arXiv subjects

M. Fornari

Publications and source records attributed to M. Fornari.

8 recordsLinked to original sources

arXiv2022

aflow++: a C++ framework for autonomous materials design

The realization of novel technological opportunities given by computational and autonomous materials design requires efficient and effective frameworks. For more than two decades, aflow++ (Automatic-Flow Framework for Materials Discovery) has provided an interconnected collection of algorithms and workflows to address this challenge. This article contains an overview of the software and some of its most heavily-used functionalities, including algorithmic details, standards, and examples. Key thrusts are highlighted: the calculation of structural, electronic, thermodynamic, and thermomechanical properties in addition to the modeling of complex materials, such as high-entropy ceramics and bulk metallic glasses. The aflow++ software prioritizes interoperability, minimizing the number of independent parameters and tolerances. It ensures consistency of results across property sets - facilitating machine learning studies. The software also features various validation schemes, offering real-time quality assurance for data generated in a high-throughput fashion. Altogether, these considerations contribute to the development of large and reliable materials databases that can ultimately deliver future materials systems

cond-mat.mtrl-sci↗

arXiv2017

AFLOW$π$: A minimalist approach to high-throughput ab initio calculations including the generation of tight-binding hamiltonians

Tight-binding models provide a conceptually transparent and computationally efficient method to represent the electronic properties of materials. With AFLOW$π$ we 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 AFLOW$π$ with the intent to simplify the self-consistent 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 $AM$N$_2$ family, and the elastic and vibrational properties of binary halides with CsCl and NaCl structure.

cond-mat.mtrl-sci↗

arXiv2016

High Throughput combinatorial method for fast and robust prediction of lattice thermal conductivity

The lack of computationally inexpensive and accurate ab-initio based methodologies to predict lattice thermal conductivity, without computing the anharmonic force constants or time-consuming ab-initio molecular dynamics, is one of the obstacles preventing the accelerated discovery of new high or low thermal conductivity materials. The Slack equation is the best alternative to other more expensive methodologies but is highly dependent on two variables: the acoustic Debye temperature, $θ_a$, and the Grüneisen parameter, $γ$. Furthermore, different definitions can be used for these two quantities depending on the model or approximation. In this article, we present a combinatorial approach to elucidate which definitions of both variables produce the best predictions of the lattice thermal conductivity, $κ_l$. A set of 42 compounds was used to test accuracy and robustness of all possible combinations. This approach is ideal for obtaining more accurate values than fast screening models based on the Debye model, while being significantly less expensive than methodologies that solve the Boltzmann transport equation.

cond-mat.mtrl-sci↗

arXiv2005

Time-Resolved Spectroscopy of Single Excitons Bound to Pairs of Te Isoelectronic Impurity Centers in ZnSe

Tellurium impurity centers in ZnSe were individually probed with time-resolved photoluminescence (PL) spectroscopy. Resolution-limited peaks with an ultra-low spatial density originate in the recombination of excitons deeply bound to isolated nearest-neighbor isoelectronic Te pairs (Te2). This interpretation is confirmed by ab-initio calculations. The peaks reveal anti-bunched photon emission and a doublet structure polarized along [110] and [-110]. We analyze the time-resolved PL decay to clarify the role of the dark states in the spin relaxation and radiative recombination of single fine-structure split excitons.

cond-mat.mtrl-sci↗

arXiv2000

Possible Coexistence of Rotational and Ferroelectric Lattice Distortions in Rhombohedral PZT

The competitions between ferroelectric and rotational instabilities in rhombohedral PZT near x = 0.5 are investigated using first principles density functional supercell calculations. As expected, we find a strong ferroelectric instability. However, we also find a substantial R-point rotational instability, close to but not as deep as the ferroelectric one. This is similar to the situation in pure PbZrO_3. These two instabilities are both strongly pressure dependent, but in opposite directions so that lattice compression of less than 1% is sufficient to change their ordering. Because of this, local stress fields due to B-site cation disorder may lead to coexistence of both types of instability are likely present in the alloy near the morphotropic phase boundary.

cond-mat.mtrl-sci↗

arXiv1999

Wannier-functions characterization of floating bonds in a-Si

We investigate the electronic structure of over-coordinated defects in amorphous silicon via density-functional total-energy calculations, with the aim of understanding the relationship between topological and electronic properties on a microscopic scale. Maximally-localized Wannier functions are computed in order to characterize the bonding and the electronic properties of these defects. The five-fold coordination defects give rise to delocalized states extending over several nearest neighbors, and therefore to very polarizable bonds and anomalously high Born effective charges for the defective atoms.

cond-mat.mtrl-sci↗

arXiv1999

Floating bonds and gap states in a-Si and a-Si:H from first principles calculations

We study in detail by means of ab-initio pseudopotential calculations the electronic structure of five-fold coordinated (T_5) defects in a-Si and a-Si:H, also during their formation and their evolution upon hydrogenation. The atom-projected densities of states (DOS) and an accurate analysis of the valence charge distribution clearly indicate the fundamental contribution of T_5 defects in originating gap states through their nearest neighbors. The interaction with hydrogen can reduce the DOS in the gap annihilating T_5 defects.

cond-mat.mtrl-sci↗

arXiv1999

Coordination defects in a-Si and a-Si:H : a characterization from first principles calculations

We study by means of first-principles pseudopotential method the coordination defects in a-Si and a-Si:H, also in their formation and their evolution upon hydrogen interaction. An accurate analysis of the valence charge distribution and of the ``electron localization function'' (ELF) allows to resolve possible ambiguities in the bonding configuration, and in particular to identify clearly three-fold (T_3) and five-fold (T_5) coordinated defects. We found that electronic states in the gap can be associated to both kind of defects, and that in both cases the interaction with hydrogen can reduce the density of states in the gap.

cond-mat.mtrl-sci↗