1,927 research outputs found
Massive Dirac fermions and spin physics in an ultrathin film of topological insulator
We study transport and optical properties of the surface states which lie in
the bulk energy gap of a thin-film topological insulator. When the film
thickness is comparable with the surface state decay length into the bulk, the
tunneling between the top and bottom surfaces opens an energy gap and form two
degenerate massive Dirac hyperbolas. Spin dependent physics emerges in the
surface bands which are vastly different from the bulk behavior. These include
the surface spin Hall effects, spin dependent orbital magnetic moment, and spin
dependent optical transition selection rule which allows optical spin
injection. We show a topological quantum phase transition where the Chern
number of the surface bands changes when varying the thickness of the thin
film.Comment: 7 pages, 5 figure
Possible singlet and triplet superconductivity on honeycomb lattice
We study the possible superconducting pairing symmetry mediated by spin and
charge fluctuations on the honeycomb lattice using the extended Hubbard model
and the random-phase-approximation method. From to doping levels,
a spin-singlet -wave is shown to be the leading
superconducting pairing symmetry when only the on-site Coulomb interaction
is considered, with the gap function being a mixture of the nearest-neighbor
and next-nearest-neighbor pairings. When the offset of the energy level between
the two sublattices exceeds a critical value, the most favorable pairing is a
spin-triplet -wave which is mainly composed of the next-nearest-neighbor
pairing. We show that the next-nearest-neighbor Coulomb interaction is also
in favor of the spin-triplet -wave pairing.Comment: 6 pages, 4 figure
Calculations of optical rotation: Influence of molecular structure
Ab initio Hartree-Fock (HF) method and Density Functional Theory (DFT) were used to calculate the optical rotation of 26 chiral compounds. The effects of theory and basis sets used for calculation, solvents influence on the geometry and values of calculated optical rotation were all discussed. The polarizable continuum model, included in the calculation, did not improve the accuracy effectively, but it was superior to γs. Optical rotation of five or sixmembered of cyclic compound has been calculated and 17 pyrrolidine or piperidine derivatives which were calculated by HF and DFT methods gave acceptable predictions. The nitrogen atom affects the calculation results dramatically, and it is necessary in the molecular structure in order to get an accurate computation result. Namely, when the nitrogen atom was substituted by oxygen atom in the ring, the calculation result deteriorated
A Parallel Batch-Dynamic Data Structure for the Closest Pair Problem
We propose a theoretically-efficient and practical parallel batch-dynamic
data structure for the closest pair problem. Our solution is based on a serial
dynamic closest pair data structure by Golin et al., and supports batches of
insertions and deletions in parallel. For a data set of size , our data
structure supports a batch of insertions or deletions of size in
expected work and depth
with high probability, and takes linear space. The key techniques for achieving
these bounds are a new work-efficient parallel batch-dynamic binary heap, and
careful management of the computation across sets of points to minimize work
and depth.
We provide an optimized multicore implementation of our data structure using
dynamic hash tables, parallel heaps, and dynamic -d trees. Our experiments
on a variety of synthetic and real-world data sets show that it achieves a
parallel speedup of up to 38.57x (15.10x on average) on 48 cores with
hyper-threading. In addition, we also implement and compare four parallel
algorithms for static closest pair problem, for which we are not aware of any
existing practical implementations. On 48 cores with hyper-threading, the
static algorithms achieve up to 51.45x (29.42x on average) speedup, and Rabin's
algorithm performs the best on average. Comparing our dynamic algorithm to the
fastest static algorithm, we find that it is advantageous to use the dynamic
algorithm for batch sizes of up to 20\% of the data set. As far as we know, our
work is the first to experimentally evaluate parallel closest pair algorithms,
in both the static and the dynamic settings
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