379 research outputs found
Silicane and germanane: tight-binding and first-principles studies
We present a first-principles and tight-binding model study of silicane and
germanane, the hydrogenated derivatives of two-dimensional silicene and
germanene. We find that the materials are stable in freestanding form, analyse
the orbital composition, and derive a tight-binding model using
first-principles calculations to fit the parameters.Comment: Published in "2D Materials
Twist-controlled Resonant Tunnelling between Monolayer and Bilayer Graphene
We investigate the current-voltage characteristics of a field-effect
tunnelling transistor comprised of both monolayer and bilayer graphene with
well-aligned crystallographic axes, separated by three layers of hexagonal
boron nitride. Using a self-consistent description of the device's
electrostatic configuration we relate the current to three distinct tunable
voltages across the system and hence produce a two-dimensional map of the I-V
characteristics in the low energy regime. We show that the use of gates either
side of the heterostructure offers a fine degree of control over the device's
rich array of characteristics, as does varying the twist between the graphene
electrodes.Comment: 5 pages including references and 3 figure
Gate induced monolayer behavior in twisted bilayer black phosphorus
Optical and electronic properties of black phosphorus strongly depend on the
number of layers and type of stacking. Using first-principles calculations
within the framework of density functional theory, we investigate the
electronic properties of bilayer black phosphorus with an interlayer twist
angle of 90. These calculations are complemented with a simple
model which is able to capture most of the low energy
features and is valid for arbitrary twist angles. The electronic spectrum of
90 twisted bilayer black phosphorus is found to be x-y isotropic in
contrast to the monolayer. However x-y anisotropy, and a partial return to
monolayer-like behavior, particularly in the valence band, can be induced by an
external out-of-plane electric field. Moreover, the preferred hole effective
mass can be rotated by 90 simply by changing the direction of the
applied electric field. In particular, a +0.4 (-0.4) V/{\AA} out-of-plane
electric field results in a 60\% increase in the hole effective mass
along the y (x) axis and enhances the ()
ratio as much as by a factor of 40. Our DFT and
simulations clearly indicate that the twist angle in combination with an
appropriate gate voltage is a novel way to tune the electronic and optical
properties of bilayer phosphorus and it gives us a new degree of freedom to
engineer the properties of black phosphorus based devices.Comment: 8 pages, 8 figure
Ballistic miniband conduction in a graphene superlattice
Rational design of artificial lattices yields effects unavailable in simple
solids, and vertical superlattices of multilayer semiconductors are already
used in optical sensors and emitters. Manufacturing lateral superlattices
remains a much bigger challenge, with new opportunities offered by the use of
moire patterns in van der Waals heterostructures of graphene and hexagonal
crystals such as boron nitride (h-BN). Experiments to date have elucidated the
novel electronic structure of highly aligned graphene/h-BN heterostructures,
where miniband edges and saddle points in the electronic dispersion can be
reached by electrostatic gating. Here we investigate the dynamics of electrons
in moire minibands by transverse electron focusing, a measurement of ballistic
transport between adjacent local contacts in a magnetic field. At low
temperatures, we observe caustics of skipping orbits extending over hundreds of
superlattice periods, reversals of the cyclotron revolution for successive
minibands, and breakdown of cyclotron motion near van Hove singularities. At
high temperatures, we study the suppression of electron focusing by inelastic
scattering
Moiré miniband features in the angle-resolved photoemission spectra of graphene/hBN heterostructures
We identify features in the angle-resolved photoemission spectra (ARPES) arising from the periodic pattern characteristic for graphene heterostructure with hexagonal boron nitride (h BN). For this, we model ARPES spectra and intensity maps for five microscopic models used previously to describe moire superlattice in graphene/h BN systems. We show that detailed analysis of these features can be used to pin down the microscopic mechanismof the interaction between graphene and h BN. We also analyze how the presence of a moire-periodic strain in graphene or scattering of photoemitted electrons off h BN can be distinguished from the miniband formation
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Conservation and flexibility in the gene regulatory landscape of heliconiine butterfly wings
Funder: Wellcome Trust; doi: http://dx.doi.org/10.13039/100004440Abstract: Background: Many traits evolve by cis-regulatory modification, by which changes to noncoding sequences affect the binding affinity for available transcription factors and thus modify the expression profile of genes. Multiple examples of cis-regulatory evolution have been described at pattern switch genes responsible for butterfly wing pattern polymorphism, including in the diverse neotropical genus Heliconius, but the identities of the factors that can regulate these switch genes have not been identified. Results: We investigated the spatial transcriptomic landscape across the wings of three closely related butterfly species, two of which have a convergently evolved co-mimetic pattern and the other having a divergent pattern. We identified candidate factors for regulating the expression of wing patterning genes, including transcription factors with a conserved expression profile in all three species, and others, including both transcription factors and Wnt pathway genes, with markedly different profiles in each of the three species. We verified the conserved expression profile of the transcription factor homothorax by immunofluorescence and showed that its expression profile strongly correlates with that of the selector gene optix in butterflies with the Amazonian forewing pattern element ‘dennis.’ Conclusion: Here we show that, in addition to factors with conserved expression profiles like homothorax, there are also a variety of transcription factors and signaling pathway components that appear to vary in their expression profiles between closely related butterfly species, highlighting the importance of genome-wide regulatory evolution between species
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
"You are just left to get on with it": qualitative study of patient and carer experiences of the transition to secondary progressive multiple sclerosis
Objectives Although the transition to secondary progressive multiple sclerosis (SPMS) is known to be a period of uncertainty for clinicians, who may find progressive disease challenging to objectively identify, little research has explored the experiences of patients and carers specifically during this transition period. Our objective was to explore what patients and their carers understand about their disease stage and describe their experiences and perspectives on the transition to SPMS.
Design Semistructured qualitative interviews and subsequent validation focus groups were analysed using inductive thematic analysis.
Setting South East Wales, UK.
Participants 20 patients with MS and 13 carers were interviewed. Eight patients and two carers participated in focus groups.
Results Four main themes around disease progression were identified. ‘Realisation’ describes how patients came to understand they had SPMS while ‘reaction’ describes their response to this realisation. The ‘realities’ of living with SPMS, including dealing with the healthcare system during this period, were described along with ‘future challenges’ envisaged by patients and carers.
Conclusions Awareness that the transition to SPMS has occurred, and subsequent emotional reactions and coping strategies, varied widely between patients and their carers. The process of diagnosing the transition was often not transparent and some individuals wanted information to help them understand what the transition to SPMS meant for them
Correcting for mobile X-band weather radar tilt using solar interference
Precise knowledge of the antenna pointing direction is a key facet to ensure the accuracy of observations from scanning weather radars. The sun is an often-used reference point to aid accurate alignment of weather radar systems and is particularly useful when observed as interference during normal scanning operations. In this study, we combine two online solar interference approaches to determine the pointing accuracy of an X-band mobile weather radar system deployed for 26 months in northern England (54.517°N, 3.615°W). During the deployment, several shifts in the tilt of the radar system are diagnosed between site visits. One extended period of time (>11 months) is shown to have a changing tilt that is independent of human intervention. To verify the corrections derived from this combined approach, quantitative precipitation estimates (QPEs) from the radar system are compared to surface observations: an approach that takes advantage of the variations in the magnitude of partial beam blockage corrections required due to tilting of the radar system close to mountainous terrain. The observed improvements in QPE performance after correction support the use of the derived tilt corrections for further applications using the corrected dataset. Finally, recommendations for future deployments are made, with particular focus on higher latitudes where solar interference spikes show more seasonality than those at mid-latitudes
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