426 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
R-matrix Floquet theory for laser-assisted electron-atom scattering
A new version of the R-matrix Floquet theory for laser-assisted electron-atom
scattering is presented. The theory is non-perturbative and applicable to a
non-relativistic many-electron atom or ion in a homogeneous linearly polarized
field. It is based on the use of channel functions built from field-dressed
target states, which greatly simplifies the general formalism.Comment: 18 pages, LaTeX2e, submitted to J.Phys.
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 âdennis.â 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 âSnowFoxâ 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
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
The transition from the adiabatic to the sudden limit in core level photoemission: A model study of a localized system
We consider core electron photoemission in a localized system, where there is
a charge transfer excitation. The system is modelled by three electron levels,
one core level and two outer levels. The model has a Coulomb interaction
between these levels and the continuum states into which the core electron is
emitted. The model is simple enough to allow an exact numerical solution, and
with a separable potential an analytic solution. We calculate the ratio
r(omega) between the weights of the satellite and the main peak as a function
of the photon energy omega. The transition from the adiabatic to the sudden
limit takes place for quite small photoelectron kinetic energies. For such
small energies, the variation of the dipole matrix element is substantial and
described by the energy scale Ed. Without the coupling to the photoelectron,
the corresponding ratio r0(omega) is determined by Ed and the satellite
excitation energy dE. When the interaction potential with the continuum states
is introduced, a new energy scale Es=1/(2Rs^2) enters, where Rs is a length
scale of the interaction potential. At threshold there is typically a (weak)
constructive interference between intrinsic and extrinsic contributions, and
the ratio r(omega)/r0(omega) is larger than its limiting value for large omega.
The interference becomes small or weakly destructive for photoelectron energies
of the order Es. For larger energies r(omega)/r0(omega) therefore typically has
a weak undershoot. If this undershoot is neglected, r(omega)/r0(omega) reaches
its limiting value on the energy scale Es.Comment: 18 pages, latex2e, 13 eps figure
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