436 research outputs found
MCViNE -- An object oriented Monte Carlo neutron ray tracing simulation package
MCViNE (Monte-Carlo VIrtual Neutron Experiment) is a versatile Monte Carlo
(MC) neutron ray-tracing program that provides researchers with tools for
performing computer modeling and simulations that mirror real neutron
scattering experiments. By adopting modern software engineering practices such
as using composite and visitor design patterns for representing and accessing
neutron scatterers, and using recursive algorithms for multiple scattering,
MCViNE is flexible enough to handle sophisticated neutron scattering problems
including, for example, neutron detection by complex detector systems, and
single and multiple scattering events in a variety of samples and sample
environments. In addition, MCViNE can take advantage of simulation components
in linear-chain-based MC ray tracing packages widely used in instrument design
and optimization, as well as NumPy-based components that make prototypes useful
and easy to develop. These developments have enabled us to carry out detailed
simulations of neutron scattering experiments with non-trivial samples in
time-of-flight inelastic instruments at the Spallation Neutron Source. Examples
of such simulations for powder and single-crystal samples with various
scattering kernels, including kernels for phonon and magnon scattering, are
presented. With simulations that closely reproduce experimental results,
scattering mechanisms can be turned on and off to determine how they contribute
to the measured scattering intensities, improving our understanding of the
underlying physics.Comment: 34 pages, 14 figure
Materials Cloud, a platform for open computational science
Materials Cloud is a platform designed to enable open and seamless sharing of
resources for computational science, driven by applications in materials
modelling. It hosts 1) archival and dissemination services for raw and curated
data, together with their provenance graph, 2) modelling services and virtual
machines, 3) tools for data analytics, and pre-/post-processing, and 4)
educational materials. Data is citable and archived persistently, providing a
comprehensive embodiment of the FAIR principles that extends to computational
workflows. Materials Cloud leverages the AiiDA framework to record the
provenance of entire simulation pipelines (calculations performed, codes used,
data generated) in the form of graphs that allow to retrace and reproduce any
computed result. When an AiiDA database is shared on Materials Cloud, peers can
browse the interconnected record of simulations, download individual files or
the full database, and start their research from the results of the original
authors. The infrastructure is agnostic to the specific simulation codes used
and can support diverse applications in computational science that transcend
its initial materials domain.Comment: 19 pages, 8 figure
Comprehensive mobility study of silicon nanowire transistors using multi-subband models
Spatial confinement is important in advanced More Moore devices, such as nanowire transistors
(NWTs), where the basic charge transport properties must be revised beyond the bulk crystal
assumptions. This work presents a comprehensive and general overview of the electron mobility in
aggressively-scaled SiNWTs in order to demonstrate the effect of quantum confinement on this topic,
establishing its dependence on numerous physical factors (shape, diameter, and orientation). The
mobility evaluation makes use of a unique simulation framework and innovative multi-subband
calculations of the scattering rates.Weshow that (1) the effect of surface roughness scattering is more
pronounced at higher sheet densities, (2) ionized impurity scattering seriously degrades the mobility
in highly-doped NWTs, and (3) the cross-section shape affects directly the subband parameters and
the mobility, with the ellipticalNWTsgiving the best performance for the same cross-sectional area.European UnionÊŒs Horizon 2020 research
and innovation programme under grant agreement No 688 101 SUPERAID7IncorporaciĂłn Fellowship scheme under grant agreement No. IJC2019-040003-I (MICINN/AEI
Quantum transport through MoS constrictions defined by photodoping
We present a device scheme to explore mesoscopic transport through molybdenum
disulfide (MoS) constrictions using photodoping. The devices are based on
van-der-Waals heterostructures where few-layer MoS flakes are partially
encapsulated by hexagonal boron nitride (hBN) and covered by a few-layer
graphene flake to fabricate electrical contacts. Since the as-fabricated
devices are insulating at low temperatures, we use photo-induced remote doping
in the hBN substrate to create free charge carriers in the MoS layer. On
top of the device, we place additional metal structures, which define the shape
of the constriction and act as shadow masks during photodoping of the
underlying MoS/hBN heterostructure. Low temperature two- and four-terminal
transport measurements show evidence of quantum confinement effects.Comment: 9 pages, 6 figure
Quantum ESPRESSO toward the exascale
Quantum ESPRESSO is an open-source distribution of computer codes for quantum-mechanical materials modeling, based on density-functional theory, pseudopotentials, and plane waves, and renowned for its performance on a wide range of hardware architectures, from laptops to massively parallel computers, as well as for the breadth of its applications. In this paper, we present a motivation and brief review of the ongoing effort to port Quantum ESPRESSO onto heterogeneous architectures based on hardware accelerators, which will overcome the energy constraints that are currently hindering the way toward exascale computing
A corner reflector of graphene Dirac fermions as a phonon-scattering sensor
Dirac fermion optics exploits the refraction of chiral fermions across
optics-inspired Klein-tunneling barriers defined by high-transparency p-n
junctions. We consider the corner reflector (CR) geometry introduced in optics
or radars. We fabricate Dirac fermion CRs using bottom-gate-defined barriers in
hBN-encapsulated graphene. By suppressing transmission upon multiple internal
reflections, CRs are sensitive to minute phonon scattering rates. We report on
doping-independent CR transmission in quantitative agreement with a simple
scattering model including thermal phonon scattering. As a new signature of
CRs, we observe Fabry-P\'erot oscillations at low temperature, consistent with
single-path reflections. Finally, we demonstrate high-frequency operation which
promotes CRs as fast phonon detectors. Our work establishes the relevance of
Dirac fermion optics in graphene and opens a route for its implementation in
topological Dirac matter.Comment: 11 pages, 4 figure
Scaleable nanomanufacturing of metasurfaces using microsphere photolithography
âThe cost-effective manufacturing of metasurfaces over large areas is a critical issue that limits their implementations. Microsphere photolithography (MPL) uses a scalable self-assembled microsphere array as an optical element to focus collimated light to nanoscale photonic jets in a photoresist layer. This dissertation investigates the fabrication capabilities, process control, and potential applications of MPL. First, the MPL concept is applied to the fabrication of metasurfaces with engineered IR absorption (e.g. perfect absorption with multiband/broadband and wavelength/polarization dependences). Improving the patterning of the photoresist requires a fundamental understanding the photochemical photonic jet interactions. The dissertation presents a model of the MPL process with a cellular automata algorithm to simulate the development process. The model accurately predicts the size and shape of the features generated from MPL. It enables the identification of fabrication conditions to improve the resolution for the MPL process. Finally, the dissertation discusses the potential for a reusable microsphere array. Control of the contact forces is critical for minimizing the gap in between the microsphere array and the substrate and maintaining the consistent performance. Overall, the dissertation provides a foundation for understanding the process-structure-performance relationships for the fabrication of metasurfaces using microsphere photolithography. The use of the MPL for the fabrication of metasurfaces, with application such as sensing and thermal management, is novel as is the modeling of the MPL processâ--Abstract, page iv
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