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

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

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

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 (hBN). For this, we model ARPES spectra and intensity maps for five microscopic models used previously to describe moire superlattice in graphene/hBN systems. We show that detailed analysis of these features can be used to pin down the microscopic mechanism of the interaction between graphene and hBN. We also analyze how the presence of a moire-periodic strain in graphene or scattering of photoemitted electrons off hBN can be distinguished from the miniband formation.Comment: 8.5 pages and 9 figures; version published in Phys. Rev.

Deriving snow water equivalent using cosmic-ray neutron sensors from the COSMOS-UK network for modelling snowmelt floods

The COSMOS-UK sensor network has the potential to provide new insights into extreme snowfall and snowmelt events in the UK and to improve the modelling of snowmelt floods. The network consist of approximately 50 measurement sites, each equipped with a Cosmic-Ray Neutron Sensor (CRNS). A number of these sites additionally include a “SnowFox” sensor for measuring snow water equivalent (SWE) and an ultrasonic snow depth sensor. Although the CRNS is currently used to produce estimates of soil moisture, it is also sensitive to water held as a snowpack. Moreover, the large (hundreds of metres) footprint of the CRNS potentially allows representative measurements of SWE even for inhomogeneous snowpacks. However, to date, there has been little attempt to produce snow products using the COSMOS-UK network, and soil moisture estimates during snowfall events are simply removed from the record. Here, a method is developed for using the COSMOS-UK network to derive snow products for the UK, where shallow, ephemeral snowpacks are the norm. The challenges posed by noise from the random nature of cosmic ray events, and the problem of separating the snow signal from moisture within the soil, are discussed. A comparison is made of SWE derived from the COSMOS-UK network and modelled using the snow hydrology component of the Grid-to-Grid (G2G) distributed hydrological model, and the effect on simulated river flows discussed

Zero-energy modes and valley asymmetry in the Hofstadter spectrum of bilayer graphene van der Waals heterostructures with hBN

We investigate the magnetic minibands of a heterostructure consisting of bilayer graphene (BLG) and hexagonal boron nitride (hBN) by numerically diagonalizing a two-band Hamiltonian that describes electrons in BLG in the presence of a moire potential. Due to inversion-symmetry breaking characteristic for the moire potential, the valley symmetry of the spectrum is broken, but despite this, the zero-energy Landau level in BLG survives, albeit with reduced degeneracy. In addition, we derive effective models for the low-energy features in the magnetic minibands and demonstrate the appearance of secondary Dirac points in the valence band, which we confirm by numerical simulations. Then, we analyze how single-particle gaps in the fractal energy spectrum produce a sequence of incompressible states observable under a variation of carrier density and magnetic field.Comment: 8 pages, 4 figure

Assessing precipitation from a dual-polarisation X-band radar campaign using the Grid-to-Grid hydrological model

A set of Quantitative Precipitation Estimates (QPEs) from a dual-polarisation X-band radar observation campaign in a mountainous area of Northern Scotland is assessed with reference to observed river flows as well as being compared to estimates from the UK C-band radar and raingauge networks. Employing estimation methods of varying complexity, the X-band QPEs are trialled as alternative inputs to Grid-to-Grid (G2G), a distributed hydrological model, to produce simulated river flows for comparison with observations. This hydrological assessment complements and extends a previous meteorological assessment that used point raingauge data only. Precipitation estimates for two periods over the observation campaign in 2016 (March to April and June to August) are assessed. During the second period, increased incorporation of dual-polarisation variables into the radar processing chain is found to be of considerable benefit, whereas during the first period the low height of the melting layer often restricts their use. As a result of the complex topography in Northern Scotland, the Lowest Usable Elevation (LUE) of the X-band radar observations is found to be a stronger indicator of the hydrological model performance than range from the radar. For catchments with an LUE of less than 3 km, the best X-band QPE typically performs better for modelling river flow than using an estimate from the UK C-band radar network. The hydrological assessment framework used here brings fresh insights into the performance of the different QPEs, as well as providing a stimulus for targeted improvements to dual-polarisation radar-based QPEs that have wider relevance beyond the case study situation

Electron quantum metamaterials in van der Waals heterostructures

In recent decades, scientists have developed the means to engineer synthetic periodic arrays with feature sizes below the wavelength of light. When such features are appropriately structured, electromagnetic radiation can be manipulated in unusual ways, resulting in optical metamaterials whose function is directly controlled through nanoscale structure. Nature, too, has adopted such techniques -- for example in the unique coloring of butterfly wings -- to manipulate photons as they propagate through nanoscale periodic assemblies. In this Perspective, we highlight the intriguing potential of designer sub-electron wavelength (as well as wavelength-scale) structuring of electronic matter, which affords a new range of synthetic quantum metamaterials with unconventional responses. Driven by experimental developments in stacking atomically layered heterostructures -- e.g., mechanical pick-up/transfer assembly -- atomic scale registrations and structures can be readily tuned over distances smaller than characteristic electronic length-scales (such as electron wavelength, screening length, and electron mean free path). Yet electronic metamaterials promise far richer categories of behavior than those found in conventional optical metamaterial technologies. This is because unlike photons that scarcely interact with each other, electrons in subwavelength structured metamaterials are charged, and strongly interact. As a result, an enormous variety of emergent phenomena can be expected, and radically new classes of interacting quantum metamaterials designed

Large tunable valley splitting in edge-free graphene quantum dots on boron nitride

Coherent manipulation of binary degrees of freedom is at the heart of modern quantum technologies. Graphene offers two binary degrees: the electron spin and the valley. Efficient spin control has been demonstrated in many solid state systems, while exploitation of the valley has only recently been started, yet without control on the single electron level. Here, we show that van-der Waals stacking of graphene onto hexagonal boron nitride offers a natural platform for valley control. We use a graphene quantum dot induced by the tip of a scanning tunneling microscope and demonstrate valley splitting that is tunable from -5 to +10 meV (including valley inversion) by sub-10-nm displacements of the quantum dot position. This boosts the range of controlled valley splitting by about one order of magnitude. The tunable inversion of spin and valley states should enable coherent superposition of these degrees of freedom as a first step towards graphene-based qubits