| 1. |
- Anker, Andy S., et al.
(författare)
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Extracting structural motifs from pair distribution function data of nanostructures using explainable machine learning
- 2022
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Ingår i: npj Computational Materials. - : Springer Science and Business Media LLC. - 2057-3960. ; 8:1
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Tidskriftsartikel (refereegranskat)abstract
- Characterization of material structure with X-ray or neutron scattering using e.g. Pair Distribution Function (PDF) analysis most often rely on refining a structure model against an experimental dataset. However, identifying a suitable model is often a bottleneck. Recently, automated approaches have made it possible to test thousands of models for each dataset, but these methods are computationally expensive and analysing the output, i.e. extracting structural information from the resulting fits in a meaningful way, is challenging. Our Machine Learning based Motif Extractor (ML-MotEx) trains an ML algorithm on thousands of fits, and uses SHAP (SHapley Additive exPlanation) values to identify which model features are important for the fit quality. We use the method for 4 different chemical systems, including disordered nanomaterials and clusters. ML-MotEx opens for a type of modelling where each feature in a model is assigned an importance value for the fit quality based on explainable ML.
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| 2. |
- Auenhammer, Robert, 1991, et al.
(författare)
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X-ray scattering tensor tomography based finite element modelling of heterogeneous materials
- 2024
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Ingår i: npj Computational Materials. - 2057-3960. ; 10:1
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Tidskriftsartikel (refereegranskat)abstract
- Among micro-scale imaging technologies of materials, X-ray micro-computed tomography has evolved as most popular choice, even though it is restricted to limited field-of-views and long acquisition times. With recent progress in small-angle X-ray scattering these downsides of conventional absorption-based computed tomography have been overcome, allowing complete analysis of the micro-architecture for samples in the dimension of centimetres in a matter of minutes. These advances have been triggered through improved X-ray optical elements and acquisition methods. However, it has not yet been shown how to effectively transfer this small-angle X-ray scattering data into a numerical model capable of accurately predicting the actual material properties. Here, a method is presented to numerically predict mechanical properties of a carbon fibre-reinforced polymer based on imaging data with a voxel-size of 100 μm corresponding to approximately fifteen times the fibre diameter. This extremely low resolution requires a completely new way of constructing the material’s constitutive law based on the fibre orientation, the X-ray scattering anisotropy, and the X-ray scattering intensity. The proposed method combining the advances in X-ray imaging and the presented material model opens for an accurate tensile modulus prediction for volumes of interest between three to six orders of magnitude larger than those conventional carbon fibre orientation image-based models can cover.
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| 3. |
- Benedek, Zsolt, et al.
(författare)
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Symmetric carbon tetramers forming spin qubits in hexagonal boron nitride
- 2023
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Ingår i: npj Computational Materials. - : NATURE PORTFOLIO. - 2057-3960. ; 9:1
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Tidskriftsartikel (refereegranskat)abstract
- Point defect quantum bits in semiconductors have the potential to revolutionize sensing at atomic scales. Currently, vacancy-related defects are at the forefront of high spatial resolution and low-dimensional sensing. On the other hand, it is expected that impurity-related defect structures may give rise to new features that could further advance quantum sensing in low dimensions. Here, we study the symmetric carbon tetramer clusters in hexagonal boron nitride and propose them as spin qubits for sensing. We utilize periodic-DFT and quantum chemistry approaches to reliably and accurately predict the electronic, optical, and spin properties of the studied defect. We show that the nitrogen-centered symmetric carbon tetramer gives rise to spin state-dependent optical signals with strain-sensitive intersystem crossing rates. Furthermore, the weak hyperfine coupling of the defect to their spin environments results in a reduced electron spin resonance linewidth that can enhance sensitivity.
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| 4. |
- Borisov, Vladislav, et al.
(författare)
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Dzyaloshinskii-Moriya interactions, Néel skyrmions and V4 magnetic clusters in multiferroic lacunar spinel GaV4S8
- 2024
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Ingår i: npj Computational Materials. - : Springer Nature. - 2057-3960. ; 10:1
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Tidskriftsartikel (refereegranskat)abstract
- Using ab initio density functional theory with static mean-field correlations, we calculate the Heisenberg and Dzyaloshinskii-Moriya interactions (DMI) for an atomistic spin Hamiltonian for the lacunar spinel, GaV4S8. The parameters describing these interactions are used in atomistic spin dynamics and micromagnetic simulations. The magnetic properties of the lacunar spinel GaV4S8, a material well-known from experiment to host magnetic skyrmions of Néel character, are simulated with these ab initio calculated parameters. The Dzyaloshinskii-Moriya contribution to the micromagnetic energy is a sum of two Lifshitz invariants, supporting the formation of Néel skyrmions and its symmetry agrees with what is usually expected for C3ν-symmetric systems. There are several conclusions one may draw from this work. One concerns the quantum nature of the magnetism, where we show that the precise magnetic state of the V4 cluster is crucial for understanding quantitatively the magnetic phase diagram. In particular, we demonstrate that a distributed-moment state of each V4 cluster explains well a variety of properties of GaV4S8, such as the band gap, observed Curie temperature and especially the stability of Néel skyrmions in the experimentally relevant temperature and magnetic-field range. In addition, we find that electronic correlations change visibly the calculated value of the DMI.
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| 5. |
- Borysov, Stanislav S., et al.
(författare)
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Online search tool for graphical patterns in electronic band structures
- 2018
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Ingår i: npj Computational Materials. - : Springer Science and Business Media LLC. - 2057-3960. ; 4
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Tidskriftsartikel (refereegranskat)abstract
- Many functional materials can be characterized by a specific pattern in their electronic band structure, for example, Dirac materials, characterized by a linear crossing of bands; topological insulators, characterized by a Mexican hat pattern or an effectively free electron gas, characterized by a parabolic dispersion. To find material realizations of these features, manual inspection of electronic band structures represents a relatively easy task for a small number of materials. However, the growing amount of data contained within modern electronic band structure databases makes this approach impracticable. To address this problem, we present an automatic graphical pattern search tool implemented for the electronic band structures contained within the Organic Materials Database. The tool is capable of finding user-specified graphical patterns in the collection of thousands of band structures from high-throughput calculations in the online regime. Using this tool, it only takes a few seconds to find an arbitrary graphical pattern within the ten electronic bands near the Fermi level for 26,739 organic crystals. The source code of the developed tool is freely available and can be adapted to any other electronic band structure database.
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| 6. |
- Bulancea Lindvall, Oscar, et al.
(författare)
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Dipolar spin relaxation of divacancy qubits in silicon carbide
- 2021
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Ingår i: npj Computational Materials. - : Nature Portfolio. - 2057-3960. ; 7:1
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Tidskriftsartikel (refereegranskat)abstract
- Divacancy spins implement qubits with outstanding characteristics and capabilities in an industrial semiconductor host. On the other hand, there are still numerous open questions about the physics of these important defects, for instance, spin relaxation has not been thoroughly studied yet. Here, we carry out a theoretical study on environmental spin-induced spin relaxation processes of divacancy qubits in the 4H polytype of silicon carbide (4H-SiC). We reveal all the relevant magnetic field values where the longitudinal spin relaxation time T-1 drops resonantly due to the coupling to either nuclear spins or electron spins. We quantitatively analyze the dependence of the T-1 time on the concentration of point defect spins and the applied magnetic field and provide an analytical expression. We demonstrate that dipolar spin relaxation plays a significant role both in as-grown and ion-implanted samples and it often limits the coherence time of divacancy qubits in 4H-SiC.
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| 7. |
- Carlsson, Adam, et al.
(författare)
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Finding stable multi-component materials by combining cluster expansion and crystal structure predictions
- 2023
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Ingår i: npj Computational Materials. - : NATURE PORTFOLIO. - 2057-3960. ; 9:1
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Tidskriftsartikel (refereegranskat)abstract
- A desired prerequisite when performing a quantum mechanical calculation is to have an initial idea of the atomic positions within an approximate crystal structure. The atomic positions combined should result in a system located in, or close to, an energy minimum. However, designing low-energy structures may be a challenging task when prior knowledge is scarce, specifically for large multi-component systems where the degrees of freedom are close to infinite. In this paper, we propose a method for identification of low-energy crystal structures within multi-component systems by combining cluster expansion and crystal structure predictions with density-functional theory calculations. Crystal structure prediction searches are applied to the Mo2AlB2 and Sc2AlB2 ternary systems to identify candidate structures, which are subsequently used to explore the quaternary (pseudo-binary) (MoxSc1-x)(2)AlB2 system through the cluster expansion formalism utilizing the ground-state search approach. Furthermore, we show that utilizing low-energy structures found within the cluster expansion ground-state search as seed structures within crystal structure predictions of (MoxSc1-x)(2)AlB2 can significantly reduce the computational demands. With this combined approach, we not only correctly identified the recently discovered Mo(4/3)Sc(2/3)AlB(2)i-MAB phase, comprised of in-plane chemical ordering of Mo and Sc and with Al in a Kagome lattice, but also predict additional low-energy structures at various concentrations. This result demonstrates that combining crystal structure prediction with cluster expansion provides a path for identifying low-energy crystal structures in multi-component systems by employing the strengths from both frameworks.
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| 8. |
- Chang, Yueqing, et al.
(författare)
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Downfolding from ab initio to interacting model Hamiltonians : comprehensive analysis and benchmarking of the DFT+cRPA approach
- 2024
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Ingår i: npj Computational Materials. - 2057-3960. ; 10:1
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Tidskriftsartikel (refereegranskat)abstract
- Model Hamiltonians are regularly derived from first principles to describe correlated matter. However, the standard methods for this contain a number of largely unexplored approximations. For a strongly correlated impurity model system, here we carefully compare a standard downfolding technique with the best possible ground-truth estimates for charge-neutral excited-state energies and wave functions using state-of-the-art first-principles many-body wave function approaches. To this end, we use the vanadocene molecule and analyze all downfolding aspects, including the Hamiltonian form, target basis, double-counting correction, and Coulomb interaction screening models. We find that the choice of target-space basis functions emerges as a key factor for the quality of the downfolded results, while orbital-dependent double-counting corrections diminish the quality. Background screening of the Coulomb interaction matrix elements primarily affects crystal-field excitations. Our benchmark uncovers the relative importance of each downfolding step and offers insights into the potential accuracy of minimal downfolded model Hamiltonians.
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| 9. |
- Cossu, Fabrizio, et al.
(författare)
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Persistent half-metallic ferromagnetism in a (111)-oriented manganite superlattice
- 2022
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Ingår i: npj Computational Materials. - : Springer Nature. - 2057-3960. ; 8:1
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Tidskriftsartikel (refereegranskat)abstract
- We employ electronic structure calculations to show that a (111)-oriented (LaMnO3)(12) divide (SrMnO3)(6) superlattice retains a half-metallic ferromagnetic character despite its large thickness. We link this behaviour to the strain and the octahedral connectivity between the layers. This also gives rise to breathing modes, which are coupled to charge and spin oscillations, whose components have a pure e(g) character. Most interestingly, the magnetisation reaches its maximum value inside the LaMnO3 region and not at the interface, which is fundamentally different from what observed for the (001) orientation. The inter-atomic exchange coupling shows that the magnetic order arises from the double-exchange mechanism, despite competing interactions inside the SrMnO3 region. Finally, the van Vleck distortions and the spin oscillations are crucially affected by the variation of Hund's exchange and charge doping, which allows us to speculate that our system behaves as a Hund's metal, creating an interesting connection between manganites and nickelates.
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| 10. |
- Dangic, Dorde, et al.
(författare)
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The origin of the lattice thermal conductivity enhancement at the ferroelectric phase transition in GeTe
- 2021
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Ingår i: npj Computational Materials. - : Nature Research. - 2057-3960. ; 7:1
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Tidskriftsartikel (refereegranskat)abstract
- The proximity to structural phase transitions in IV-VI thermoelectric materials is one of the main reasons for their large phonon anharmonicity and intrinsically low lattice thermal conductivity kappa. However, the kappa of GeTe increases at the ferroelectric phase transition near 700 K. Using first-principles calculations with the temperature dependent effective potential method, we show that this rise in kappa is the consequence of negative thermal expansion in the rhombohedral phase and increase in the phonon lifetimes in the high-symmetry phase. Strong anharmonicity near the phase transition induces non-Lorentzian shapes of the phonon power spectra. To account for these effects, we implement a method of calculating kappa based on the Green-Kubo approach and find that the Boltzmann transport equation underestimates kappa near the phase transition. Our findings elucidate the influence of structural phase transitions on kappa and provide guidance for design of better thermoelectric materials.
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