arXiv subjects

Rico Friedrich

Publications and source records attributed to Rico Friedrich.

14 recordsLinked to original sources

arXiv2026

Towards Non-van der Waals 2D Topological Insulators

Non-van der Waals two-dimensional (2D) materials derived from strongly bonded non-layered crystals have recently emerged as a novel and rising platform for nanoscale research. While uncovering and tuning their (opto-)electronic, catalytic, and magnetic properties has been the focus of intense research, the impact of spin-orbit coupling (SOC) onto their electronic structure has not yet been explored in detail. Studying these effects is, however, particularly relevant due to their surface cation termination and the presence of heavy elements in several representative compounds. Here, we investigate the effect of SOC onto the electronic structure of 2D AgBiO3, NaBiO3, and SbTlO3. While the first two systems show negligible band renormalization upon inclusion of relativistic effects around the band gap, SbTlO3 showcases a large SOC induced splitting (229meV) for the lowest conduction bands associated with a band inversion. Substitution of Tl with Pb forming SbPbO3 brings the band-inverted feature to the Fermi level. Analysis of topological invariants and investigation of edge states of zig-zag and armchair ribbons within the 200meV gap confirms the topological nature of the band splitting. Our work thus establishes a foundation for the systematic study of robust non-van der Waals 2D topological insulators.

cond-mat.mtrl-sci↗

arXiv2026

Perspective: Towards sustainable exploration of chemical spaces with machine learning

Artificial intelligence is transforming molecular and materials science, but its growing computational and data demands raise critical sustainability challenges. In this Perspective, we examine resource considerations across the AI-driven discovery pipeline--from quantum-mechanical (QM) data generation and model training to automated, self-driving research workflows--building on discussions from the ``SusML workshop: Towards sustainable exploration of chemical spaces with machine learning'' held in Dresden, Germany. In this context, the availability of large quantum datasets has enabled rigorous benchmarking and rapid methodological progress, while also incurring substantial energy and infrastructure costs. We highlight emerging strategies to enhance efficiency, including general-purpose machine learning (ML) models, multi-fidelity approaches, model distillation, and active learning. Moreover, incorporating physics-based constraints within hierarchical workflows, where fast ML surrogates are applied broadly and high-accuracy QM methods are used selectively, can further optimize resource use without compromising reliability. Equally important is bridging the gap between idealized computational predictions and real-world conditions by accounting for synthesizability and multi-objective design criteria, which is essential for practical impact. Finally, we argue that sustainable progress will rely on open data and models, reusable workflows, and domain-specific AI systems that maximize scientific value per unit of computation, enabling efficient and responsible discovery of technological materials and therapeutics.

cs.LG↗

arXiv2025

Exfoliation and Cleavage of Crystals from a Universal Potential

Exfoliation and cleavage create two-dimensional (2D) materials and surfaces with physical and chemical properties distinct from their bulk parents. The rising class of non-van der Waals (non-vdW) 2D materials derived from non-layered crystals provides a fascinating new platform - greatly expanding the landscape of low-dimensional materials. Current computational models, however, provide limited guidance: existing descriptors are largely tailored to vdW layered systems. Here, we introduce a general framework predicting crystal cleavage and exfoliable 2D subunits directly from bulk structures. At its core is a universal eXfoliation and Cleavage Potential (XCP) enabling large-scale screening of diverse materials at negligible computational cost. Applying this approach, we obtain 37,208 cleavable surfaces and candidate non-vdW 2D materials from which we investigate 2,377 likely exfoliable ones using high-throughput density functional theory. We identify sheets with square and rectangular lattices, semiconducting systems exhibiting an indirect-to-direct band-gap transition upon exfoliation, and first non-vdW 2D metals. Our study thus opens a systematic route to explore and design new 2D materials with unprecedented chemical and structural diversity.

cond-mat.mtrl-sci↗

arXiv2025

AFLOW4: heading toward disorder

AFLOW4 is the latest iteration of the AFLOW toolkit, specifically tailored to study high-entropy disordered materials. This upgrade includes innovative features like the Soliquidy module, based on the Euclidean transport cost between disordered and ordered material states. AFLOW4 can calculate dielectric functions to understand optical and electronic properties of disordered ceramics. The newly introduced human-readable data export feature ensures the uncomplicated incorporation of AFLOW4 in diverse automated workflows. Features relevant to high-entropy research, like prototype identification, partial occupation method, convex hull calculation, and enthalpy corrections based on local atomic environments, have been improved and exhibit substantial speed-up. Together, these enhancements represent a step forward for AFLOW as a valuable tool for research of high-entropy materials.

cond-mat.mtrl-sci↗

arXiv2025

Non-van der Waals Heterostructures

Beyond the study of individual materials, their interfaces and arising functionality are crucial enablers of fundamental science and technological progress. Recently, the arena of two-dimensional (2D) materials was extended to so-called non-van der Waals (non-vdW) compounds derived from strongly bonded non-layered bulk crystals. These nanosystems have advantageous active surfaces and thus now complement - through their chemically bonded heterostructure (HS) interfaces - the established space of weakly interacting vdW HSs. Here, we study a wide range of 55 candidate non-vdW HSs with extensive density functional calculations and find that the resulting interfacial chemical hybridization and bonding is the key factor determining their electronic and magnetic properties. It gives rise to the formation of unique hybrid interface bands, strong magnetic coupling, and substantial electronic as well as magnetic moire surface property modulations upon twisting. Our work thus provides a valuable platform for charting the new interface class of non-vdW HSs.

cond-mat.mtrl-sci↗

arXiv2023

A priori procedure to establish spinodal decomposition in alloys

Spinodal decomposition can improve a number of essential properties in materials, especially hardness. Yet, the theoretical prediction of the onset of this phenomenon (e.g., temperature) and its microstructure (e.g., wavelength) often requires input parameters coming from costly and time-consuming experimental efforts, hindering rational materials optimization. Here, we present a procedure where such parameters are not derived from experiments. First, we calculate the spinodal temperature by modeling nucleation in the solid solution while approaching the spinode boundary. Then, we compute the spinodal wavelength self-consistently using a few reasonable approximations. Our results show remarkable agreement with experiments and, for NiRh, the calculated yield strength due to spinodal microstructures surpasses even those of Ni-based superalloys. We believe that this procedure will accelerate the exploration of the complex materials experiencing spinodal decomposition, critical for their macroscopic properties.

cond-mat.mtrl-sci↗

arXiv2023

AFLOW-CCE for the thermodynamics of ionic materials

Accurate thermodynamic stability predictions enable data-driven computational materials design. Standard density functional theory (DFT) approximations have limited accuracy with average errors of a few hundred meV/atom for ionic materials such as oxides and nitrides. Thus, insightful correction schemes as given by the coordination corrected enthalpies (CCE) method, based on an intuitive parameterization of DFT errors with respect to coordination numbers and cation oxidation states present a simple, yet accurate solution to enable materials stability assessments. Here, we illustrate the computational capabilities of our AFLOW-CCE software by utilizing our previous results for oxides and introducing new results for nitrides. The implementation reduces the deviations between theory and experiment to the order of the room temperature thermal energy scale, i.e. ~25 meV/atom. The automated corrections for both materials classes are freely available within the AFLOW ecosystem via the AFLOW-CCE module, requiring only structural inputs.

cond-mat.mtrl-sci↗

arXiv2023

Magnetic State Control of Non-van der Waals 2D Materials by Hydrogenation

Controlling the magnetic state of two-dimensional (2D) materials is crucial for spintronic applications. By employing data-mining and autonomous density functional theory calculations, we demonstrate the switching of magnetic properties of 2D non-van der Waals materials upon hydrogen passivation. The magnetic configurations are tuned to states with flipped and enhanced moments. For 2D CdTiO$_3$ - a nonmagnetic compound in the pristine case - we observe an onset of ferromagnetism upon hydrogenation. Further investigation of the magnetization density of the pristine and passivated systems provides a detailed analysis of modified local spin symmetries and the emergence of ferromagnetism. Our results indicate that selective surface passivation is a powerful tool for tailoring magnetic properties of nanomaterials such as non-vdW 2D compounds.

cond-mat.mtrl-sci↗

arXiv2022

A New Group of Two-Dimensional Non-van der Waals Materials with Ultra Low Exfoliation Energies

The exfoliation energy - quantifying the energy required to extract a two-dimensional (2D) sheet from the surface of a bulk material - is a key parameter determining the synthesizability of 2D compounds. Here, using ab initio calculations, we present a new group of non-van der Waals 2D materials derived from non-layered crystals which exhibit ultra low exfoliation energies. In particular for sulfides, surface relaxations are essential to correctly describe the associated energy gain needed to obtain reliable results. Taking into account long-range dispersive interactions has only a minor effect on the energetics and ultimately proves that the exfoliation energies are close to the ones of traditional van der Waals bound 2D compounds. The candidates with the lowest energies, 2D SbTlO$_3$ and MnNaCl$_3$, exhibit appealing electronic, potential topological, and magnetic features as evident from the calculated band structures making these systems an attractive platform for fundamental and applied nanoscience.

cond-mat.mtrl-sci↗

arXiv2022

aflow.org: A Web Ecosystem of Databases, Software and Tools

To enable materials databases supporting computational and experimental research, it is critical to develop platforms that both facilitate access to the data and provide the tools used to generate/analyze it - all while considering the diversity of users' experience levels and usage needs. The recently formulated FAIR principles (Findable, Accessible, Interoperable, and Reusable) establish a common framework to aid these efforts. This article describes aflow_org, a web ecosystem developed to provide FAIR - compliant access to the AFLOW databases. Graphical and programmatic retrieval methods are offered, ensuring accessibility for all experience levels and data needs. aflow_org goes beyond data-access by providing applications to important features of the AFLOW software, assisting users in their own calculations without the need to install the entire high-throughput framework. Outreach commitments to provide AFLOW tutorials and materials science education to a global and diverse audiences will also be presented.

cond-mat.mtrl-sci↗

arXiv2021

Data-driven quest for two-dimensional non-van der Waals materials

Two-dimensional (2D) materials are frequently associated with the sheets which form bulk layered compounds bonded by van der Waals (vdW) forces. The anisotropy and weak interaction between the sheets have also been the main criteria in the computational search for new 2D systems, which predicted about 2000 exfoliable compounds. However, several representatives of a new type of non-vdW 2D systems, such as hematene or ilmenene, which have no layered 3D analogues, and which, unlike, e.g. silicene, do not need to strongly interact with the substrate to be stable, were recently manufactured. The family of non-vdW 2D materials is an attractive playground for data-driven high-throughput approaches as computational design principles are still missing. Here, we outline a new set of 8 binary and 20 ternary candidates by filtering the AFLOW-ICSD database according to the structural prototype of the first two template systems realized in experiment. All materials show a strong structural relaxation upon confinement to 2D which is essential to correctly estimate the bonding strength between facets. The oxidation state of the cations at the surface of the sheets is demonstrated to regulate the inter-facet binding energy with low oxidation states leading to weak bonding. We anticipate this descriptor to be highly useful to obtain novel 2D materials, providing clear guidelines for experiments. Our calculations further indicate that the materials exhibit a vast range of appealing electronic, optical and magnetic properties, which is expected to also make them attractive for potential applications particularly in spintronics.

cond-mat.mtrl-sci↗

arXiv2020

The Tin Pest Problem as a Test of Density Functionals Using High-Throughput Calculations

At ambient pressure tin transforms from its ground-state semi-metal $α$-Sn (diamond structure) phase to the compact metallic $β$-Sn phase at 13$^\circ$C (286K). There may be a further transition to the simple hexagonal $γ$-Sn above 450K. These relatively low transition temperatures are due to the small energy differences between the structures, $\approx 20$\,meV/atom between $α$- and $β$-Sn. This makes tin an exceptionally sensitive test of the accuracy of density functionals and computational methods. Here we use the high-throughput Automatic-FLOW (AFLOW) method to study the energetics of tin in multiple structures using a variety of density functionals. We look at the successes and deficiencies of each functional. As no functional is completely satisfactory, we look Hubbard U corrections and show that the Coulomb interaction can be chosen to predict the correct phase transition temperature. We also discuss the necessity of testing high-throughput calculations for convergence for systems with small energy differences.

cond-mat.mtrl-sci↗

arXiv2019

Coordination corrected ab initio formation enthalpies

The correct calculation of formation enthalpy is one of the enablers of ab-initio computational materials design. For several classes of systems (e.g. oxides) standard density functional theory produces incorrect values. Here we propose the "Coordination Corrected Enthalpies" method (CCE), based on the number of nearest neighbor cation-anion bonds, and also capable of correcting relative stability of polymorphs. CCE uses calculations employing the Perdew, Burke and Ernzerhof (PBE), Local Density Approximation (LDA) and Strongly Constrained and Appropriately Normed (SCAN) exchange correlation functionals, in conjunction with a quasiharmonic Debye model to treat zero-point vibrational and thermal effects. The benchmark, performed on binary and ternary oxides (halides), shows very accurate room temperature results for all functionals, with the smallest mean absolute error of 27 (24) meV/atom obtained with SCAN. The zero-point vibrational and thermal contributions to the formation enthalpies are small and with different signs - largely cancelling each other.

cond-mat.mtrl-sci↗

arXiv2018

Spinodal superlattices of topological insulators

Spinodal decomposition is proposed for stabilizing self-assembled interfaces between topological insulators (TIs) by combining layers of iso-structural and iso-valent TlBi$X_2$ ($X$=S, Se, Te) materials. The composition range for gapless states is addressed concurrently to the study of thermodynamically driven boundaries. By tailoring composition, the TlBiS$_2$-TlBiTe$_2$ system might produce both spinodal superlattices and two dimensional eutectic microstructures, either concurrently or separately. The dimensions and topological nature of the metallic channels are determined by following the spatial distribution of the charge density and the spin-texture. The results validate the proof of concept for obtaining spontaneously forming two-dimensional TI-conducting channels embedded into three-dimensional insulating environments without any vacuum interfaces. Since spinodal decomposition is a controllable kinetic phenomenon, its leverage could become the long-sought enabler for effective TI technological deployment.

cond-mat.mtrl-sci↗