1,500 research outputs found
Development of simulation package 'ELSES' for extra-large-scale electronic-structure calculation
An early-stage version of simulation package ' ELSES' (Extra-Large-Scale
Electronic-Structure calculation) is developed for electronic structure and
dynamics of large systems, particularly, nm-scale or 10nm-scale systems
(www.elses.jp). Input and output files are written in the Extensible Markup
Language (XML) style for general users. Related pre-/post-simulation tools are
also available. Practical work flow and example are described. A test
calculation of GaAs bulk system is shown to demonstrate that the present code
can handle systems with more than one atom species. Several future aspects are
also discussed.Comment: 7 Pages, 4 figures. A PDF file in better graphics is available at
http://fujimac.t.u-tokyo.ac.jp/lses/index_e.htm
Small values of signed harmonic sums
For every \u3c4 08R and every integer N, let mN(\u3c4) be the minimum of the distance of \u3c4 from the sums 11n=1Nsn/n, where s1,\u2026,sn 08 121,+1. We prove that mN(\u3c4
Unravelling the roles of size, ligands and pressure in the piezochromic properties of CdS nanocrystals
Understanding the effects of pressure-induced deformations on the optoelectronic properties of nanomaterials is important not only from the fundamental point of view, but also for po- tential applications such as stress sensors and electromechanical devices. Here we describe the novel insights into these piezochromic ef- fects gained from using a linear-scaling den- sity functional theory framework and an elec- tronic enthalpy scheme, which allow us to ac- curately characterize the electronic structure of CdS nanocrystals with a zincblende-like core of experimentally relevant size. In particu- lar we focus on unravelling the complex inter- play of size and surface (phenyl) ligands with pressure. We show that pressure-induced de- formations are not simple isotropic scaling of the original structures and that the change in HOMO-LUMO gap with pressure results from two competing factors: (i) a bulk-like linear in- crease due to compression, which is offset by (ii) distortions/disorder and, to a lesser ex- tent, orbital hybridization induced by ligands affecting the frontier orbitals. Moreover, we observe that the main peak in the optical ab- sorption spectra is systematically red-shifted or blue-shifted, as pressure is increased up to 5 GPa, depending on the presence or absence of phenyl ligands. These heavily hybridize the frontier orbitals, causing a reduction in over- lap and oscillator strength, so that at zero pres- sure the lowest energy transition involves deeper hole orbitals than in the case of hydrogen- capped nanocrystals; the application of pressure induces greater delocalisation over the whole nanocrystals bringing the frontier hole orbitals into play and resulting in an unexpected red shift for the phenyl-capped nanocrystals, in part caused by distortions. In response to a growing interest in relatively small nanocrystals that can be difficult to accurately characterize with ex- perimental techniques, this work exemplifies the detailed understanding of structure-property re- lationships under pressure that can be obtained for realistic nanocrystals with state-of-the-art first principles methods and used for the charac- terization and design of devices based on these and similar nanomaterials
Simulations of nanocrystals under pressure: Combining electronic enthalpy and linear-scaling density-functional theory.
We present an implementation in a linear-scaling density-functional theory
code of an electronic enthalpy method, which has been found to be natural and
efficient for the ab initio calculation of finite systems under hydrostatic
pressure. Based on a definition of the system volume as that enclosed within an
electronic density isosurface [Phys. Rev. Lett., 94, 145501 (2005)], it
supports both geometry optimizations and molecular dynamics simulations. We
introduce an approach for calibrating the parameters defining the volume in the
context of geometry optimizations and discuss their significance. Results in
good agreement with simulations using explicit solvents are obtained,
validating our approach. Size-dependent pressure-induced structural
transformations and variations in the energy gap of hydrogenated silicon
nanocrystals are investigated, including one comparable in size to recent
experiments. A detailed analysis of the polyamorphic transformations reveals
three types of amorphous structures and their persistence on depressurization
is assessed.Comment: 11 pages and 13 figures (accepted for publication by The Journal of
Chemical Physics on the 29th of July 2013
Structural relaxations in electronically excited poly(para-phenylene)
Structural relaxations in electronically excited poly(para-phenylene) are
studied using many-body perturbation theory and density-functional-theory
methods. A sophisticated description of the electron-hole interaction is
required to describe the energies of the excitonic states, but we show that the
structural relaxations associated with exciton formation can be obtained quite
accurately within a constrained density-functional-theory approach. We find
that the structural relaxations in the low-energy excitonic states extend over
about 8 monomers, leading to an energy reduction of 0.22 eV and a Stokes shift
of 0.40 eV.Comment: 4 pages, 3 figure
Role of m6A RNA Methylation in Thyroid Cancer Cell Lines
N6-methyladenosine (m6A) is the most abundant internal modification of RNA in eukaryotic cells, and, in recent years, it has gained increasing attention. A good amount of data support the involvement of m6A modification in tumorigenesis, tumor progression, and metastatic dissemination. However, the role of this RNA modification in thyroid cancer still remains poorly investigated. In this study, m6A-related RNA methylation profiles are compared between a normal thyroid cell line and different thyroid cancer cell lines. With this approach, it was possible to identify the different patterns of m6A modification in different thyroid cancer models. Furthermore, by silencing METTL3, which is the main player in the RNA methylation machinery, it was possible to evaluate the impact of m6A modification on gene expression in an anaplastic thyroid cancer model. This experimental approach allowed us to identify DDI2 as a gene specifically controlled by the m6A modification in anaplastic thyroid cancer cell lines. Altogether, these data are a proof of concept that RNA methylation widely occurs in thyroid cancer cell models and open a way forward in the search for new molecular patterns for diagnostic discrimination between benign and malignant lesions
Shadows of quantum machine learning
Quantum machine learning is often highlighted as one of the most promising
uses for a quantum computer to solve practical problems. However, a major
obstacle to the widespread use of quantum machine learning models in practice
is that these models, even once trained, still require access to a quantum
computer in order to be evaluated on new data. To solve this issue, we suggest
that following the training phase of a quantum model, a quantum computer could
be used to generate what we call a classical shadow of this model, i.e., a
classically computable approximation of the learned function. While recent
works already explore this idea and suggest approaches to construct such shadow
models, they also raise the possibility that a completely classical model could
be trained instead, thus circumventing the need for a quantum computer in the
first place. In this work, we take a novel approach to define shadow models
based on the frameworks of quantum linear models and classical shadow
tomography. This approach allows us to show that there exist shadow models
which can solve certain learning tasks that are intractable for fully classical
models, based on widely-believed cryptography assumptions. We also discuss the
(un)likeliness that all quantum models could be shadowfiable, based on common
assumptions in complexity theory.Comment: 7 + 16 pages, 5 figure
Risk of internet addiction in adolescents: A confrontation between traditional teaching and online teaching
Background: The technological evolution has given the opportunities to develop new models of education, like online teaching. However, Internet Problematic Use and Internet Addiction are becoming frequently represented among adolescents with a prevalence that varies worldwide from 2% to 20% of the high school population. Objective: The aim of this study was to analyse the risk of Internet Addiction in a High Schools student sample comparing two different types of schools (online and traditional teaching) and analyzing the associations between pathological use of Internet and socio-demographic factors connected to the different educational orientations and to the daily usage of Internet. Methods: Students were enrolled from four different orientation school programs (different high school, technical and economical Institute, vocational schools). Each student completed a self-reported test to collect socio-demographic data and th Internet Addiction Test (IAT) from K. Young to assess the risk of Internet Addiction. The Mann-Whitney test for quantitative variables was used for statistical analysis. Results: 522 students were enrolled, 243 students from online teaching and 279 from traditional teaching schools. Internet Addiction was observed in 1,16% of the total sample, while 53.83% of subjects was at risk of development Internet Addiction. No significant difference was found between the two different types of teaching, nor considering gender. Considering the amount of time spent on the web in portion of the sample at risk of developing Internet Addiction, the Traditional Teaching group spent between 4 and 7 hours a day on the Web, while the Online Teaching group between 1 to 3 hours/daily. However, no statistically significant difference was found. Conclusion: Although our data demonstrate that there is no clear association between online education and problematic use of Internet, the excessive use of Internet is linked to a massive waste of personal energy in terms of time and social life
Structural, electronic, and dynamical properties of amorphous gallium arsenide: a comparison between two topological models
We present a detailed study of the effect of local chemical ordering on the
structural, electronic, and dynamical properties of amorphous gallium arsenide.
Using the recently-proposed ``activation-relaxation technique'' and empirical
potentials, we have constructed two 216-atom tetrahedral continuous random
networks with different topological properties, which were further relaxed
using tight-binding molecular dynamics. The first network corresponds to the
traditional, amorphous, Polk-type, network, randomly decorated with Ga and As
atoms. The second is an amorphous structure with a minimum of wrong (homopolar)
bonds, and therefore a minimum of odd-membered atomic rings, and thus
corresponds to the Connell-Temkin model. By comparing the structural,
electronic, and dynamical properties of these two models, we show that the
Connell-Temkin network is energetically favored over Polk, but that most
properties are little affected by the differences in topology. We conclude that
most indirect experimental evidence for the presence (or absence) of wrong
bonds is much weaker than previously believed and that only direct structural
measurements, i.e., of such quantities as partial radial distribution
functions, can provide quantitative information on these defects in a-GaAs.Comment: 10 pages, 7 ps figures with eps
Unravelling the roles of size, ligands and pressure in the piezochromic properties of CdS nanocrystals
Understanding the effects of pressure-induced deformations on the optoelectronic properties of nanomaterials is important not only from the fundamental point of view, but also for potential applications such as stress sensors and electromechanical devices. Here we describe the novel insights into these piezochromic effects gained from using a linear-scaling density functional theory framework and an electronic enthalpy scheme, which allow us to accurately characterize the electronic structure of CdS nanocrystals with a zincblende-like core of experimentally relevant size. In particular we focus on unravelling the complex interplay of size and surface (phenyl) ligands with pressure. We show that pressure-induced deformations are not simple isotropic scaling of the original structures and that the change in HOMO-LUMO gap with pressure results from two competing factors: (i) a bulk-like linear increase due to compression, which is offset by (ii) distortions/disorder and, to a lesser extent, orbital hybridization induced by ligands affecting the frontier orbitals. Moreover, we observe that the main peak in the optical absorption spectra is systematically red-shifted or blue-shifted, as pressure is increased up to 5 GPa, depending on the presence or absence of phenyl ligands. These heavily hybridize the frontier orbitals, causing a reduction in overlap and oscillator strength, so that at zero pressure the lowest energy transition involves deeper hole orbitals than in the case of hydrogencapped nanocrystals; the application of pressure induces greater delocalisation over the whole nanocrystals bringing the frontier hole orbitals into play and resulting in an unexpected red shift for the phenyl-capped nanocrystals, in part caused by distortions. In response to a growing interest in relatively small nanocrystals that can be difficult to accurately characterize with experimental techniques, this work exemplifies the detailed understanding of structure-property relationships under pressure that can be obtained for realistic nanocrystals with state-of-the-art first principles methods and used for the characterization and design of devices based on these and similar nanomaterials
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