76 research outputs found
Field-induced insulating states in a graphene superlattice
We report on high-field magnetotransport (B up to 35 T) on a gated
superlattice based on single-layer graphene aligned on top of hexagonal boron
nitride. The large-period moir\'e modulation (15 nm) enables us to access the
Hofstadter spectrum in the vicinity of and above one flux quantum per
superlattice unit cell (Phi/Phi_0 = 1 at B = 22 T). We thereby reveal, in
addition to the spin-valley antiferromagnet at nu = 0, two insulating states
developing in positive and negative effective magnetic fields from the main nu
= 1 and nu = -2 quantum Hall states respectively. We investigate the field
dependence of the energy gaps associated with these insulating states, which we
quantify from the temperature-activated peak resistance. Referring to a simple
model of local Landau quantization of third generation Dirac fermions arising
at Phi/Phi_0 = 1, we describe the different microscopic origins of the
insulating states and experimentally determine the energy-momentum dispersion
of the emergent gapped Dirac quasi-particles
Excess resistivity in graphene superlattices caused by umklapp electron-electron scattering
Umklapp processes play a fundamental role as the only intrinsic mechanism
that allows electrons to transfer momentum to the crystal lattice and,
therefore, provide a finite electrical resistance in pure metals. However,
umklapp scattering has proven to be elusive in experiment as it is easily
obscured by other dissipation mechanisms. Here we show that electron-electron
umklapp scattering dominates the transport properties of
graphene-on-boron-nitride superlattices over a wide range of temperatures and
carrier densities. The umklapp processes cause giant excess resistivity that
rapidly increases with increasing the superlattice period and are responsible
for deterioration of the room-temperature mobility by more than an order of
magnitude as compared to standard, non-superlattice graphene devices. The
umklapp scattering exhibits a quadratic temperature dependence accompanied by a
pronounced electron-hole asymmetry with the effect being much stronger for
holes rather than electrons. Aside from fundamental interest, our results have
direct implications for design of possible electronic devices based on
heterostructures featuring superlattices
Engineering Graphene Flakes for Wearable Textile Sensors via Highly Scalable and Ultrafast Yarn Dyeing Technique
© 2019 American Chemical Society. Multifunctional wearable e-textiles have been a focus of much attention due to their great potential for healthcare, sportswear, fitness, space, and military applications. Among them, electroconductive textile yarn shows great promise for use as next-generation flexible sensors without compromising the properties and comfort of usual textiles. However, the current manufacturing process of metal-based electroconductive textile yarn is expensive, unscalable, and environmentally unfriendly. Here we report a highly scalable and ultrafast production of graphene-based flexible, washable, and bendable wearable textile sensors. We engineer graphene flakes and their dispersions in order to select the best formulation for wearable textile application. We then use a high-speed yarn dyeing technique to dye (coat) textile yarn with graphene-based inks. Such graphene-based yarns are then integrated into a knitted structure as a flexible sensor and could send data wirelessly to a device via a self-powered RFID or a low-powered Bluetooth. The graphene textile sensor thus produced shows excellent temperature sensitivity, very good washability, and extremely high flexibility. Such a process could potentially be scaled up in a high-speed industrial setup to produce tonnes (∼1000 kg/h) of electroconductive textile yarns for next-generation wearable electronics applications
Holographic reconstruction of the interlayer distance of bilayer two-dimensional crystal samples from their convergent beam electron diffraction patterns
The convergent beam electron diffraction (CBED) patterns of twisted bilayer
samples exhibit interference patterns in their CBED spots. Such interference
patterns can be treated as off-axis holograms and the phase of the scattered
waves, meaning the interlayer distance can be reconstructed. A detailed
protocol of the reconstruction procedure is provided in this study. In
addition, we derive an exact formula for reconstructing the interlayer distance
from the recovered phase distribution, which takes into account the different
chemical compositions of the individual monolayers. It is shown that one
interference fringe in a CBED spot is sufficient to reconstruct the distance
between the layers, which can be practical for imaging samples with a
relatively small twist angle or when probing small sample regions. The quality
of the reconstructed interlayer distance is studied as a function of the twist
angle. At smaller twist angles, the reconstructed interlayer distance
distribution is more precise and artefact free. At larger twist angles,
artefacts due to the moir\'e structure appear in the reconstruction. A method
for the reconstruction of the average interlayer distance is presented. As for
resolution, the interlayer distance can be reconstructed by the holographic
approach at an accuracy of 0.5 A, which is a few hundred times better than the
intrinsic z-resolution of diffraction limited resolution, as expressed through
the spread of the measured k-values. Moreover, we show that holographic CBED
imaging can detect variations as small as 0.1 A in the interlayer distance,
though the quantitative reconstruction of such variations suffers from large
errors
Swimming dynamics of a micro-organism in a couple stress fluid : a rheological model of embryological hydrodynamic propulsion
Mathematical simulations of embryological fluid dynamics are fundamental to improving clinical understanding of the intricate mechanisms underlying sperm locomotion. The strongly rheological nature of reproductive fluids has been established for a number of decades. Complimentary to clinical studies, mathematical models of reproductive hydrodynamics provide a deeper understanding of the intricate mechanisms involved in spermatozoa locomotion which can be of immense benefit in clarifying fertilization processes. Although numerous non-Newtonian studies of spermatozoa swimming dynamics in non-Newtonian media have been communicated, very few have addressed the micro-structural characteristics of embryological media. This family of micro-continuum models include Eringen’s micro-stretch theory, Eringen’s microfluid and micropolar constructs and V.K. Stokes’ couple-stress fluid model, all developed in the 1960s. In the present paper we implement the last of these models to examine the problem of micro-organism (spermatozoa) swimming at low Reynolds number in a homogenous embryological fluid medium with couple stress effects. The micro-organism is modeled as with Taylor’s classical approach, as an infinite flexible sheet on whose surface waves of lateral displacement are propagated. The swimming speed of the sheet and rate of work done by it are determined as function of the parameters of orbit and the couple stress fluid parameter (α). The perturbation solutions are validated with a Nakamura finite difference algorithm. The perturbation solutions reveal that the normal beat pattern is effective for both couple stress and Newtonian fluids only when the amplitude of stretching wave is small. The swimming speed is observed to decrease with couple stress fluid parameter tending to its Newtonian limit as alpha tends to infinity. However the rate of work done by the sheet decreases with α and approaches asymptotically to its Newtonian value. The present solutions also provide a good benchmark for more advanced numerical simulations of micro-organism swimming in couple-stress rheological biofluids
Mechanical Properties of Atomically Thin Tungsten Dichalcogenides::WS2, WSe2, and WTe2
Two-dimensional (2D) tungsten disulfide (WS), tungsten diselenide
(WSe), and tungsten ditelluride (WTe) draw increasing attention due to
their attractive properties deriving from the heavy tungsten and chalcogenide
atoms, but their mechanical properties are still mostly unknown. Here, we
determine the intrinsic and air-aged mechanical properties of mono-, bi-, and
trilayer (1-3L) WS, WSe and WTe using a complementary suite of
experiments and theoretical calculations. High-quality 1L WS has the
highest Young's modulus (302.4+-24.1 GPa) and strength (47.0+-8.6 GPa) of the
entire family, overpassing those of 1L WSe (258.6+-38.3 and 38.0+-6.0 GPa,
respectively) and WTe (149.1+-9.4 and 6.4+-3.3 GPa, respectively). However,
the elasticity and strength of WS decrease most dramatically with increased
thickness among the three materials. We interpret the phenomenon by the
different tendencies for interlayer sliding in equilibrium state and under
in-plane strain and out-of-plane compression conditions in the indentation
process, revealed by finite element method (FEM) and density functional theory
(DFT) calculations including van der Waals (vdW) interactions. We also
demonstrate that the mechanical properties of the high-quality 1-3L WS and
WSe are largely stable in the air for up to 20 weeks. Intriguingly, the
1-3L WSe shows increased modulus and strength values with aging in the air.
This is ascribed to oxygen doping, which reinforces the structure. The present
study will facilitate the design and use of 2D tungsten dichalcogenides in
applications, such as strain engineering and flexible field-effect transistors
(FETs)
Graphene hot-electron light bulb: incandescence from hBN-encapsulated graphene in air
The excellent electronic and mechanical properties of graphene allow it to
sustain very large currents, enabling its incandescence through Joule heating
in suspended devices. Although interesting scientifically and promising
technologically, this process is unattainable in ambient environment, because
graphene quickly oxidises at high temperatures. Here, we take the performance
of graphene-based incandescent devices to the next level by encapsulating
graphene with hexagonal boron nitride (hBN). Remarkably, we found that the hBN
encapsulation provides an excellent protection for hot graphene filaments even
at temperatures well above 2000 K. Unrivalled oxidation resistance of hBN
combined with atomically clean graphene/hBN interface allows for a stable light
emission from our devices in atmosphere for many hours of continuous operation.
Furthermore, when confined in a simple photonic cavity, the thermal emission
spectrum is modified by a cavity mode, shifting the emission to the visible
range spectrum. We believe our results demonstrate that hBN/graphene
heterostructures can be used to conveniently explore the technologically
important high-temperature regime and to pave the way for future optoelectronic
applications of graphene-based systems
Long-range ballistic transport of Brown-Zak fermions in graphene superlattices
In quantizing magnetic fields, graphene superlattices exhibit a complex fractal spectrum often referred to as the Hofstadter butterfly. It can be viewed as a collection of Landau levels that arise from quantization of Brown-Zak minibands recurring at rational (p/q) fractions of the magnetic flux quantum per superlattice unit cell. Here we show that, in graphene-on-boron-nitride superlattices, Brown-Zak fermions can exhibit mobilities above 106 cm2 V−1 s−1 and the mean free path exceeding several micrometers. The exceptional quality of our devices allows us to show that Brown-Zak minibands are 4q times degenerate and all the degeneracies (spin, valley and mini-valley) can be lifted by exchange interactions below 1 K. We also found negative bend resistance at 1/q fractions for electrical probes placed as far as several micrometers apart. The latter observation highlights the fact that Brown-Zak fermions are Bloch quasiparticles propagating in high fields along straight trajectories, just like electrons in zero field
Regulation of Classical Cadherin Membrane Expression and F-Actin Assembly by Alpha-Catenins, during Xenopus Embryogenesis
Alpha (α)-E-catenin is a component of the cadherin complex, and has long been thought to provide a link between cell surface cadherins and the actin skeleton. More recently, it has also been implicated in mechano-sensing, and in the control of tissue size. Here we use the early Xenopus embryos to explore functional differences between two α-catenin family members, α-E- and α-N-catenin, and their interactions with the different classical cadherins that appear as tissues of the embryo become segregated from each other. We show that they play both cadherin-specific and context-specific roles in the emerging tissues of the embryo. α-E-catenin interacts with both C- and E-cadherin. It is specifically required for junctional localization of C-cadherin, but not of E-cadherin or N-cadherin at the neurula stage. α-N-cadherin interacts only with, and is specifically required for junctional localization of, N-cadherin. In addition, α -E-catenin is essential for normal tissue size control in the non-neural ectoderm, but not in the neural ectoderm or the blastula. We also show context specificity in cadherin/ α-catenin interactions. E-cadherin requires α-E-catenin for junctional localization in some tissues, but not in others, during early development. These specific functional cadherin/alpha-catenin interactions may explain the basis of cadherin specificity of actin assembly and morphogenetic movements seen previously in the neural and non-neural ectoderm
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