302 research outputs found
The Antiquity of the Avesta
Paper read before the Bombay Branch of the Royal Asiatic Society, read 26th June 1896. Dr. Gerson Da Cunha in the Chair
Linear-scaling density-functional simulations of charged point defects in Al2O3 using hierarchical sparse matrix algebra
We present calculations of formation energies of defects in an ionic solid (Al(2)O(3)) extrapolated to the dilute limit, corresponding to a simulation cell of infinite size. The large-scale calculations required for this extrapolation are enabled by developments in the approach to parallel sparse matrix algebra operations, which are central to linear-scaling density-functional theory calculations. The computational cost of manipulating sparse matrices, whose sizes are determined by the large number of basis functions present, is greatly improved with this new approach. We present details of the sparse algebra scheme implemented in the ONETEP code using hierarchical sparsity patterns, and demonstrate its use in calculations on a wide range of systems, involving thousands of atoms on hundreds to thousands of parallel processes
A converse approach to the calculation of NMR shielding tensors
We introduce an alternative approach to the first-principles calculation of
NMR shielding tensors. These are obtained from the derivative of the orbital
magnetization with respect to the application of a microscopic, localized
magnetic dipole. The approach is simple, general, and can be applied to either
isolated or periodic systems. Calculated results for simple hydrocarbons,
crystalline diamond, and liquid water show very good agreement with established
methods and experimental results.Comment: submitted to Phys. Rev. Lett; second half completely rewritten to
address referee's comment
Emergence of non-centrosymmetric topological insulating phase in BiTeI under pressure
The spin-orbit interaction affects the electronic structure of solids in
various ways. Topological insulators are one example where the spin-orbit
interaction leads the bulk bands to have a non-trivial topology, observable as
gapless surface or edge states. Another example is the Rashba effect, which
lifts the electron-spin degeneracy as a consequence of spin-orbit interaction
under broken inversion symmetry. It is of particular importance to know how
these two effects, i.e. the non-trivial topology of electronic states and
Rashba spin splitting, interplay with each other. Here we show, through
sophisticated first-principles calculations, that BiTeI, a giant bulk Rashba
semiconductor, turns into a topological insulator under a reasonable pressure.
This material is shown to exhibit several unique features such as, a highly
pressure-tunable giant Rashba spin splitting, an unusual pressure-induced
quantum phase transition, and more importantly the formation of strikingly
different Dirac surface states at opposite sides of the material.Comment: 5 figures are include
Robotic Wireless Sensor Networks
In this chapter, we present a literature survey of an emerging, cutting-edge,
and multi-disciplinary field of research at the intersection of Robotics and
Wireless Sensor Networks (WSN) which we refer to as Robotic Wireless Sensor
Networks (RWSN). We define a RWSN as an autonomous networked multi-robot system
that aims to achieve certain sensing goals while meeting and maintaining
certain communication performance requirements, through cooperative control,
learning and adaptation. While both of the component areas, i.e., Robotics and
WSN, are very well-known and well-explored, there exist a whole set of new
opportunities and research directions at the intersection of these two fields
which are relatively or even completely unexplored. One such example would be
the use of a set of robotic routers to set up a temporary communication path
between a sender and a receiver that uses the controlled mobility to the
advantage of packet routing. We find that there exist only a limited number of
articles to be directly categorized as RWSN related works whereas there exist a
range of articles in the robotics and the WSN literature that are also relevant
to this new field of research. To connect the dots, we first identify the core
problems and research trends related to RWSN such as connectivity,
localization, routing, and robust flow of information. Next, we classify the
existing research on RWSN as well as the relevant state-of-the-arts from
robotics and WSN community according to the problems and trends identified in
the first step. Lastly, we analyze what is missing in the existing literature,
and identify topics that require more research attention in the future
Emergent quantum confinement at topological insulator surfaces
Bismuth-chalchogenides are model examples of three-dimensional topological
insulators. Their ideal bulk-truncated surface hosts a single spin-helical
surface state, which is the simplest possible surface electronic structure
allowed by their non-trivial topology. They are therefore widely
regarded ideal templates to realize the predicted exotic phenomena and
applications of this topological surface state. However, real surfaces of such
compounds, even if kept in ultra-high vacuum, rapidly develop a much more
complex electronic structure whose origin and properties have proved
controversial. Here, we demonstrate that a conceptually simple model,
implementing a semiconductor-like band bending in a parameter-free
tight-binding supercell calculation, can quantitatively explain the entire
measured hierarchy of electronic states. In combination with circular dichroism
in angle-resolved photoemission (ARPES) experiments, we further uncover a rich
three-dimensional spin texture of this surface electronic system, resulting
from the non-trivial topology of the bulk band structure. Moreover, our study
reveals how the full surface-bulk connectivity in topological insulators is
modified by quantum confinement.Comment: 9 pages, including supplementary information, 4+4 figures. A high
resolution version is available at
http://www.st-andrews.ac.uk/~pdk6/pub_files/TI_quant_conf_high_res.pd
Low-Energy Effective Hamiltonian and the Surface States of Ca_3PbO
The band structure of Ca_3PbO, which possesses a three-dimensional massive
Dirac electron at the Fermi energy, is investigated in detail. Analysis of the
orbital weight distributions on the bands obtained in the first-principles
calculation reveals that the bands crossing the Fermi energy originate from the
three Pb-p orbitals and three Ca-dx2y2 orbitals. Taking these Pb-p and Ca-dx2y2
orbitals as basis wave functions, a tight-binding model is constructed. With
the appropriate choice of the hopping integrals and the strength of the
spin-orbit coupling, the constructed model sucessfully captures important
features of the band structure around the Fermi energy obtained in the
first-principles calculation. By applying the suitable basis transformation and
expanding the matrix elements in the series of the momentum measured from a
Dirac point, the low-energy effective Hamiltonian of this model is explicitely
derived and proved to be a Dirac Hamiltonain. The origin of the mass term is
also discussed. It is shown that the spin-orbit coupling and the orbitals other
than Pb-p and Ca-dx2y2 orbitals play important roles in making the mass term
finite. Finally, the surface band structures of Ca_3PbO for several types of
surfaces are investigated using the constructed tight-binding model. We find
that there appear nontrivial surface states that cannot be explained as the
bulk bands projected on the surface Brillouin zone. The relation to the
topological insulator is also discussed.Comment: 11 page
Linear-scaling density-functional simulations of charged point defects in Al2O3 using hierarchical sparse matrix algebra
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