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...
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...
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...
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.
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...
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...
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...
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.
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Papers by Marco Fornari