1,186 research outputs found
Manipulating thermal conductivity through substrate coupling
We report a new approach to the thermal conductivity manipulation --
substrate coupling. Generally, the phonon scattering with substrates can
decrease the thermal conductivity, as observed in recent experiments. However,
we find that at certain regions, the coupling to substrates can increase the
thermal conductivity due to a reduction of anharmonic phonon scattering induced
by shift of the phonon band to the low wave vector. In this way, the thermal
conductivity can be efficiently manipulated via coupling to different
substrates, without changing or destroying the material structures. This idea
is demonstrated by calculating the thermal conductivity of modified
double-walled carbon nanotubes and also by the ice nanotubes coupled within
carbon nanotubes.Comment: 5 figure
Finite temperature properties of clusters by replica exchange metadynamics: the water nonamer
We introduce an approach for the accurate calculation of thermal properties
of classical nanoclusters. Based on a recently developed enhanced sampling
technique, replica exchange metadynamics, the method yields the true free
energy of each relevant cluster structure, directly sampling its basin and
measuring its occupancy in full equilibrium. All entropy sources, whether
vibrational, rotational anharmonic and especially configurational -- the latter
often forgotten in many cluster studies -- are automatically included. For the
present demonstration we choose the water nonamer (H2O)9, an extremely simple
cluster which nonetheless displays a sufficient complexity and interesting
physics in its relevant structure spectrum. Within a standard TIP4P potential
description of water, we find that the nonamer second relevant structure
possesses a higher configurational entropy than the first, so that the two free
energies surprisingly cross for increasing temperature.Comment: J. Am. Chem. Soc. 133, 2535-2540 (2011
Thermal conductivity of epitaxial graphene nanoribbons on SiC: effect of substrate
We study the effect of SiC substrate on thermal conductivity of epitaxial
graphene nanoribbons (GNRs) using the nonequilibrium molecular dynamics method.
We show that the substrate has strong interaction with single-layer GNRs during
the thermal transport, which largely reduces the thermal conductivity. The
thermal conductivity characteristics of suspended GNRs are well preserved in
the second GNR layers of bilayer GNR, which has a weak van der Waals
interaction with the underlying structures. The out-of-plane phonon mode is
found to play a critical role on the thermal conductivity variation of the
second GNR layer induced by the underlying structures.Comment: 5 pages, 4 figures, 1 tabl
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
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