23 research outputs found
A fundamental mechanism for carbon-film lubricity identified by means of ab initio molecular dynamics
Different hypotheses have been proposed to explain the mechanism for the
extremely low friction coefficient of carbon coatings and its undesired
dependence on air humidity. A decisive atomistic insight is still lacking
because of the difficulties in monitoring what actually happens at the buried
sliding interface. Here we perform large-scale ab initio molecular dynamics
simulations of both undoped and silicon-doped carbon films sliding in the
presence of water. We observe the tribologically-induced surface hydroxylation
and subsequent formation of a thin film of water molecules bound to the
OH-terminated surface by hydrogen bonds. The comparative analysis of
silicon-incorporating and clean surfaces, suggests that this two-step process
can be the key phenomenon to provide high slipperiness to the carbon coatings.
The water layer is, in fact, expected to shelter the carbon surface from direct
solid-on-solid contact and make any counter surface slide extremely easily on
it. The present insight into the wettability of carbon-based films can be
useful for designing new coatings for biomedical and energy-saving applications
with environmental adaptability.Comment: 22 pages, 4 figures, 1 tabl
Insigths into the tribochemistry of silicon-doped carbon based films by ab initio analysis of water/surface interactions
Diamond and diamond-like carbon (DLC) are used as coating materials for
numerous applications, ranging from biomedicine to tribology. Recently, it has
been shown that the hydrophilicity of the carbon films can be enhanced by
silicon doping, which highly improves their biocompatibility and frictional
performances. Despite the relevance of these properties for applications, a
microscopic understanding on the effects of silicon is still lacking. Here we
apply ab initio calculations to study the interaction of water molecules with
Si-incorporated C(001) surfaces. We find that the presence of Si dopants
considerably increases the energy gain for water chemisorption and decreases
the energy barrier for water dissociation by more than 50%. We provide a
physical rational for the phenomenon by analysing the electronic charge
displacements occuring upon adsorption. We also show that once hydroxylated,
the surface is able to bind further water molecules much strongly than the
clean surface via hydrogen-bond networks. This two-step process is consistent
with and can explain the enhanced hydrophilic character observed in
carbon-based films doped by silicon
Spatial regression-based transfer learning for prediction problems
Although spatial prediction is widely used for urban and environmental
monitoring, its accuracy is often unsatisfactory if only a small number of
samples are available in the study area. The objective of this study was to
improve the prediction accuracy in such a case through transfer learning using
larger samples obtained outside the study area. Our proposal is to pre-train
latent spatial-dependent processes, which are difficult to transfer, and apply
them as additional features in the subsequent transfer learning. The proposed
method is designed to involve local spatial dependence and can be implemented
easily. This spatial-regression-based transfer learning is expected to achieve
a higher and more stable prediction accuracy than conventional learning, which
does not explicitly consider local spatial dependence. The performance of the
proposed method was examined using land price and crime predictions. These
results suggest that the proposed method successfully improved the accuracy and
stability of these spatial predictions
Scalable model selection for spatial additive mixed modeling: application to crime analysis
A rapid growth in spatial open datasets has led to a huge demand for
regression approaches accommodating spatial and non-spatial effects in big
data. Regression model selection is particularly important to stably estimate
flexible regression models. However, conventional methods can be slow for large
samples. Hence, we develop a fast and practical model-selection approach for
spatial regression models, focusing on the selection of coefficient types that
include constant, spatially varying, and non-spatially varying coefficients. A
pre-processing approach, which replaces data matrices with small inner products
through dimension reduction dramatically accelerates the computation speed of
model selection. Numerical experiments show that our approach selects the model
accurately and computationally efficiently, highlighting the importance of
model selection in the spatial regression context. Then, the present approach
is applied to open data to investigate local factors affecting crime in Japan.
The results suggest that our approach is useful not only for selecting factors
influencing crime risk but also for predicting crime events. This scalable
model selection will be key to appropriately specifying flexible and
large-scale spatial regression models in the era of big data. The developed
model selection approach was implemented in the R package spmoran
Fast simulation for optical systems addressing the curse of dimensionality of multi-photons in quantum mechanics
Photons are an elementary particle of light, whose detailed understanding
plays a key in unraveling the mysteries of quantum mechanics. However, its
counter-intuitive quantum nature makes it challenging to gain insights into its
dynamics, particularly in complex systems. Simulation is a promising tool to
resolve this issue, but previous methods are limited by the curse of
dimensionality, namely, that the number of bases increases exponentially in the
number of photons. Here we mitigate this dimensionality scaling by focusing on
optical system composed of linear-optical objects. We decompose the time
evolutionary operator on multiple photons into a group of time evolution
operators acting on a single photon. Since the dimension of a single-photon
time evolution operator is exponentially smaller than that of a multi-photon
one in the number of photons, the decomposition enables the multi-photon
simulations to be performed at a much lower computational cost. We apply this
method to basic single- and multi-photon phenomena, such as Hong-Ou-Mandel
interference and violation of the Bell-CHSH inequality, and confirm that the
calculated properties are quantitatively comparable to the experimental
results. Furthermore, our method visualizes the spatial propagation of photons
hence provides insights that aid experiment designs for quantum-enabled
technologies.Comment: 17 pages, 6 figures, 1 tabl
Quantum topology optimization of ground structures using noisy intermediate-scale quantum devices
To arrive at some viable product design, product development processes
frequently use numerical simulations and mathematical programming techniques.
Topology optimization, in particular, is one of the most promising techniques
for generating insightful design choices. Topology optimization problems reduce
to an NP-hard combinatorial optimization problem, where the combination of the
existence or absence of the material at some positions is optimized. In this
study, we examine the usage of quantum computers as a potential solution to
topology optimization problems. The proposed method consists of two variational
quantum algorithms (VQAs): the first solves the state equilibrium equation for
all conceivable material configurations, while the second amplifies the
likelihood of an optimal configuration in quantum superposition using the first
VQA's quantum state. Several experiments, including a real device experiment,
show that the proposed method successfully obtained the optimal configurations.
These findings suggest that quantum computers could be a potential tool for
solving topology optimization problems and they open the window to the
near-future product designs.Comment: 16 pages, 6 figure
Creation of crystal structure reproducing X-ray diffraction pattern without using database
When a sample's X-ray diffraction pattern (XRD) is measured, the
corresponding crystal structure is usually determined by searching for similar
XRD patterns in the database. However, if a similar XRD pattern is not found,
it is tremendously laborious to identify the crystal structure even for
experts. This case commonly happens when researchers develop novel and complex
materials. In this study, we propose a crystal structure creation scheme that
reproduces a given XRD pattern. We employed a combinatorial inverse design
method using an evolutionary algorithm and crystal morphing (Evolv&Morph)
supported by Bayesian optimization, which maximizes the similarity of the XRD
patterns between target one and those of the created crystal structures. For
sixteen different crystal structure systems with twelve simulated and four
powder target XRD patterns, Evolv&Morph successfully created crystal structures
with the same XRD pattern as the target (cosine similarity > 99% for the
simulated ones and > 96% the experimentally-measured ones). Furthermore, the
present method has merits in that it is an automated crystal structure creation
scheme, not dependent on a database. We believe that Evolv&Morph can be applied
not only to determine crystal structures but also to design materials for
specific properties.Comment: 18 pages, 5 figures, 2 tables, submitted to npjC