arXiv subjects

Claudia Draxl

Publications and source records attributed to Claudia Draxl.

At least 91 records · Page 5Linked to original sources

arXiv2020

Structural, electronic, and optical properties of periodic graphene/h-BN van der Waals heterostructures

The emerging interest in van der Waals heterostructures as new materials for opto-electronics and photonics poses questions about their stability and structure-property relations. In the framework of density-functional and many-body perturbation theory, we investigate the structural, electronic, and optical properties of periodic heterostructures formed by graphene and hexagonal boron nitride (h-BN). To understand how the constituents affect each other depending on the layer stacking, we examine 12 commensurate arrangements. We find that interaction with graphene improves the stability of bulk h-BN also in those configurations that are predicted to be energetically metastable. In return, the interaction with h-BN can open a band gap of a few hundred meV in graphene. Its actual size can be tuned by the arrangement of the layers. In the semiconducting configurations, the character and spatial distribution of optical excitations are affected by the specific stacking, that determines the electronic states involved in the transitions. Remarkably, six out of the 12 explored heterostructures remain semi-metallic.

cond-mat.mtrl-sci↗

arXiv2020

The role of the interface in controlling the epitaxial relationship between orthorhombic $\text{LaInO}_\text{3}$ and cubic $\text{BaSnO}_\text{3}$

Epitaxial perovskite oxide interfaces with different symmetry of the epitaxial layers have attracted considerable attention due to the emergence of novel behaviors and phenomena. In this paper, we show by aberration corrected transmission electron microscopy that orthorhombic $\text{LaInO}_\text{3}$ films grow in form of three different types of domains on the cubic $\text{BaSnO}_\text{3}$ pseudosubstrate. Quantitative evaluation of our TEM data shows that $c_{pc}$-oriented and $a_{pc}/b_{pc}$-oriented domains are present with similar probability. While continuum elasticity theory suggests that $c_{pc}$-oriented domains should exhibit a significantly higher strain energy density than $a_{pc}/b_{pc}$-oriented domains, density functional calculations confirm that $c_{pc}$- and $a_{pc}$-oriented domains on $\text{BaSnO}_\text{3}$ have similar energies.

cond-mat.mtrl-sci↗

arXiv2020

Band gap renormalization, carrier mobilities, and the electron-phonon self-energy in crystalline naphthalene

Organic molecular crystals are expected to feature appreciable electron-phonon interactions that influence their electronic properties at zero and finite temperature. In this work, we report first-principles calculations and an analysis of the electron-phonon self-energy in naphthalene crystals. We compute the zero-point renormalization and temperature dependence of the fundamental band gap, and the resulting scattering lifetimes of electronic states near the valence- and conduction-band edges employing density functional theory. Further, our calculated phonon renormalization of the $GW$-corrected quasiparticle band structure predicts a fundamental band gap of 5 eV for naphthalene at room temperature, in good agreement with experiments. From our calculated phonon-induced electron lifetimes, we obtain the temperature-dependent mobilities of electrons and holes in good agreement with experimental measurements at room temperatures. Finally, we show that an approximate energy self-consistent computational scheme for the electron-phonon self-energy leads to the prediction of strong satellite bands in the electronic band structure. We find that a single calculation of the self-energy can reproduce the self-consistent results of the band gap renormalization and electrical mobilities for naphthalene, provided that the on-the-mass-shell approximation is used, i.e., if the self-energy is evaluated at the bare eigenvalues.

cond-mat.mtrl-sci↗

arXiv2020

MoTe2 as a natural hyperbolic material across the visible and the ultraviolet region

Hyperbolic materials are of particular interest for the next generation of photonic and optoelectronic devices. Since artificial metamaterials are intrinsically limited by the size of their nanostructured components, there has been a hunt for natural hyperbolic materials in the last few years. In a first-principles work based on density-functional theory and many-body perturbation theory, we investigate the fundamental dielectric response of MoTe2 in monolayer, bilayer, and bulk form, and find that it is a natural type-II hyperbolic material with low losses between 3 and 6 eV. Going from the monolayer to the bulk, the energy window of hyperbolic dispersion is blue-shifted by a few tenths of an eV. We show that excitonic effects and optical anisotropy play a major role in the hyperbolic behavior of MoTe2. Our results confirm the potential of layered materials as hyperbolic media for opto-electronics, photonics, and nano-imaging applications.

cond-mat.mtrl-sci↗

arXiv2020

High Performance Solution of Skew-symmetric Eigenvalue Problems with Applications in Solving the Bethe-Salpeter Eigenvalue Problem

We present a high-performance solver for dense skew-symmetric matrix eigenvalue problems. Our work is motivated by applications in computational quantum physics, where one solution approach to solve the so-called Bethe-Salpeter equation involves the solution of a large, dense, skew-symmetric eigenvalue problem. The computed eigenpairs can be used to compute the optical absorption spectrum of molecules and crystalline systems. One state-of-the art high-performance solver package for symmetric matrices is the ELPA (Eigenvalue SoLvers for Petascale Applications) library. We extend the methods available in ELPA to skew-symmetric matrices. This way, the presented solution method can benefit from the optimizations available in ELPA that make it a well-established, efficient and scalable library, such as GPU support. We compare performance and scalability of our method to the only available high-performance approach for skew-symmetric matrices, an indirect route involving complex arithmetic. In total, we achieve a performance that is up to 3.67 higher than the reference method using Intel's ScaLAPACK implementation. The runtime to solve the Bethe-Salpeter-Eigenvalue problem can be improved by a factor of 10. Our method is freely available in the current release of the ELPA library.

math.NA↗

arXiv2020

Excitation Pathways in Resonant Inelastic X-Ray Scattering of Solids

We present a novel derivation of resonant inelastic x-ray scattering (RIXS) which yields an expression for the RIXS cross section in terms of emission pathways between intermediate and final many-body states. Thereby electron-hole interactions are accounted for, as obtained from full diagonalization of the Bethe-Salpeter equation within an all-electron first-principles framework. We demonstrate our approach with the emission spectra of the fluorine K edge in LiF, and provide an in-depth analysis of the pathways that determine the spectral shape. Excitoninc effects are shown to play a crucial role in both the valence and core regime.

cond-mat.mtrl-sci↗

arXiv2020

Assessment of approaches for dispersive forces employing graphone as a case study

We have studied two interchange layer systems, (i) free standing partly hydrogenated graphene (graphone), and (ii) graphone on the Nickel (111) surface, to assess various density functional theory based computational schemes incorporating van der Waals forces. The various van der Waals methods have been employed ranging from the semiempirical force-field-like correction of Grimme, through non-local van der Waals density functionals, up to the functionals involving exact exchange and the random phase approximation for the correlation. Generally, all computational schemes lead to a similar qualitative picture of hydrogen layer physisorption and chemisorption to graphene. The largest discrepancies between the approaches emerge for the energetics of the investigated systems. Our studies shed light on the physical mechanisms of graphene hydrogenation both in vacuum and in the proximity of metallic surface. In particular, it is revealed that the adsorption of hydrogen atoms affects the nature of the bonding between graphene and the Ni(111) surface, from the weak to strong semi-covalent bonding. On the other hand, it turns out that the adsorption of hydrogen layer to graphene is stronger in the presence of the metallic surface.

cond-mat.mtrl-sci↗

arXiv2019

Photoemission Signatures of Non-Equilibrium Carrier Dynamics from First Principles

Time- and angle-resolved photoemission spectroscopy (tr-ARPES) constitutes a powerful tool to inspect the dynamics and thermalization of hot carriers. The identification of the processes that drive the dynamics, however, is challenging even for the simplest systems owing to the coexistence of several relaxation mechanisms. Here, we devise a Green's function formalism for predicting the tr-ARPES spectral function and establish the origin of carrier thermalization entirely from first principles. The predictive power of this approach is demonstrated by an excellent agreement with experiments for graphene over time scales ranging from a few tens of femtoseconds up to several picoseconds. Our work provides compelling evidence of a non-equilibrium dynamics dominated by the establishment of a hot-phonon regime.

cond-mat.mtrl-sci↗

arXiv2019

Maximally localized Wannier functions within the (L)APW+LO method

We present a robust algorithm that computes (maximally localized) Wannier functions (WFs) without the need of providing an initial guess. Instead, a suitable starting point is constructed automatically from so-called local orbitals which are fundamental building blocks of the basis set within (linearized) augmented planewave methods. Our approach is applied to a vast variety of materials such as metals, bulk and low-dimensional semiconductors, and complex inorganic-organic hybrid interfaces. For the interpolation of electronic single-particle energies, an accuracy in the meV range can be easily achieved. We exemplify the capabilities of our method by the calculation of the joint density of states in aluminum, (generalized) Kohn-Sham and quasi-particle band structures in various semiconductors, and the electronic structure of $β$-Ga$_2$O$_3$, including electron and hole effective masses.

cond-mat.mtrl-sci↗

arXiv2019

Energy-level alignment at organic/inorganic interfaces from first principles: Example of poly(\emph{para}-phenylene) / rock-salt ZnO(100)

By means of full-potential all-electron density-functional theory and many-body perturbation theory, we compute the band alignment at a prototypical hybrid inorganic/organic interface. The electronic properties of a model system built of poly(\emph{para}-phenylene) and \emph{rs}-ZnO are studied in two different geometries, employing several approaches of increasing sophistication. To this extent, we explore models for predicting the level alignment, which are based on the knowledge of the electronic structure of the individual constituents and are commonly used for semiconductor interfaces. For their evaluation in the context of hybrid materials, we perform an \textit{ab-initio} study of the entire system, including a quasiparticle description of the electronic structure within the $G_0W_0$ approximation. Based on this, we quantify the impact of structure, charge redistribution, orbital hybridization, and molecular polarization on the band offsets and the alignment type. We highlight not only known limitations of predicting the level alignment at a hybrid inorganic/organic interface by simple models, but also demonstrate how structural details of the interface components impact the results.

cond-mat.mtrl-sci↗

arXiv2019

Electronic and optical excitations of two-dimensional ZrS$_2$ and HfS$_2$ and their heterostructure

In a first-principles study based on density-functional theory and many-body perturbation theory we investigate the electronic properties and the optical excitations of ZrS$_2$ and HfS$_2$ monolayers and their van der Waals heterostructure. Both materials have an indirect quasi-particle band gap, which amounts to about 2.8 eV in ZrS$_2$ and to 2.6 eV in HfS$_2$. In both systems the valence-band maximum is at $Γ$ and the conduction-band minimum at M. Spin-orbit coupling induces a splitting of about 100 meV at the $Γ$ point in the valence band, while it does not affect the conduction band. The optical absorption spectra are dominated by excitonic peaks, with binding energies between 0.6 eV and 0.8 eV. The ZrS$_2$/HfS$_2$ heterobilayer exhibits a peculiar type-I level alignment with a large degree of hybridization between the two monolayers in the valence band, while the conduction bands retain either ZrS$_2$ or HfS$_2$ character, respectively. As a consequence, both the electron and the hole components of the first exciton are localized in the ZrS$_2$ monolayer with non-vanishing probability of finding the hole also in the HfS$_2$ sheet.

cond-mat.mtrl-sci↗

arXiv2019

Challenges for Verifying and Validating Scientific Software in Computational Materials Science

Many fields of science rely on software systems to answer different research questions. For valid results researchers need to trust the results scientific software produces, and consequently quality assurance is of utmost importance. In this paper we are investigating the impact of quality assurance in the domain of computational materials science (CMS). Based on our experience in this domain we formulate challenges for validation and verification of scientific software and their results. Furthermore, we describe directions for future research that can potentially help dealing with these challenges.

cs.SE↗

arXiv2019

Thermally-Enhanced Fröhlich Coupling in SnSe

To gain insight into the peculiar temperature dependence of the thermoelectric material SnSe, we employ many-body perturbation theory and explore the influence of the electron-phonon interaction on its electronic and transport properties. We show that a lattice dynamics characterized by soft highly-polar phonons induces a large thermal enhancement of the Fröhlich interaction. We account for these phenomena in ab-initio calculations of the photoemission spectrum and electrical conductivity at finite temperature, unraveling the mechanisms behind recent experimental data. Our results reveal a complex interplay between lattice thermal expansion and Fröhlich coupling, providing a new rationale for the in-silico prediction of transport coefficients of high-performance thermoelectrics.

cond-mat.mtrl-sci↗

arXiv2019

Vibrational Effects in X-ray Absorption Spectra of 2D Layered Materials

With the examples of the C $K$-edge in graphite and the B $K$-edge in hexagonal BN, we demonstrate the impact of vibrational coupling and lattice distortions on the X-ray absorption near-edge structure (XANES) in 2D layered materials. Theoretical XANES spectra are obtained by solving the Bethe-Salpeter equation of many-body perturbation theory, including excitonic effects through the correlated motion of core-hole and excited electron. We show that accounting for zero-point motion is important for the interpretation and understanding of the measured X-ray absorption fine structure in both materials, in particular for describing the $σ^*$-peak structure.

cond-mat.mtrl-sci↗

arXiv2019

Bethe-Salpeter equation for absorption and scattering spectroscopy: Implementation in the $\texttt{exciting}$ code

The Bethe-Salpeter equation for the electron-hole correlation function is the state-of-the-art formalism for optical and core spectroscopy in condensed matter. Solutions of this equation yield the full dielectric response, including both the absorption and the inelastic scattering spectra. Here, we present an efficient implementation within the all-electron full-potential code $\texttt{exciting}$, which employs the linearized augmented plane-wave (L)APW+LO basis set. Being an all-electron code, $\texttt{exciting}$ allows the calculation of optical and core excitations on the same footing. The implementation fully includes the effects of finite momentum transfer which may occur in inelastic x-ray spectroscopy and electron energy-loss spectroscopy. Our implementation does not require the application of the Tamm-Dancoff approximation that is commonly employed in the determination of absorption spectra in condensed matter. The interface with parallel linear-algebra libraries enables the calculation for complex systems. The capability of our implementation to compute, analyze, and interpret the results of different spectroscopic techniques is demonstrated by selected examples of prototypical inorganic and organic semiconductors and insulators.

cond-mat.mtrl-sci↗

arXiv2019

Big-Data-Driven Materials Science and its FAIR Data Infrastructure

This chapter addresses the forth paradigm of materials research -- big-data driven materials science. Its concepts and state-of-the-art are described, and its challenges and chances are discussed. For furthering the field, Open Data and an all-embracing sharing, an efficient data infrastructure, and the rich ecosystem of computer codes used in the community are of critical importance. For shaping this forth paradigm and contributing to the development or discovery of improved and novel materials, data must be what is now called FAIR -- Findable, Accessible, Interoperable and Re-purposable/Re-usable. This sets the stage for advances of methods from artificial intelligence that operate on large data sets to find trends and patterns that cannot be obtained from individual calculations and not even directly from high-throughput studies. Recent progress is reviewed and demonstrated, and the chapter is concluded by a forward-looking perspective, addressing important not yet solved challenges.

cond-mat.mtrl-sci↗

arXiv2018

Polaron-induced band renormalization due to linear and quadratic electron-phonon coupling

We present a novel approach to electron-lattice interaction beyond the linear-coupling regime. Based on the solution of a Holstein-Peierls-type model, we derive explicit analytical expressions for the eigenvalue spectrum of the Hamiltonian, resulting in a narrowing of bands as a function of temperature. Our approach enables the intuitive interpretation in terms of quasiparticles, i.e. polaron bands and dressed-phonon frequencies. Being nonperturbative, the formalism also applies in the strong-coupling case. We apply it to the organic crystal naphthalene, with the coupling strengths obtained by \textit{ab initio} calculations.

cond-mat.mtrl-sci↗

arXiv2018

Hybrid Organic-Inorganic Perovskites as Promising Substrates for Pt Single-Atom Catalysts

Single-atom catalysts (SACs) combine the best of two worlds by bridging heterogeneous and homogeneous catalysis. The superior catalytic properties of SACs, however, can hardly be exploited without a suitable substrate. Here, we explore the possibility of using hybrid organic-inorganic perovskites as supporting materials for single transition-metal atoms. By means of first-principles calculations, we predict that single Pt atoms can be incorporated into methylammonium lead iodide surfaces by replacing the methylammonium groups at the outermost layer. The iodide anions at the surface provide potentially uniform anchoring sites for the Pt atoms and donate electrons, generating negatively-charged Pt$_{1}^{δ-}$ species that allow for preferential O$_{2}$ adsorption in the presence of CO. Such Pt sites are able to catalyze CO oxidation and may also play a role in CO$_{2}$ reduction. The fundamental understanding generated here will shed light on potential applications of hybrid perovskites in the field of (photo)catalysis.

cond-mat.mtrl-sci↗