29 research outputs found
Effective mass theory of monolayer \delta-doping in the high-density limit
Monolayer \delta-doped structures in silicon have attracted renewed interest
with their recent incorporation into atomic-scale device fabrication strategies
as source and drain electrodes and in-plane gates. Modeling the physics of
\delta-doping at this scale proves challenging, however, due to the large
computational overhead associated with ab initio and atomistic methods. Here,
we develop an analytical theory based on an effective mass approximation. We
specifically consider the Si:P materials system, and the limit of high donor
density, which has been the subject of recent experiments. In this case,
metallic behavior including screening tends to smooth out the local disorder
potential associated with random dopant placement. While smooth potentials may
be difficult to incorporate into microscopic, single-electron analyses, the
problem is easily treated in the effective mass theory by means of a jellium
approximation for the ionic charge. We then go beyond the analytic model,
incorporating exchange and correlation effects within a simple numerical model.
We argue that such an approach is appropriate for describing realistic,
high-density, highly disordered devices, providing results comparable to
density functional theory, but with greater intuitive appeal, and lower
computational effort. We investigate valley coupling in these structures,
finding that valley splitting in the low-lying \Gamma band grows much more
quickly than the \Gamma-\Delta band splitting at high densities. We also find
that many-body exchange and correlation corrections affect the valley splitting
more strongly than they affect the band splitting
Microscopy as a statistical, Rényi-Ulam, half-lie game: a new heuristic search strategy to accelerate imaging
Finding a fluorescent target in a biological environment is a common and pressing microscopy problem. This task is formally analogous to the canonical search problem. In ideal (noise-free, truthful) search problems, the well-known binary search is optimal. The case of half-lies, where one of two responses to a search query may be deceptive, introduces a richer, Rényi-Ulam problem and is particularly relevant to practical microscopy. We analyse microscopy in the contexts of Rényi-Ulam games and half-lies, developing a new family of heuristics. We show the cost of insisting on verification by positive result in search algorithms; for the zero-half-lie case bisectioning with verification incurs a 50% penalty in the average number of queries required. The optimal partitioning of search spaces directly following verification in the presence of random half-lies is determined. Trisectioning with verification is shown to be the most efficient heuristic of the family in a majority of cases
Correlating the Energetics and Atomic Motions of the Metal-Insulator Transition of M1 Vanadium Dioxide
Materials that undergo reversible metal-insulator transitions are obvious
candidates for new generations of devices. For such potential to be realised,
the underlying microscopic mechanisms of such transitions must be fully
determined. In this work we probe the correlation between the energy landscape
and electronic structure of the metal-insulator transition of vanadium dioxide
and the atomic motions occurring using first principles calculations and high
resolution X-ray diffraction. Calculations find an energy barrier between the
high and low temperature phases corresponding to contraction followed by
expansion of the distances between vanadium atoms on neighbouring sub-lattices.
X-ray diffraction reveals anisotropic strain broadening in the low temperature
structure's crystal planes, however only for those with spacings affected by
this compression/expansion. GW calculations reveal that traversing this barrier
destabilises the bonding/anti-bonding splitting of the low temperature phase.
This precise atomic description of the origin of the energy barrier separating
the two structures will facilitate more precise control over the transition
characteristics for new applications and devices.Comment: 11 Pages, 8 Figure
Chiminey: Reliable Computing and Data Management Platform in the Cloud
The enabling of scientific experiments that are embarrassingly parallel, long
running and data-intensive into a cloud-based execution environment is a
desirable, though complex undertaking for many researchers. The management of
such virtual environments is cumbersome and not necessarily within the core
skill set for scientists and engineers. We present here Chiminey, a software
platform that enables researchers to (i) run applications on both traditional
high-performance computing and cloud-based computing infrastructures, (ii)
handle failure during execution, (iii) curate and visualise execution outputs,
(iv) share such data with collaborators or the public, and (v) search for
publicly available data.Comment: Preprint, ICSE 201
A study of size-dependent properties of MoS2 monolayer nanoflakes using density-functional theory
Novel physical phenomena emerge in ultra-small sized nanomaterials. We study the limiting small-size-dependent properties of MoS2 monolayer rhombic nanoflakes using density-functional theory on structures of size up to Mo35S70 (1.74 nm). We investigate the structural and electronic properties as functions of the lateral size of the nanoflakes, finding zigzag is the most stable edge configuration, and that increasing size is accompanied by greater stability. We also investigate passivation of the structures to explore realistic settings, finding increased HOMO-LUMO gaps and energetic stability. Understanding the size-dependent properties will inform efforts to engineer electronic structures at the nano-scale