages, indicating resonant tunneling through
magnetic (spin-polarized) defect states. The spin polarization of the defect
states is attributed to exchange coupling of a paramagnetic impurity in the
few-atomic-layer thick hBN to the ferromagnetic electrodes. This is confirmed
by excellent agreement with theoretical modelling. Our findings should be taken
into account in analyzing tunneling processes in hBN-based magnetic devices.
More generally, our study shows the potential of using atomically thin hBN
barriers with defects to engineer the magnetoresistance of MTJs and to achieve
spin filtering, opening the door towards exploiting the spin degree of freedom
in current studies of point defects as quantum emitters
Electric Field Control of Soliton Motion and Stacking in Trilayer Graphene
The crystal structure of a material plays an important role in determining
its electronic properties. Changing from one crystal structure to another
involves a phase transition which is usually controlled by a state variable
such as temperature or pressure. In the case of trilayer graphene, there are
two common stacking configurations (Bernal and rhombohedral) which exhibit very
different electronic properties. In graphene flakes with both stacking
configurations, the region between them consists of a localized strain soliton
where the carbon atoms of one graphene layer shift by the carbon-carbon bond
distance. Here we show the ability to move this strain soliton with a
perpendicular electric field and hence control the stacking configuration of
trilayer graphene with only an external voltage. Moreover, we find that the
free energy difference between the two stacking configurations scales
quadratically with electric field, and thus rhombohedral stacking is favored as
the electric field increases. This ability to control the stacking order in
graphene opens the way to novel devices which combine structural and electrical
properties
Measuring valley polarization in two-dimensional materials with second-harmonic spectroscopy
A population imbalance at different valleys of an electronic system lowers
its effective rotational symmetry. We introduce a technique to measure such
imbalance - a valley polarization - that exploits the unique fingerprints of
this symmetry reduction in the polarization-dependent second-harmonic
generation (SHG). We present the principle and detection scheme in the context
of hexagonal two-dimensional crystals, which include graphene-based systems and
the family of transition metal dichalcogenides, and provide a direct
experimental demonstration using a 2H-MoSe monolayer at room temperature.
We deliberately use the simplest possible setup, where a single pulsed laser
beam simultaneously controls the valley imbalance and tracks the SHG process.
We further developed a model of the transient population dynamics which
analytically describes the valley-induced SHG rotation in very good agreement
with the experiment. In addition to providing the first experimental
demonstration of the effect, this work establishes a conceptually simple,
com-pact and transferable way of measuring instantaneous valley polarization,
with direct applicability in the nascent field of valleytronics
Visualizing the Effect of an Electrostatic Gate with Angle-Resolved Photoemission Spectroscopy
Electrostatic gating is pervasive in materials science, yet its effects on
the electronic band structure of materials has never been revealed directly by
angle-resolved photoemission spectroscopy (ARPES), the technique of choice to
non-invasively probe the electronic band structure of a material. By means of a
state-of-the-art ARPES setup with sub-micron spatial resolution, we have
investigated a heterostructure composed of Bernal-stacked bilayer graphene
(BLG) on hexagonal boron nitride and deposited on a graphite flake. By voltage
biasing the latter, the electric field effect is directly visualized on the
valence band as well as on the carbon 1s core level of BLG. The band gap
opening of BLG submitted to a transverse electric field is discussed and the
importance of intralayer screening is put forward. Our results pave the way for
new studies that will use momentum-resolved electronic structure information to
gain insight on the physics of materials submitted to the electric field
effect
Near-field photocurrent nanoscopy on bare and encapsulated graphene
Opto-electronic devices utilizing graphene have already demonstrated unique
capabilities, which are much more difficult to realize with conventional
technologies. However, the requirements in terms of material quality and
uniformity are very demanding. A major roadblock towards high-performance
devices are the nanoscale variations of graphene properties, which strongly
impact the macroscopic device behaviour. Here, we present and apply
opto-electronic nanoscopy to measure locally both the optical and electronic
properties of graphene devices. This is achieved by combining scanning
near-field infrared nanoscopy with electrical device read-out, allowing
infrared photocurrent mapping at length scales of tens of nanometers. We apply
this technique to study the impact of edges and grain boundaries on spatial
carrier density profiles and local thermoelectric properties. Moreover, we show
that the technique can also be applied to encapsulated graphene/hexagonal boron
nitride (h-BN) devices, where we observe strong charge build-up near the edges,
and also address a device solution to this problem. The technique enables
nanoscale characterization for a broad range of common graphene devices without
the need of special device architectures or invasive graphene treatment
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