arXiv subjects

Claudia Draxl

Publications and source records attributed to Claudia Draxl.

At least 55 records · Page 3Linked to original sources

arXiv2023

Extrapolation to complete basis-set limit in density-functional theory by quantile random-forest models

The numerical precision of density-functional-theory (DFT) calculations depends on a variety of computational parameters, one of the most critical being the basis-set size. The ultimate precision is reached with an infinitely large basis set, i.e., in the limit of a complete basis set (CBS). Our aim in this work is to find a machine-learning model that extrapolates finite basis-size calculations to the CBS limit. We start with a data set of 63 binary solids investigated with two all-electron DFT codes, exciting and FHI-aims, which employ very different types of basis sets. A quantile-random-forest model is used to estimate the total-energy correction with respect to a fully converged calculation as a function of the basis-set size. The random-forest model achieves a symmetric mean absolute percentage error of lower than 25% for both codes and outperforms previous approaches in the literature. Our approach also provides prediction intervals, which quantify the uncertainty of the models' predictions.

physics.comp-ph↗

arXiv2023

Enhancing superconductivity with resonant anti-shielding and topological plasmon-polarons

By employing ab initio Migdal-Eliashberg calculations, we predict a 4-fold enhancement of the superconducting critical temperature of MgB$_{2}$ when proximity-coupled to the topological crystal Bi$_{2}$Se$_{3}$. We support this result with calculations using the general Leavens scaling method. We show that this effect is a result of dynamic resonant anti-shielding of Cooper pairs by plasmon polarons of Dirac electrons in the topological crystal. Our calculations show that such superconductivity enhancement varies strongly with Coulomb coupling between plasmon polarons and Cooper pairs, with a pronounced maximum of $\textit{T}_{c}$ at a critical value of the coupling parameter. This feature is universal, and so can occur in other superconductor-topological crystal combinations, including with non-phonon mediated superconductors. We discuss methods to experimentally optimize the key coupling parameter.

cond-mat.supr-con↗

arXiv2022

Novel Approach to Structural Relaxation of Materials in Optically Excited States

We present a first-principles method for relaxing a material's geometry in an optically excited state. This method, based on the Bethe-Salpeter equation, consists of solving coupled equations for exciton wavefunctions and atomic displacements. Our approach allows for structural relaxation of excited states to be achieved through a single iteration. As results, one obtains not only energy and wavefunction of the thus modified, i.e. self-trapped, exciton, but also the mechanism of relaxation in terms of atomic displacements in the respective phonon eigenmodes. We demonstrate and evaluate our formalism with the example of the three molecules CO, H$_{2}$O, and NH$_{3}$.

physics.comp-ph↗

arXiv2022

Lightshow: a Python package for generating computational x-ray absorption spectroscopy input files

First-principles computational spectroscopy is a critical tool for interpreting experiment, performing structure refinement, and developing new physical understanding. Systematically setting up input files for different simulation codes and a diverse class of materials is a challenging task with a very high barrier-to-entry, given the complexities and nuances of each individual simulation package. This task is non-trivial even for experts in the electronic structure field and nearly formidable for non-expert researchers. Lightshow solves this problem by providing a uniform abstraction for writing computational x-ray spectroscopy input files for multiple popular codes, including FEFF, VASP, OCEAN, EXCITING and XSPECTRA. Its extendable framework will also allow the community to easily add new functions and to incorporate new simulation codes.

cond-mat.mtrl-sci↗

arXiv2022

Gauge Invariance of the Thermal Conductivity in the Quantum Regime

The widely used linear-response (LR) theory of thermal conduction in the quantum regime rests on the yet unproven assumption, that the thermal conductivity is invariant with respect to the gauge of the energy density of the system. This assumption manifests itself clearly in, e.g., Hardy's formulation of the heat-flux operator [Phys. Rev. 132, 168 (1963)]. In this work, we rigorously prove this assumption. Our proof, being valid for the nuclear and electronic subsystems, not only puts the state-of-the-art theory on solid grounds, but also enables going beyond the scope of the widely-used Boltzmann transport equation (BTE) within the LR framework for many-body systems.

cond-mat.mtrl-sci↗

arXiv2022

Roadmap on Electronic Structure Codes in the Exascale Era

Electronic structure calculations have been instrumental in providing many important insights into a range of physical and chemical properties of various molecular and solid-state systems. Their importance to various fields, including materials science, chemical sciences, computational chemistry and device physics, is underscored by the large fraction of available public supercomputing resources devoted to these calculations. As we enter the exascale era, exciting new opportunities to increase simulation numbers, sizes, and accuracies present themselves. In order to realize these promises, the community of electronic structure software developers will however first have to tackle a number of challenges pertaining to the efficient use of new architectures that will rely heavily on massive parallelism and hardware accelerators. This roadmap provides a broad overview of the state-of-the-art in electronic structure calculations and of the various new directions being pursued by the community. It covers 14 electronic structure codes, presenting their current status, their development priorities over the next five years, and their plans towards tackling the challenges and leveraging the opportunities presented by the advent of exascale computing.

cond-mat.mtrl-sci↗

arXiv2022

Advancing descriptor search in materials science: feature engineering and selection strategies

A main goal of data-driven materials research is to find optimal low-dimensional descriptors, allowing us to predict a physical property, and to interpret them in a human-understandable way. In this work, we advance methods to identify descriptors out of a large pool of candidate features by means of compressed sensing. To this extent, we develop schemes for engineering appropriate candidate features that are based on simple basic properties of building blocks that constitute the materials and that are able to represent a multi-component system by scalar numbers. Cross-validation based feature-selection methods are developed for identifying the most relevant features, thereby focusing on high generalizability. We apply our approaches to an \textit{ab initio} dataset of ternary group-IV compounds to obtain a set of descriptors for predicting lattice constants and energies of mixing. In particular, we introduce simple complexity measures in terms of involved algebraic operations as well as the amount of utilized basic properties.

cond-mat.mtrl-sci↗

arXiv2022

All-electron many-body approach to resonant inelastic x-ray scattering

We present a formalism for the resonant inelastic x-ray scattering (RIXS) cross section. The resulting compact expression in terms of polarizability matrix elements, particularly lends itself to the implementation in an all-electron many-body perturbation theory (MBPT) framework, which is realized in the full-potential package exciting. With the carbon K edge RIXS of diamond and the oxygen K edge RIXS of $β-\mathrm{Ga_2 O_3}$, respectively, we demonstrate the importance of electron-hole correlation and atomic coherence in the RIXS spectra.

cond-mat.mtrl-sci↗

arXiv2022

FAIR data enabling new horizons for materials research

The prosperity and lifestyle of our society are very much governed by achievements in condensed matter physics, chemistry and materials science, because new products for sectors such as energy, the environment, health, mobility and information technology (IT) rely largely on improved or even new materials. Examples include solid-state lighting, touchscreens, batteries, implants, drug delivery and many more. The enormous amount of research data produced every day in these fields represents a gold mine of the twenty-first century. This gold mine is, however, of little value if these data are not comprehensively characterized and made available. How can we refine this feedstock; that is, turn data into knowledge and value? For this, a FAIR (findable, accessible, interoperable and reusable) data infrastructure is a must. Only then can data be readily shared and explored using data analytics and artificial intelligence (AI) methods. Making data 'findable and AI ready' (a forward-looking interpretation of the acronym) will change the way in which science is carried out today. In this Perspective, we discuss how we can prepare to make this happen for the field of materials science.

cond-mat.mtrl-sci↗

arXiv2022

Similarity of materials and data-quality assessment by fingerprinting

Identifying similar materials, i.e., those sharing a certain property or feature, requires interoperable data of high quality. It also requires means to measure similarity. We demonstrate how a spectral fingerprint as a descriptor, combined with a similarity metric, can be used for establishing quantitative relationships between materials data, thereby serving multiple purposes. This concerns, for instance, the identification of materials exhibiting electronic properties similar to a chosen one. The same approach can be used for assessing uncertainty in data that potentially come from different sources. Selected examples show how to quantify differences between measured optical spectra or the impact of methodology and computational parameters on calculated properties, like the the density of states or excitonic spectra. Moreover, combining the same fingerprint with a clustering approach allows us to explore materials spaces in view of finding (un)expected trends or patterns. In all cases, we provide physical reasoning behind the findings of the automatized assessment of data.

cond-mat.mtrl-sci↗

arXiv2022

Influence of spin-orbit coupling on chemical bonding

The influence of spin-orbit interaction on chemical bonds in elemental solids and homonuclear dimers is analyzed by means of density-functional-theory calculations. Employing highly precise all-electron full-potential methodology, our results represent benchmark quality. Comparison of the scalar- and fully-relativistic approaches for elemental solids shows that the spin-orbit interaction may contract or expand the volume of the considered material. The largest variation of the volume is obtained for Au, Tl, I, Bi, Po and Hg, exhibiting changes between 1.0--7.6\%. Using the tight-binding model, we show for diatomic molecules that the nature of this effect lies in the angular rearrangement of bonding and antibonding orbitals introduced by spin-orbit coupling. Such an angular rearrangement appears in partially filled $p$- or $d$-orbitals in heavy elements. Finally, we discuss the impact of the relativistic effects on the chemical bonding in single-layer iodides and transition metal dichalcogenides

cond-mat.mtrl-sci↗

arXiv2022

A consistent picture of excitations in cubic BaSnO$_{3}$ revealed by combining theory and experiment

Among the transparent conducting oxides, the perovskite barium stannate is most promising for various electronic applications due to its outstanding carrier mobility achieved at room temperature. However, most of its important characteristics, such as band gaps, effective masses, and absorption edge, remain controversial. Here, we provide a fully consistent picture by combining state-of-the-art {\it ab initio} methodology with forefront electron energy-loss spectroscopy and optical absorption measurements. Valence electron energy-loss spectra, featuring signals originating from band gap transitions, are acquired on defect-free sample regions of a BaSnO$_{3}$ single crystal. These high-energy-resolution measurements are able to capture also very weak excitations below the optical gap, attributed to indirect transitions. By temperature-dependent optical absorption measurements, we assess band-gap renormalization effects induced by electron-phonon coupling. Overall, we find for the effective electronic mass, the direct and the indirect gap, the optical gap, as well as the absorption onsets and spectra, excellent agreement between both experimental techniques and the theoretical many-body results, supporting also the picture of a phonon-mediated mechanism where indirect transitions are activated by phonon-induced symmetry lowering. This work demonstrates a fruitful connection between different high-level theoretical and experimental methods for exploring the characteristics of advanced materials.

cond-mat.mtrl-sci↗

arXiv2022

How ferroelectric BaTiO$_3$ can tune a two-dimensional electron gas at the interface of LaInO3 and BaSnO$_3$: a first-principles study

The emerging interest in two-dimensional electron gases (2DEGs), formed at interfaces between two insulating oxide perovskites poses crucial fundamental question in view of future electronic devices. In the framework of density-functional theory, we investigate the possibility to control the characteristics of the 2DEG formed at the LaInO$_3$/BaSnO$_3$ interface by including a ferroelectric BaTiO$_3$ layer. To do so, we examine how the orientation of the ferroelectric polarization impacts density and confinement of the 2DEG. We find that aligning the ferroelectric polarization toward (outward) the LaInO$_3$/BaSnO$_3$ interface leads to an accumulation (depletion) of the interfacial 2DEG. Varying its magnitude, we find a linear effect on the 2DEG charge density that is confined within the BaSnO$_3$ side. Analysis of the optimized geometries revels that inclusion of the BaTiO$_3$ block makes structural distortions at the LaInO$_3$/BaSnO$_3$ less pronounced, which, in turn, enhances the 2DEG density. Thicker ferroelectric layers allow for reaching higher polarization magnitudes. We discuss the mechanisms behind all these findings and rationalize how the characteristics of both 2DEG and 2D hole gases can be controlled in the considered heterostructures. Overall, our results can be generalized to other combinations of ferroelectric, polar, and nonpolar materials.

cond-mat.mtrl-sci↗

arXiv2022

Numerical Quality Control for DFT-based Materials Databases

Electronic-structure theory is a strong pillar of materials science. Many different computer codes that employ different approaches are used by the community to solve various scientific problems. Still, the precision of different packages has only recently been scrutinized thoroughly, focusing on a specific task, namely selecting a popular density functional, and using unusually high, extremely precise numerical settings for investigating 71 monoatomic crystals. Little is known, however, about method- and code-specific uncertainties that arise under numerical settings that are commonly used in practice. We shed light on this issue by investigating the deviations in total and relative energies as a function of computational parameters. Using typical settings for basis sets and k-grids, we compare results for 71 elemental and 63 binary solids obtained by three different electronic-structure codes that employ fundamentally different strategies. On the basis of the observed trends, we propose a simple, analytical model for the estimation of the errors associated with the basis-set incompleteness. We cross-validate this model using ternary systems obtained from the NOMAD Repository and discuss how our approach enables the comparison of the heterogeneous data present in computational materials databases.

physics.comp-ph↗

arXiv2022

Ehrenfest dynamics implemented in the all-electron package exciting

Ehrenfest Dynamics combined with real-time time-dependent density functional theory has proven to be a reliable tool to study non-adiabatic molecular dynamics with a reasonable computational cost. Among other possibilities, it allows for assessing in real time electronic excitations generated by ultra-fast laser pulses, as e.g., in pump-probe spectroscopy, and their coupling to the nuclear vibrations even beyond the linear regime. In this work, we present its implementation in the all-electron full-potential package exciting. Three cases are presented as examples: diamond and cubic BN relaxed after an initial lattice distortion, and cubic BN exposed to a laser pulse. Comparison with the Octopus code exhibits good agreement.

cond-mat.mtrl-sci↗

arXiv2022

Robust excitons across the phase transition of two-dimensional hybrid perovskites

Two-dimensional halide perovskites are among intensely studied materials platforms profiting from solution based growth and chemical flexibility. They feature exceptionally strong interactions among electronic, optical as well as vibrational excitations and hold a great potential for future optoelectronic applications. A key feature for these materials is the occurrence of structural phase transitions that can impact their functional properties, including the electronic band gap and optical response dominated by excitons. However, to what extent the phase-transitions in two-dimensional perovskites alter the fundamental exciton properties remains barely explored so far. Here, we study the influence of the phase transition on both exciton binding energy and exciton diffusion, demonstrating their robust nature across the phase transition. These findings are unexpected in view of the associated substantial changes of the free carrier masses, strongly contrast broadly considered effective mass and drift-diffusion transport mechanisms, highlighting the unusual nature of excitons in two-dimensional perovskites.

cond-mat.mes-hall↗

arXiv2022

Tuning two-dimensional electron (hole) gases at LaInO$_{3}$/BaSnO$_{3}$ interfaces: Impact of polar distortions, termination, and thickness

Two-dimensional election gases (2DEG), arising due to quantum confinement at interfaces between transparent conducting oxides, have received tremendous attention in view of electronic applications. The challenge is to find a material system that exhibits both a high charge-carrier density and mobility, at and even above room temperature. Here, we explore the potential of interfaces formed by two lattice-matched wide-gap oxides of emerging interest, $\textit{i.e.}$, the polar, orthorhombic perovskite LaInO$_{3}$ and the non-polar, cubic perovskite BaSnO$_{3}$, employing density-functional theory and many-body theory. We demonstrate that this material combination exhibits all the key features for reaching the goal. For periodic heterostructures, we find that the polar discontinuity at the interface is mainly compensated by electronic relaxation through charge transfer from the LaInO$_{3}$ to the BaSnO$_{3}$ side. This leads to the formation of a 2DEG hosted by the highly-dispersive Sn-$s$-derived conduction band and a 2D hole gas of O-$p$ character, strongly localized inside LaInO$_{3}$. Remarkably, structural distortions through octahedra tilts induce a depolarization field counteracting the polar discontinuity, and thus increasing the $critical$ (minimal) LaInO$_{3}$ thickness, $t_c$, required for the formation of a 2DEG. These polar distortions decrease with increasing LaInO$_{3}$ thickness, enhancing the polar discontinuity and leading to a 2DEG density of 0.5 electron per unit-cell surface. Interestingly, in non-periodic heterostructures, these distortions lead to a decrease of $t_c$, thereby enhancing and delocalizing the 2DEG. We rationalize how polar distortions, termination, and thickness can be exploited in view of tailoring the 2DEG characteristics, and why this material is superior to the most studied prototype LaAlO$_{3}$/SrTiO$_{3}$.

cond-mat.mtrl-sci↗

arXiv2022

Two-dimensional electron gas at LaInO$_3$/BaSnO$_3$ interfaces controlled by a ferroelectric layer

With the example of LaInO$_{3}$/BaSnO$_3$, we demonstrate how both density and distribution of a two-dimensional electron gas (2DEG) formed at the interface between these perovskite oxides, can be efficiently controlled by a ferroelectric functional material. A polarization induced in a BaTiO$_3$ layer pointing toward the interface enhances the polar discontinuity which, in turn, significantly increases the 2DEG density and confinement, while, the opposite polarization depletes the 2DEG. Our predictions and analysis, based on first-principles calculations, can serve as a guide for designing such material combinations to be used in electronic devices.

cond-mat.mtrl-sci↗