This document is confidential and is proprietary to the American Chemical Society and its authors... more This document is confidential and is proprietary to the American Chemical Society and its authors. Do not copy or disclose without written permission. If you have received this item in error, notify the sender and delete all copies. A tunable structural family with ultralow thermal conductivity: copper deficient Cu1-x□xPb1-xBi1+xS3
B Bader Theory for charge analysis 77 Bibliography 89 Introduction The wide eeort of the past dec... more B Bader Theory for charge analysis 77 Bibliography 89 Introduction The wide eeort of the past decade to understand physical properties of the semiconductor heterojunctions has provided many fundamental and applica-tive results 1, 2]. The experience acquired on simple systems like isovalent common anion heterojunctions, e.g. GaAs/AlAs, has been successfully extended to heterovalent systems and also, in part, to metal-semiconductor contacts 92]. Also junctions between materials with diierent crystalline structure have been partially studied but, to our knowledge, the properties between crystalline and amorphous semiconductors is an almost unexplored eld. Stimulated by recent measurements made on c-Si(100)=a-Si 1?x C x : H by the experimental group of Evangelisti in Rome 3, 4, 5], we have faced in this thesis the interesting problem of a junction between amorphous (a-Si) and crystalline silicon (c-Si), which is nowadays a subject of interest. The modern silicon solar cells 6], for inst...
computation of material properties by abinitio methods has become the foundation of an effective ... more computation of material properties by abinitio methods has become the foundation of an effective approach to materials design. One of the major challenges in mapping the materials genome is in developing efficient computational tools that are cost-effective and accurate at the same time. In this talk, we discuss the newly developed ACBN0 pseudo-hybrid Hubbard density functional where the Hubbard energy within the DFT + U formulation is calculated self consistently. The U depends on the electron density and depends both on the geometry and chemical environment of the system. We show that ACBN0 improves the description of both the structural and electronic properties in a range of complex materials from Zn/Cd based chalcogenides to the TMOs. The magnetic properties are better described compared to the LDA/GGA functionals. We will also discuss the application of the ACBN0 approach to surfaces, doped and multi-valent systems where it is possible to evaluate U for different sites and chemical bonding. For all the complex materials studied here, we find that the electronic properties are significantly improved over the DFT values and the accuracy is at par with the HSE values at a fraction of the computational cost.
Recent research in materials science opens exciting perspectives to design novel quantum material... more Recent research in materials science opens exciting perspectives to design novel quantum materials and devices, but it calls for quantitative predictions of properties which are not accessible in standard first principles packages. PAOFLOW is a software tool that constructs tight-binding Hamiltonians from self-consistent electronic wavefunctions by projecting onto a set of atomic orbitals. The electronic structure provides numerous materials properties that otherwise would have to be calculated via phenomenological models. In this paper, we describe recent redesign of the code as well as the new features and improvements in performance. In particular, we have implemented symmetry operations for unfolding equivalent k-points, which drastically reduces the runtime requirements of first principles calculations, and we have provided internal routines of projections onto atomic orbitals enabling generation of real space atomic orbitals. Moreover, we have included models for non-constant relaxation time in electronic transport calculations, doubling the real space dimensions of the Hamiltonian as well as the construction of Hamiltonians directly from analytical models. Importantly, PAOFLOW has been now converted into a Python package, and is streamlined for use directly within other Python codes. The new object oriented design treats PAOFLOW's computational routines as class methods, providing an API for explicit control of each calculation.
this talk, we will present our results of the performance of the recently developed ACBN0 pseudo-... more this talk, we will present our results of the performance of the recently developed ACBN0 pseudo-hybrid Hubbard density functional in predicting the electronic and structural properties of the Zn-and Cd-based semiconductors. ACBN0 is a fast, accurate and parameterfree extension of traditional DFT+U proved to correct the band gap in transition metal compounds. Within ACBN0, the values of U and J are functionals of the electron density and depend directly on the chemical environment and crystalline field. 1) We will compare the structural and electronic properties of ZnX and CdX (X=0,S,Se,Te) semiconductors calculated in rs,wz and zb phases using ACBN0 with the results obtained by semi-local PBE, hybrid HSE06 functionals and experiments whenever available. Our results demonstrate that the lattice constants, bulk modulii and band-gaps are more accurately described by ACBN0 compared to the PBE functionals. Overall, we show that ACBN0 is a powerful tool which preserves the accuracy of the HSE calculations with higher computational efficiency.
Ferroelectrics are important technological materials with wide-ranging applications in electronic... more Ferroelectrics are important technological materials with wide-ranging applications in electronics, communication, health, and energy. While lead-based ferroelectrics have remained the predominant mainstay of industry for decades, environmentally friendly lead-free alternatives are limited due to relatively low Curie temperatures (T C) and/or high cost in many cases. Efforts have been made to enhance T C through strain engineering, often involving energy-intensive and expensive fabrication of thin epitaxial films on lattice-mismatched substrates. Here, a relatively simple and scalable sol-gel synthesis route to fabricate polycrystalline (Ba 0.85 Ca 0.15)(Zr 0.1 Ti 0.9)O 3 nanowires within porous templates is presented, with an observed enhancement of T C up to ≈300 °C as compared to ≈90 °C in the bulk. By combining experiments and theoretical calculations, this effect is attributed to the volume reduction in the template-grown nanowires that modifies the balance between different structural instabilities. The results offer a cost-effective solution-based approach for strain-tuning in a promising lead-free ferroelectric system, thus widening their current applicability.
PAOFLOW is a utility for the analysis and characterization of materials properties from the outpu... more PAOFLOW is a utility for the analysis and characterization of materials properties from the output of electronic structure calculations. By exploiting an efficient procedure to project the full plane-wave solution on a reduced space of atomic orbitals, PAOFLOW facilitates the calculation of a plethora of quantities such as diffusive, anomalous and spin Hall conductivities, magnetic and spin circular dichroism, and Z 2 topological invariants and more. The computational cost associated with post-processing first principles calculations is negligible. This code, written entirely in Python under GPL 3.0 or later, opens the way to the high-throughput computational characterization of materials at an unprecedented scale.
One of the most accurate approaches for calculating lattice thermal conductivity, κ ' , is solvin... more One of the most accurate approaches for calculating lattice thermal conductivity, κ ' , is solving the Boltzmann transport equation starting from third-order anharmonic force constants. In addition to the underlying approximations of ab-initio parameterization, two main challenges are associated with this path: high computational costs and lack of automation in the frameworks using this methodology, which affect the discovery rate of novel materials with ad-hoc properties. Here, the Automatic Anharmonic Phonon Library (AAPL) is presented. It efficiently computes interatomic force constants by making effective use of crystal symmetry analysis, it solves the Boltzmann transport equation to obtain κ ' , and allows a fully integrated operation with minimum user intervention, a rational addition to the current high-throughput accelerated materials development framework AFLOW. An "experiment vs. theory" study of the approach is shown, comparing accuracy and speed with respect to other available packages, and for materials characterized by strong electron localization and correlation. Combining AAPL with the pseudo-hybrid functional ACBN0 is possible to improve accuracy without increasing computational requirements.
Optical and transport properties of materials depend heavily upon features of electronic band str... more Optical and transport properties of materials depend heavily upon features of electronic band structures in proximity to energy extrema in the Brillouin zone (BZ). Such features are generally described in terms of multi-dimensional quadratic expansions and corresponding definitions of effective masses. Multi-dimensional expansions, however, are permissible only under strict conditions that are typically violated by degenerate bands and even some non-degenerate bands. Suggestive terms such as "band warping" or "corrugated energy surfaces" have been used to refer to such situations and ad hoc methods have been developed to treat them. While numerical calculations may reflect such features, a complete theory of band warping has not been developed. We develop a generally applicable theory, based on radial expansions, and a corresponding definition of angular effective mass. Our theory also accounts for effects of band non-parabolicity and anisotropy, which hitherto h...
Given an edge-weighted graph, the maximum edge weight clique (MEWC) problem is to find a clique t... more Given an edge-weighted graph, the maximum edge weight clique (MEWC) problem is to find a clique that maximizes the sum of edge weights within the corresponding complete subgraph. This problem generalizes the classical maximum clique problem and finds many real-world applications in molecular biology, broadband network design, pattern recognition and robotics, information retrieval, marketing, and bioinformatics among other areas. The main goal of this chapter is to provide an up-to-date review of mathematical optimization formulations and solution approaches for the MEWC problem. Information on standard benchmark instances and state-of-the-art computational results is also included.
One of the most accurate approaches for calculating lattice thermal conductivity, $\kappa_l$, is ... more One of the most accurate approaches for calculating lattice thermal conductivity, $\kappa_l$, is solving the Boltzmann transport equation starting from third-order anharmonic force constants. In addition to the underlying approximations of ab-initio parameterization, two main challenges are associated with this path. High computational costs and lack of automation in the frameworks using this methodology affect the discovery rate of novel materials with ad-hoc properties. Here, we present the Automatic-Anharmonic-Phonon-Library, AAPL. It efficiently computes interatomic force constants by making effective use of crystal symmetry analysis, it solves the Boltzmann transport equation to obtain $\kappa_l$, and allows a fully integrated operation with minimum user intervention, a rational addition to the current high-throughput accelerated materials development framework AFLOW. We show an "experiment versus theory" study of the approach, we compare accuracy and speed with respe...
High-throughput (HT) DFT computations facilitates the understanding and the design of materials w... more High-throughput (HT) DFT computations facilitates the understanding and the design of materials with novel properties. In this work, we use our HT infrastructure, AFLOWπ, to compute the electronic structure and related properties for mineral in the clay family: lizardite (Mg 3 (Si 2 O 5)(OH) 4), talc (Mg 3 (Si 2 O 5) 2 (OH) 2), kaolinite(Al 2 (Si 2 O 5)(OH) 4) and pyrophyllite (Al 2 (Si 2 O 5) 2 (OH) 2). Using these four prototypes, we studied the effect of chemical substitutions in 48 different compositions. We computed the formation energies, optimal lattice parameters, elastic constants and the band structures using ACBN0, a pseudo hybrid Hubbard density functional, all of which is incorporated in the AFLOWπ framework. One main result shows that Ni-substituted lizardite (Ni 3 (Si 2 O 5)(OH) 4) is structurally stable and is a promising candidate in spintronic applications as spin filter.
Pathologies associated with calcified tissue, such as osteoporosis, demand in vivo and/or in situ... more Pathologies associated with calcified tissue, such as osteoporosis, demand in vivo and/or in situ spectroscopic analysis to assess the role of chemical substitutions in the inorganic component. High energy X-ray or NMR spectroscopies are often impractical or damaging in biomedical conditions. Low energy spectroscopies, such as IR and Raman techniques, are often the best alternative. In apatite biominerals, the vibrational signatures of the phosphate group are generally used as fingerprint of the materials although they provide only limited information. Here, we have used first principles calculations to unravel the complexity of the complete vibrational spectra of apatites. We determined the spectroscopic features of all the phonon modes of fluor-apatite, hydroxy-apatite, and carbonated fluoroapatite beyond the analysis of the phosphate groups, focusing on the effect of local corrections induced by the crystalline environment and the specific mineral composition. This provides a cle...
High performance thermoelectric materials are key to the development of an energy efficient techn... more High performance thermoelectric materials are key to the development of an energy efficient technology. Unfortunately, the design and tailoring of materials for thermoelectric energy conversion is a formidable task. Electrical and heat transport coefficients must satisfy contradictory requirements that depend on the details of the electronic structure, the anharmonic terms in the vibrations, and the effects of chemical disorder and defects. We have exploited the capability of computational methods based on density functional theory to predict the thermoelectric properties of novel chemical compositions. I will discuss heuristic design rules for efficient thermoelectric energy conversion materials as derived from standard electronic structure calculations and also applications to skutterudite and oxide materials. Speaker: Edward Brown Michigan State University September 28, 2009 “Journey to the Core of a Neutron Star” Abstract: Neutron stars are composed of the densest observable mat...
Tight-binding models provide a conceptually transparent and computationally efficient method to r... more Tight-binding models provide a conceptually transparent and computationally efficient method to represent the electronic properties of materials. With AFLOWpwe introduce a framework for high-throughput first principles calculations that automatically generates tight-binding hamiltonians without any additional input. Several additional features are included in AFLOWp with the intent to simplify the selfconsistent calculation of Hubbard U corrections, the calculations of phonon dispersions, elastic properties, complex dielectric constants, and electronic transport coefficients. As examples we show how to compute the optical properties of layered nitrides in the AMN2 family, and the elastic and vibrational properties of binary halides with CsCl and NaCl structure. 2017 Elsevier B.V. All rights reserved.
Si 1−x Ge x alloys are among the most used materials for power electronics and quantum technology... more Si 1−x Ge x alloys are among the most used materials for power electronics and quantum technology. In most engineering models the parameters used to simulate the material and its electronic transport properties are derived from experimental results using simple semiempirical approaches. In this paper, we present a high-throughput study of the electron transport properties in Si 1−x Ge x alloys, based on the combination of atomistic first principles calculations and statistical analysis. Our results clarify the effects of the Ge concentration and of disorder on the properties of the Si 1−x Ge x alloy. We discuss the results in comparison with existing semiempirical methods and we provide a Ge-dependent set of transport parameters that can be used in device modeling.
Hyperbolic metamaterials (HMMs) are highly anisotropic optical materials that behave as metals or... more Hyperbolic metamaterials (HMMs) are highly anisotropic optical materials that behave as metals or as dielectrics depending on the direction of propagation of light. They are becoming essential for a plethora of applications, ranging from aerospace to automotive, from wireless to medical and IoT. These applications often work in harsh environments or may sustain remarkable external stresses. This calls for materials that show enhanced optical properties as well as tailorable mechanical properties. Depending on their specific use, both hard and ultrasoft materials could be required, although the combination with optical hyperbolic response is rarely addressed. Here, we demonstrate the possibility to combine optical hyperbolicity and tunable mechanical properties in the same (meta)material, focusing on the case of extreme mechanical hardness. Using high-throughput calculations from first principles and effective medium theory, we explored a large class of layered materials with hyperbolic optical activity in the near-IR and visible range, and we identified a reduced number of ultrasoft and hard HMMs among more than 1800 combinations of transition metal rocksalt crystals. Once validated by the experiments, this new class of metamaterials may foster previously unexplored optical/mechanical applications.
This document is confidential and is proprietary to the American Chemical Society and its authors... more This document is confidential and is proprietary to the American Chemical Society and its authors. Do not copy or disclose without written permission. If you have received this item in error, notify the sender and delete all copies. A tunable structural family with ultralow thermal conductivity: copper deficient Cu1-x□xPb1-xBi1+xS3
B Bader Theory for charge analysis 77 Bibliography 89 Introduction The wide eeort of the past dec... more B Bader Theory for charge analysis 77 Bibliography 89 Introduction The wide eeort of the past decade to understand physical properties of the semiconductor heterojunctions has provided many fundamental and applica-tive results 1, 2]. The experience acquired on simple systems like isovalent common anion heterojunctions, e.g. GaAs/AlAs, has been successfully extended to heterovalent systems and also, in part, to metal-semiconductor contacts 92]. Also junctions between materials with diierent crystalline structure have been partially studied but, to our knowledge, the properties between crystalline and amorphous semiconductors is an almost unexplored eld. Stimulated by recent measurements made on c-Si(100)=a-Si 1?x C x : H by the experimental group of Evangelisti in Rome 3, 4, 5], we have faced in this thesis the interesting problem of a junction between amorphous (a-Si) and crystalline silicon (c-Si), which is nowadays a subject of interest. The modern silicon solar cells 6], for inst...
computation of material properties by abinitio methods has become the foundation of an effective ... more computation of material properties by abinitio methods has become the foundation of an effective approach to materials design. One of the major challenges in mapping the materials genome is in developing efficient computational tools that are cost-effective and accurate at the same time. In this talk, we discuss the newly developed ACBN0 pseudo-hybrid Hubbard density functional where the Hubbard energy within the DFT + U formulation is calculated self consistently. The U depends on the electron density and depends both on the geometry and chemical environment of the system. We show that ACBN0 improves the description of both the structural and electronic properties in a range of complex materials from Zn/Cd based chalcogenides to the TMOs. The magnetic properties are better described compared to the LDA/GGA functionals. We will also discuss the application of the ACBN0 approach to surfaces, doped and multi-valent systems where it is possible to evaluate U for different sites and chemical bonding. For all the complex materials studied here, we find that the electronic properties are significantly improved over the DFT values and the accuracy is at par with the HSE values at a fraction of the computational cost.
Recent research in materials science opens exciting perspectives to design novel quantum material... more Recent research in materials science opens exciting perspectives to design novel quantum materials and devices, but it calls for quantitative predictions of properties which are not accessible in standard first principles packages. PAOFLOW is a software tool that constructs tight-binding Hamiltonians from self-consistent electronic wavefunctions by projecting onto a set of atomic orbitals. The electronic structure provides numerous materials properties that otherwise would have to be calculated via phenomenological models. In this paper, we describe recent redesign of the code as well as the new features and improvements in performance. In particular, we have implemented symmetry operations for unfolding equivalent k-points, which drastically reduces the runtime requirements of first principles calculations, and we have provided internal routines of projections onto atomic orbitals enabling generation of real space atomic orbitals. Moreover, we have included models for non-constant relaxation time in electronic transport calculations, doubling the real space dimensions of the Hamiltonian as well as the construction of Hamiltonians directly from analytical models. Importantly, PAOFLOW has been now converted into a Python package, and is streamlined for use directly within other Python codes. The new object oriented design treats PAOFLOW's computational routines as class methods, providing an API for explicit control of each calculation.
this talk, we will present our results of the performance of the recently developed ACBN0 pseudo-... more this talk, we will present our results of the performance of the recently developed ACBN0 pseudo-hybrid Hubbard density functional in predicting the electronic and structural properties of the Zn-and Cd-based semiconductors. ACBN0 is a fast, accurate and parameterfree extension of traditional DFT+U proved to correct the band gap in transition metal compounds. Within ACBN0, the values of U and J are functionals of the electron density and depend directly on the chemical environment and crystalline field. 1) We will compare the structural and electronic properties of ZnX and CdX (X=0,S,Se,Te) semiconductors calculated in rs,wz and zb phases using ACBN0 with the results obtained by semi-local PBE, hybrid HSE06 functionals and experiments whenever available. Our results demonstrate that the lattice constants, bulk modulii and band-gaps are more accurately described by ACBN0 compared to the PBE functionals. Overall, we show that ACBN0 is a powerful tool which preserves the accuracy of the HSE calculations with higher computational efficiency.
Ferroelectrics are important technological materials with wide-ranging applications in electronic... more Ferroelectrics are important technological materials with wide-ranging applications in electronics, communication, health, and energy. While lead-based ferroelectrics have remained the predominant mainstay of industry for decades, environmentally friendly lead-free alternatives are limited due to relatively low Curie temperatures (T C) and/or high cost in many cases. Efforts have been made to enhance T C through strain engineering, often involving energy-intensive and expensive fabrication of thin epitaxial films on lattice-mismatched substrates. Here, a relatively simple and scalable sol-gel synthesis route to fabricate polycrystalline (Ba 0.85 Ca 0.15)(Zr 0.1 Ti 0.9)O 3 nanowires within porous templates is presented, with an observed enhancement of T C up to ≈300 °C as compared to ≈90 °C in the bulk. By combining experiments and theoretical calculations, this effect is attributed to the volume reduction in the template-grown nanowires that modifies the balance between different structural instabilities. The results offer a cost-effective solution-based approach for strain-tuning in a promising lead-free ferroelectric system, thus widening their current applicability.
PAOFLOW is a utility for the analysis and characterization of materials properties from the outpu... more PAOFLOW is a utility for the analysis and characterization of materials properties from the output of electronic structure calculations. By exploiting an efficient procedure to project the full plane-wave solution on a reduced space of atomic orbitals, PAOFLOW facilitates the calculation of a plethora of quantities such as diffusive, anomalous and spin Hall conductivities, magnetic and spin circular dichroism, and Z 2 topological invariants and more. The computational cost associated with post-processing first principles calculations is negligible. This code, written entirely in Python under GPL 3.0 or later, opens the way to the high-throughput computational characterization of materials at an unprecedented scale.
One of the most accurate approaches for calculating lattice thermal conductivity, κ ' , is solvin... more One of the most accurate approaches for calculating lattice thermal conductivity, κ ' , is solving the Boltzmann transport equation starting from third-order anharmonic force constants. In addition to the underlying approximations of ab-initio parameterization, two main challenges are associated with this path: high computational costs and lack of automation in the frameworks using this methodology, which affect the discovery rate of novel materials with ad-hoc properties. Here, the Automatic Anharmonic Phonon Library (AAPL) is presented. It efficiently computes interatomic force constants by making effective use of crystal symmetry analysis, it solves the Boltzmann transport equation to obtain κ ' , and allows a fully integrated operation with minimum user intervention, a rational addition to the current high-throughput accelerated materials development framework AFLOW. An "experiment vs. theory" study of the approach is shown, comparing accuracy and speed with respect to other available packages, and for materials characterized by strong electron localization and correlation. Combining AAPL with the pseudo-hybrid functional ACBN0 is possible to improve accuracy without increasing computational requirements.
Optical and transport properties of materials depend heavily upon features of electronic band str... more Optical and transport properties of materials depend heavily upon features of electronic band structures in proximity to energy extrema in the Brillouin zone (BZ). Such features are generally described in terms of multi-dimensional quadratic expansions and corresponding definitions of effective masses. Multi-dimensional expansions, however, are permissible only under strict conditions that are typically violated by degenerate bands and even some non-degenerate bands. Suggestive terms such as "band warping" or "corrugated energy surfaces" have been used to refer to such situations and ad hoc methods have been developed to treat them. While numerical calculations may reflect such features, a complete theory of band warping has not been developed. We develop a generally applicable theory, based on radial expansions, and a corresponding definition of angular effective mass. Our theory also accounts for effects of band non-parabolicity and anisotropy, which hitherto h...
Given an edge-weighted graph, the maximum edge weight clique (MEWC) problem is to find a clique t... more Given an edge-weighted graph, the maximum edge weight clique (MEWC) problem is to find a clique that maximizes the sum of edge weights within the corresponding complete subgraph. This problem generalizes the classical maximum clique problem and finds many real-world applications in molecular biology, broadband network design, pattern recognition and robotics, information retrieval, marketing, and bioinformatics among other areas. The main goal of this chapter is to provide an up-to-date review of mathematical optimization formulations and solution approaches for the MEWC problem. Information on standard benchmark instances and state-of-the-art computational results is also included.
One of the most accurate approaches for calculating lattice thermal conductivity, $\kappa_l$, is ... more One of the most accurate approaches for calculating lattice thermal conductivity, $\kappa_l$, is solving the Boltzmann transport equation starting from third-order anharmonic force constants. In addition to the underlying approximations of ab-initio parameterization, two main challenges are associated with this path. High computational costs and lack of automation in the frameworks using this methodology affect the discovery rate of novel materials with ad-hoc properties. Here, we present the Automatic-Anharmonic-Phonon-Library, AAPL. It efficiently computes interatomic force constants by making effective use of crystal symmetry analysis, it solves the Boltzmann transport equation to obtain $\kappa_l$, and allows a fully integrated operation with minimum user intervention, a rational addition to the current high-throughput accelerated materials development framework AFLOW. We show an "experiment versus theory" study of the approach, we compare accuracy and speed with respe...
High-throughput (HT) DFT computations facilitates the understanding and the design of materials w... more High-throughput (HT) DFT computations facilitates the understanding and the design of materials with novel properties. In this work, we use our HT infrastructure, AFLOWπ, to compute the electronic structure and related properties for mineral in the clay family: lizardite (Mg 3 (Si 2 O 5)(OH) 4), talc (Mg 3 (Si 2 O 5) 2 (OH) 2), kaolinite(Al 2 (Si 2 O 5)(OH) 4) and pyrophyllite (Al 2 (Si 2 O 5) 2 (OH) 2). Using these four prototypes, we studied the effect of chemical substitutions in 48 different compositions. We computed the formation energies, optimal lattice parameters, elastic constants and the band structures using ACBN0, a pseudo hybrid Hubbard density functional, all of which is incorporated in the AFLOWπ framework. One main result shows that Ni-substituted lizardite (Ni 3 (Si 2 O 5)(OH) 4) is structurally stable and is a promising candidate in spintronic applications as spin filter.
Pathologies associated with calcified tissue, such as osteoporosis, demand in vivo and/or in situ... more Pathologies associated with calcified tissue, such as osteoporosis, demand in vivo and/or in situ spectroscopic analysis to assess the role of chemical substitutions in the inorganic component. High energy X-ray or NMR spectroscopies are often impractical or damaging in biomedical conditions. Low energy spectroscopies, such as IR and Raman techniques, are often the best alternative. In apatite biominerals, the vibrational signatures of the phosphate group are generally used as fingerprint of the materials although they provide only limited information. Here, we have used first principles calculations to unravel the complexity of the complete vibrational spectra of apatites. We determined the spectroscopic features of all the phonon modes of fluor-apatite, hydroxy-apatite, and carbonated fluoroapatite beyond the analysis of the phosphate groups, focusing on the effect of local corrections induced by the crystalline environment and the specific mineral composition. This provides a cle...
High performance thermoelectric materials are key to the development of an energy efficient techn... more High performance thermoelectric materials are key to the development of an energy efficient technology. Unfortunately, the design and tailoring of materials for thermoelectric energy conversion is a formidable task. Electrical and heat transport coefficients must satisfy contradictory requirements that depend on the details of the electronic structure, the anharmonic terms in the vibrations, and the effects of chemical disorder and defects. We have exploited the capability of computational methods based on density functional theory to predict the thermoelectric properties of novel chemical compositions. I will discuss heuristic design rules for efficient thermoelectric energy conversion materials as derived from standard electronic structure calculations and also applications to skutterudite and oxide materials. Speaker: Edward Brown Michigan State University September 28, 2009 “Journey to the Core of a Neutron Star” Abstract: Neutron stars are composed of the densest observable mat...
Tight-binding models provide a conceptually transparent and computationally efficient method to r... more Tight-binding models provide a conceptually transparent and computationally efficient method to represent the electronic properties of materials. With AFLOWpwe introduce a framework for high-throughput first principles calculations that automatically generates tight-binding hamiltonians without any additional input. Several additional features are included in AFLOWp with the intent to simplify the selfconsistent calculation of Hubbard U corrections, the calculations of phonon dispersions, elastic properties, complex dielectric constants, and electronic transport coefficients. As examples we show how to compute the optical properties of layered nitrides in the AMN2 family, and the elastic and vibrational properties of binary halides with CsCl and NaCl structure. 2017 Elsevier B.V. All rights reserved.
Si 1−x Ge x alloys are among the most used materials for power electronics and quantum technology... more Si 1−x Ge x alloys are among the most used materials for power electronics and quantum technology. In most engineering models the parameters used to simulate the material and its electronic transport properties are derived from experimental results using simple semiempirical approaches. In this paper, we present a high-throughput study of the electron transport properties in Si 1−x Ge x alloys, based on the combination of atomistic first principles calculations and statistical analysis. Our results clarify the effects of the Ge concentration and of disorder on the properties of the Si 1−x Ge x alloy. We discuss the results in comparison with existing semiempirical methods and we provide a Ge-dependent set of transport parameters that can be used in device modeling.
Hyperbolic metamaterials (HMMs) are highly anisotropic optical materials that behave as metals or... more Hyperbolic metamaterials (HMMs) are highly anisotropic optical materials that behave as metals or as dielectrics depending on the direction of propagation of light. They are becoming essential for a plethora of applications, ranging from aerospace to automotive, from wireless to medical and IoT. These applications often work in harsh environments or may sustain remarkable external stresses. This calls for materials that show enhanced optical properties as well as tailorable mechanical properties. Depending on their specific use, both hard and ultrasoft materials could be required, although the combination with optical hyperbolic response is rarely addressed. Here, we demonstrate the possibility to combine optical hyperbolicity and tunable mechanical properties in the same (meta)material, focusing on the case of extreme mechanical hardness. Using high-throughput calculations from first principles and effective medium theory, we explored a large class of layered materials with hyperbolic optical activity in the near-IR and visible range, and we identified a reduced number of ultrasoft and hard HMMs among more than 1800 combinations of transition metal rocksalt crystals. Once validated by the experiments, this new class of metamaterials may foster previously unexplored optical/mechanical applications.
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