116 research outputs found
Electrical conductivity from first principles
Die zuverlässige Berechnung der elektrischen Leitfähigkeit vieler Materialien aus ersten Prinzipien erfordert die Berücksichtigung der anharmonischen Gitterdynamik. Der ab initio Kubo-Greenwood (KG)-Ansatz, der die KG-Leitfähigkeitsformel und die ab initio-Molekulardynamik kombiniert, scheint vielversprechend zu sein, da er die Anharmonizität des Gitters auf natürliche Weise berücksichtigt. Seine Anwendung auf kristalline Materialien hat jedoch bisher nur wenig Beachtung gefunden. Diese Arbeit beschreibt den KG-Ansatz und stellt eine numerische Implementierung dieses Ansatzes für den harmonischen Kristall Si und den anharmonischen Kristall SnSe vor. Die Fallstudie für Si zeigt erhebliche numerische Schwierigkeiten bei den KG-Berechnungen auf. Insbesondere behindert die erforderliche dichte k-Punkt-Abtastung die Konvergenz in Superzellengröße und macht die Berechnungen nur innerhalb der (semi-)lokalen Dichtefunktionaltheorie (DFT) durchführbar. Außerdem führt die notwendige Einführung eines Verbreiterungsparameters (η) zu einer erheblichen Unsicherheit bei der Bestimmung der Leitfähigkeit. Um diese Probleme zu lösen, werden rechnerisch effiziente Strategien diskutiert, darunter: (i) der "Scherenoperator"-Ansatz zur Korrektur des DFT-Bandlückenproblems; (ii) das "Optimal-η-Schema" zur Wahl eines geeigneten Wertes von η; und (iii) die Finite-Size-Scaling-Methode zur Ableitung der Leitfähigkeit in der thermodynamischen Grenze. Es wird festgestellt, dass die KG-Berechnungen mit diesen Strategien Leitfähigkeiten in angemessener Übereinstimmung mit den Experimenten ergeben. Der Vergleich mit früheren ab initio Boltzmann-Transportberechnungen zeigt jedoch, dass das η-Problem und die Frage der Konvergenz in Superzellengröße weiter verbesserte Konzepte erfordern. Die Fallstudie für SnSe zeigt sehr ähnliche numerische Schwierigkeiten wie im Fall von Si. Es werden Einblicke in die Auswirkung der Anharmonizität auf die Konvergenz der Superzellengröße gegeben.Reliable first-principles calculation of the electrical conductivity in many materials requires accounting for the anharmonic lattice dynamics. The ab initio Kubo-Greenwood (KG) approach, which combines the KG conductivity formula and ab initio molecular dynamics, appears to be promising because it naturally includes lattice anharmonicity. However, its application to crystalline materials has so far received very little attention. This thesis describes the KG approach and presents a numerical implementation of this approach for the harmonic crystal Si and the anharmonic crystal SnSe. The case study for Si identifies considerable numerical difficulties in the KG calculations. In particular, the dense k-point sampling required hinders supercell-size convergence and makes the calculations only feasible within (semi)local density-functional theory (DFT). Besides, the necessary introduction of a broadening parameter (η) introduces a significant uncertainty in determining the conductivity. To address these issues, computationally efficient strategies are discussed, including: (i) the "scissor operator" approach to correct the DFT band-gap problem; (ii) the "optimal-η scheme" to choose an appropriate value of η; and (iii) the finite-size scaling method to deduce the conductivity in the thermodynamic limit. It is found that with these strategies, the KG calculations yield conductivities in reasonable agreement with experiment. Yet, comparison with previous ab initio Boltzmann transport calculations shows that the η problem and the issue of supercell-size convergence still require improved concepts. The case study for SnSe shows very similar numerical difficulties as in the case of Si. Insights into the effect of anharmonicity on the supercell-size convergence are provided
First-principles study of intrinsic and hydrogen point defects in the earth-abundant photovoltaic absorber Zn3P2
Zinc phosphide (Zn3P2) has had a long history of scientific interest largely
because of its potential for earth-abundant photovoltaics. To realize
high-efficiency Zn3P2 solar cells, it is critical to understand and control
point defects in this material. Using hybrid functional calculations, we assess
the energetics and electronic behavior of intrinsic point defects and hydrogen
impurities in Zn3P2. All intrinsic defects are found to act as compensating
centers in p-type Zn3P2 and have deep levels in the band gap, except for zinc
vacancies which are shallow acceptors and can act as a source of doping. Our
work highlights that zinc vacancies rather than phosphorus interstitials are
likely to be the main source of p-type doping in as-grown Zn3P2. We also show
that Zn-poor and P-rich growth conditions, which are usually used for enhancing
p-type conductivity of Zn3P2, will facilitate the formation of certain
deep-level defects (P_Zn and P_i) which might be detrimental to solar cell
efficiency. For hydrogen impurities, which are frequently present in the growth
environment of Zn3P2, we study interstitial hydrogen and hydrogen complexes
with vacancies. The results suggest small but beneficial effects of hydrogen on
the electrical properties of Zn3P2
Multiagent Bidirectionally-Coordinated Nets: Emergence of Human-level Coordination in Learning to Play StarCraft Combat Games
Many artificial intelligence (AI) applications often require multiple
intelligent agents to work in a collaborative effort. Efficient learning for
intra-agent communication and coordination is an indispensable step towards
general AI. In this paper, we take StarCraft combat game as a case study, where
the task is to coordinate multiple agents as a team to defeat their enemies. To
maintain a scalable yet effective communication protocol, we introduce a
Multiagent Bidirectionally-Coordinated Network (BiCNet ['bIknet]) with a
vectorised extension of actor-critic formulation. We show that BiCNet can
handle different types of combats with arbitrary numbers of AI agents for both
sides. Our analysis demonstrates that without any supervisions such as human
demonstrations or labelled data, BiCNet could learn various types of advanced
coordination strategies that have been commonly used by experienced game
players. In our experiments, we evaluate our approach against multiple
baselines under different scenarios; it shows state-of-the-art performance, and
possesses potential values for large-scale real-world applications.Comment: 10 pages, 10 figures. Previously as title: "Multiagent
Bidirectionally-Coordinated Nets for Learning to Play StarCraft Combat
Games", Mar 201
Large Language Model for Multi-objective Evolutionary Optimization
Multiobjective evolutionary algorithms (MOEAs) are major methods for solving
multiobjective optimization problems (MOPs). Many MOEAs have been proposed in
the past decades, of which the search operators need a carefully handcrafted
design with domain knowledge. Recently, some attempts have been made to replace
the manually designed operators in MOEAs with learning-based operators (e.g.,
neural network models). However, much effort is still required for designing
and training such models, and the learned operators might not generalize well
on new problems. To tackle the above challenges, this work investigates a novel
approach that leverages the powerful large language model (LLM) to design MOEA
operators. With proper prompt engineering, we successfully let a general LLM
serve as a black-box search operator for decomposition-based MOEA (MOEA/D) in a
zero-shot manner. In addition, by learning from the LLM behavior, we further
design an explicit white-box operator with randomness and propose a new version
of decomposition-based MOEA, termed MOEA/D-LO. Experimental studies on
different test benchmarks show that our proposed method can achieve competitive
performance with widely used MOEAs. It is also promising to see the operator
only learned from a few instances can have robust generalization performance on
unseen problems with quite different patterns and settings. The results reveal
the potential benefits of using pre-trained LLMs in the design of MOEAs
Multicharged optical vortices induced in a dissipative atomic vapor system
We investigate numerically the dynamics of optical vortex beams carrying
different topological charges, launched in a dissipative three level ladder
type nonlinear atomic vapor. We impose the electromagnetically induced
transparency (EIT) condition on the medium. Linear, cubic, and quintic
susceptibilities, considered simultaneously with the dressing effect, are
included in the analysis. Generally, the beams slowly expand during propagation
and new vortices are induced, commonly appearing in oppositely charged pairs.
We demonstrate that not only the form and the topological charge of the
incident beam, but also its growing size in the medium greatly affect the
formation and evolution of vortices. We formulate common rules for finding the
number of induced vortices and the corresponding rotation directions, stemming
from the initial conditions of various incident beams, as well as from the
dynamical aspects of their propagation. The net topological charge of the
vortex is conserved during propagation, as it should be, but the total number
of charges is not necessarily same as the initial number, because of the
complex nature of the system. When the EIT condition is lifted, an enhancement
region of beam dynamics if reached, in which the dynamics and the expansion of
the beam greatly accelerate. In the end, we discuss the liquid like behavior of
light evolution in this dissipative system and propose a potential experimental
scheme for observing such a behavior.Comment: 9 pages, 6 figures, 3 table
Analysis of necroptosis-related prognostic genes and immune infiltration in idiopathic pulmonary fibrosis
BackgroundIPF is an undetermined, progressive lung disease. Necroptosis is a type of programmed apoptosis, which involved in the pathogenesis of lung diseases like COPD and ARDS. However, necroptosis in IPF have not been adequately studied. This study aimed to investigate the necroptosis in IPF and the relationship between necroptosis and immune infiltration, to construct a prognostic prediction model of IPF based on necroptosis-related genes.MethodsGSE110147 was downloaded from the GEO database and utilized to analyze the expression of necroptosis-related differentially expressed genes (NRDEGs). Then NRDEGs were used to construct protein-protein interaction (PPI) networks in the STRING database, and Cytoscape software was used to identify and visualize hub genes. Necroptosis-related prognosticgenes were explored in GSE70866, and a prognostic prediction model was constructed. The ImmuCellAI algorithm was utilized to analyze the landscape of immune infiltration in GSE110147. The single-cell RNA sequencing dataset GSE122960 was used to explore the association between necroptosis and type II alveolar epithelial cells (AT II) in IPF. The GSE213001 and GSE93606 were used for external validation. The expression of prognostic genes was quantified using RT-qPCRin the IPF A549 cell model, and was further verified by western blotting in the bleomycin-induced pulmonary fibrosis mouse model.ResultsIt was observed that necroptosis-related signaling pathways were abundantly enriched in IPF. 29 NRDEGs were screened, of which 12 showed consistent expression trends in GSE213001. Spearman correlation analysis showed that the expression of NRDEGs was positively correlated with the infiltration of proinflammatory immune cells, and negatively correlated with the infiltration of anti-inflammatory immune cells. NRDEGs, including MLKL, were highly expressed in AT II of fibrotic lung tissue. A necroptosis-related prediction model was constructed based on 4 NRDEGsby the cox stepwise regression. In the validation dataset GSE93606, the prognostic prediction model showed good applicability. The verification results of RT-qPCR and western blotting showed the reliability of most of the conclusions.ConclusionsThis study revealed that necroptosis existed in IPF and might occur in AT II. Necroptosis was associated with immune infiltration, suggesting that necroptosis of AT II might involve in IPF by activating immune infiltration and immune response
Discovery of the Zintl-phosphide BaCdP as a long carrier lifetime and stable solar absorber
Thin-film photovoltaics offers a path to significantly decarbonize our energy
production. Unfortunately, current materials commercialized or under
development as thin-film solar cell absorbers are far from optimal as they show
either low power conversion efficiency or issues with earth-abundance and
stability. Entirely new and disruptive materials platforms are rarely
discovered as the search for new solar absorbers is traditionally slow and
serendipitous. Here, we use first principles high-throughput screening to
accelerate this process. We identify new solar absorbers among known inorganic
compounds using considerations on band gap, carrier transport, optical
absorption but also on intrinsic defects which can strongly limit the carrier
lifetime and ultimately the solar cell efficiency. Screening about 40,000
materials, we discover the Zintl-phosphide BaCdP as a potential
high-efficiency solar absorber. Follow-up experimental work confirms the
predicted promises of BaCdP highlighting an optimal band gap for
visible absorption, bright photoluminescence, and long carrier lifetime of up
to 30 ns even for unoptimized powder samples. Importantly, BaCdP
does not contain any critical elements and is highly stable in air and water.
Our work opens an avenue for a new family of stable, earth-abundant,
high-performance Zintl-based solar absorbers. It also demonstrates how recent
advances in first principles computation can accelerate the search of
photovoltaic materials by combining high-throughput screening with experiment
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