1,462 research outputs found
LMSFC: A Novel Multidimensional Index based on Learned Monotonic Space Filling Curves
The recently proposed learned indexes have attracted much attention as they
can adapt to the actual data and query distributions to attain better search
efficiency. Based on this technique, several existing works build up indexes
for multi-dimensional data and achieve improved query performance. A common
paradigm of these works is to (i) map multi-dimensional data points to a
one-dimensional space using a fixed space-filling curve (SFC) or its variant
and (ii) then apply the learned indexing techniques. We notice that the first
step typically uses a fixed SFC method, such as row-major order and z-order. It
definitely limits the potential of learned multi-dimensional indexes to adapt
variable data distributions via different query workloads. In this paper, we
propose a novel idea of learning a space-filling curve that is carefully
designed and actively optimized for efficient query processing. We also
identify innovative offline and online optimization opportunities common to
SFC-based learned indexes and offer optimal and/or heuristic solutions.
Experimental results demonstrate that our proposed method, LMSFC, outperforms
state-of-the-art non-learned or learned methods across three commonly used
real-world datasets and diverse experimental settings.Comment: Extended Version. Accepted by VLDB 202
The effects of halogen elements on the opening of an icosahedral B12 framework
The fully halogenated or hydrogenated B12X12 (X = H, F, Cl, Br and I)
clusters are confirmed to be icosahedral. On the other hand, the bare B12
cluster is shown to have a planar structure. A previous study showed that a
transformation from an icosahedron to a plane happens when 5 to 7 iodine atoms
are substituted. Later, the transition was confirmed to be seven iodine
substitutions based on an infrared spectroscopy study. In this study, we
investigated the effects of different halogen atoms on the opening of the B12
icosahedral cage by means of density functional theory calculations. We found
that the halogen elements do not have significant effects on the geometries of
the clusters. The computed IR spectra show similar representative peaks for all
halogen substituted clusters. Interestingly, we found a blue-shift in the IR
spectra with the increase in the mass of the halogen atoms. Further, we
compared the Gibbs free energies at different temperatures for different
halogen atoms. The results show that the Gibbs free energy differences between
open and close structures of B12X7 become larger when heavier halogen atoms are
present. This interesting finding was subsequently investigated by energy
decomposition analysis
Topological Dirac states beyond orbitals for silicene on SiC(0001) surface
The discovery of intriguing properties related to the Dirac states in
graphene has spurred huge interest in exploring its two-dimensional group-IV
counterparts, such as silicene, germanene, and stanene. However, these
materials have to be obtained via synthesizing on substrates with strong
interfacial interactions, which usually destroy their intrinsic
()-orbital Dirac states. Here we report a theoretical study on the
existence of Dirac states arising from the orbitals instead of
orbitals in silicene on 4H-SiC(0001), which survive in spite of the strong
interfacial interactions. We also show that the exchange field together with
the spin-orbital coupling give rise to a detectable band gap of 1.3 meV. Berry
curvature calculations demonstrate the nontrivial topological nature of such
Dirac states with a Chern number , presenting the potential of realizing
quantum anomalous Hall effect for silicene on SiC(0001). Finally, we construct
a minimal effective model to capture the low-energy physics of this system.
This finding is expected to be also applicable to germanene and stanene, and
imply great application potentials in nanoelectronics.Comment: 6 Figures , Accepted by Nano Letter
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