541 research outputs found
Magnetoresistance of compensated semimetals in confined geometries
Two-component conductors -- e.g., semi-metals and narrow band semiconductors
-- often exhibit unusually strong magnetoresistance in a wide temperature
range. Suppression of the Hall voltage near charge neutrality in such systems
gives rise to a strong quasiparticle drift in the direction perpendicular to
the electric current and magnetic field. This drift is responsible for a strong
geometrical increase of resistance even in weak magnetic fields. Combining the
Boltzmann kinetic equation with sample electrostatics, we develop a microscopic
theory of magnetotransport in two and three spatial dimensions. The compensated
Hall effect in confined geometry is always accompanied by electron-hole
recombination near the sample edges and at large-scale inhomogeneities. As the
result, classical edge currents may dominate the resistance in the vicinity of
charge compensation. The effect leads to linear magnetoresistance in two
dimensions in a broad range of parameters. In three dimensions, the
magnetoresistance is normally quadratic in the field, with the linear regime
restricted to rectangular samples with magnetic field directed perpendicular to
the sample surface. Finally, we discuss the effects of heat flow and
temperature inhomogeneities on the magnetoresistance.Comment: 22 pages, 7 figures, published versio
Assessment of the impacts of clear-cutting on soil loss by water erosion in Italian forests: First comprehensive monitoring and modelling approach
Abstract As a member of the European Union, Italy has committed to the maintenance and protection of its forests based on sustainable forest development and management practices. According to Eurostat, Italy has the seventh largest forest surface available for wood supply in the EU-28, which is equal to 8.086 million hectares. For 2012, the Italian National Institute of Statistics estimated the total roundwood production of Italy to be 7.7 million m3, from a harvested forest surface of 61,038 ha. Large parts of the country's forests, mainly located in vulnerable mountainous landscapes that are highly sensitive to environmental changes, are subject to anthropogenic disturbance driven by wood supply interests. Despite the extensive logging activities and the well-known impacts that such management practices have on the soil-related forest ecosystems, there is a lack of spatially and temporally explicit information about the removal of trees. Hence, this study aims to: i) assess the soil loss by water erosion in Italian forest areas, ii) map forest harvests and iii) evaluate the effects of logging activities in terms of soil loss by means of comprehensive remote sensing and GIS modelling techniques. The study area covers about 785.6 × 104 ha, which corresponds to the main forest units of the CORINE land cover 2006 database (i.e. broad-leaved forests, coniferous forests and mixed forests). Annual forest logging activities were mapped using Landsat imagery. Validation procedures were applied. A revised version of the Universal Soil Loss Equation (USLE) was used to predict the soil loss potential due to rill and inter-rill processes. To ensure a thorough modelling approach, the input parameters were calculated using the original methods reported in the USDA handbooks. The derived high-resolution data regarding forest cover change shows that 317,535 ha (4.04% of the total forest area in Italy) were harvested during the period under review. The predicted long-term annual average soil loss rate was 0.54 Mg ha− 1 yr− 1. The average rate of soil loss in forests that remained undisturbed during the modelled period is equal to 0.33 Mg ha− 1 yr− 1. Notably, about half of the soil loss (45.3%) was predicted for the logged areas, even though these cover only about 10.6% of the Italian forests. The identified erosion hotspots may represent a serious threat for the soil-related forest ecosystems, and are in contrast to the EC Thematic Strategy for Soil Protection and Water Framework Directive
Coulomb interaction in graphene: Relaxation rates and transport
We analyze the inelastic electron-electron scattering in undoped graphene
within the Keldysh diagrammatic approach. We demonstrate that finite
temperature strongly affects the screening properties of graphene, which, in
turn, influences the inelastic scattering rates as compared to the
zero-temperature case. Focussing on the clean regime, we calculate the quantum
scattering rate which is relevant for dephasing of interference processes. We
identify an hierarchy of regimes arising due to the interplay of a plasmon
enhancement of the scattering and finite-temperature screening of the
interaction. We further address the energy relaxation and transport scattering
rates in graphene. We find a non-monotonic energy dependence of the inelastic
relaxation rates in clean graphene which is attributed to the resonant
excitation of plasmons. Finally, we discuss the temperature dependence of the
conductivity at the Dirac point in the presence of both interaction and
disorder. Our results complement the kinetic-equation and hydrodynamic
approaches for the collision-limited conductivity of clean graphene and can be
generalized to the treatment of physics of inelastic processes in strongly
non-equilibrium setups.Comment: 28 pages, 16 figure
SchNet - a deep learning architecture for molecules and materials
Deep learning has led to a paradigm shift in artificial intelligence,
including web, text and image search, speech recognition, as well as
bioinformatics, with growing impact in chemical physics. Machine learning in
general and deep learning in particular is ideally suited for representing
quantum-mechanical interactions, enabling to model nonlinear potential-energy
surfaces or enhancing the exploration of chemical compound space. Here we
present the deep learning architecture SchNet that is specifically designed to
model atomistic systems by making use of continuous-filter convolutional
layers. We demonstrate the capabilities of SchNet by accurately predicting a
range of properties across chemical space for \emph{molecules and materials}
where our model learns chemically plausible embeddings of atom types across the
periodic table. Finally, we employ SchNet to predict potential-energy surfaces
and energy-conserving force fields for molecular dynamics simulations of small
molecules and perform an exemplary study of the quantum-mechanical properties
of C-fullerene that would have been infeasible with regular ab initio
molecular dynamics
Generation of Relativistic Electron Bunches with Arbitrary Current Distribution via Transverse-to-Longitudinal Phase Space Exchange
We propose a general method for tailoring the current distribution of
relativistic electron bunches. The technique relies on a recently proposed
method to exchange the longitudinal phase space emittance with one of the
transverse emittances. The method consists of transversely shaping the bunch
and then converting its transverse profile into a current profile via a
transverse-to-longitudinal phase-space-exchange beamline. We show that it is
possible to tailor the current profile to follow, in principle, any desired
distributions. We demonstrate, via computer simulations, the application of the
method to generate trains of microbunches with tunable spacing and
linearly-ramped current profiles. We also briefly explore potential
applications of the technique.Comment: 13 pages, 17 figure
SchNetPack 2.0: A neural network toolbox for atomistic machine learning
SchNetPack is a versatile neural networks toolbox that addresses both the
requirements of method development and application of atomistic machine
learning. Version 2.0 comes with an improved data pipeline, modules for
equivariant neural networks as well as a PyTorch implementation of molecular
dynamics. An optional integration with PyTorch Lightning and the Hydra
configuration framework powers a flexible command-line interface. This makes
SchNetPack 2.0 easily extendable with custom code and ready for complex
training task such as generation of 3d molecular structures
Coulomb drag in graphene near the Dirac point
We study Coulomb drag in double-layer graphene near the Dirac point. A
particular emphasis is put on the case of clean graphene, with transport
properties dominated by the electron-electron interaction. Using the quantum
kinetic equation framework, we show that the drag becomes -independent in
the clean limit, , where is temperature and
impurity scattering rate. For stronger disorder (or lower temperature), , where is the interaction strength, the kinetic
equation agrees with the leading-order () perturbative result. At
still lower temperatures, (diffusive regime) this contribution
gets suppressed, while the next-order () contribution becomes
important; it yields a peak centered at the Dirac point with a magnitude that
grows with lowering .Comment: 3 figures, with expanded Supplemental Material attached as an
appendi
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