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Thomas Olsen

Publications and source records attributed to Thomas Olsen.

At least 37 records · Page 2Linked to original sources
arXiv2024

GPAW: An open Python package for electronic-structure calculations

We review the GPAW open-source Python package for electronic structure calculations. GPAW is based on the projector-augmented wave method and can solve the self-consistent density functional theory (DFT) equations using three different wave-function representations, namely real-space grids, plane waves, and numerical atomic orbitals. The three representations are complementary and mutually independent and can be connected by transformations via the real-space grid. This multi-basis feature renders GPAW highly versatile and unique among similar codes. By virtue of its modular structure, the GPAW code constitutes an ideal platform for implementation of new features and methodologies. Moreover, it is well integrated with the Atomic Simulation Environment (ASE) providing a flexible and dynamic user interface. In addition to ground-state DFT calculations, GPAW supports many-body GW band structures, optical excitations from the Bethe-Salpeter Equation (BSE), variational calculations of excited states in molecules and solids via direct optimization, and real-time propagation of the Kohn-Sham equations within time-dependent DFT. A range of more advanced methods to describe magnetic excitations and non-collinear magnetism in solids are also now available. In addition, GPAW can calculate non-linear optical tensors of solids, charged crystal point defects, and much more. Recently, support of GPU acceleration has been achieved with minor modifications of the GPAW code thanks to the CuPy library. We end the review with an outlook describing some future plans for GPAW.

cond-mat.mtrl-sci↗
arXiv2024

Linear magnetoelectricity in the Zintl phase pnictides (Ba, Ca, Sr)$\mathrm{Mn}_2\mathrm{(P, As, Sb)}_2$ from first principles calculations

We report a comprehensive set of density functional theory calculations on the family of layered antiferromagnetic manganese pnictides (Ba, Ca, Sr)$\mathrm{Mn}_2\mathrm{(P, As, Sb)}_2$. We characterize all components to the linear magnetoelectric (ME) tensor $α$ which are parsed into their contributions from spin and orbital moments for both lattice-mediated and their clamped-ion electronic analogs. Our main results show that the orbital magnetization components cannot be neglected in these systems. The ME response is dominated by electronic effects with total $α$ values exceeding those of the prototypical $\mathrm{Cr}_2\mathrm{O}_3$ (i.e. $α\simeq$ 6.79 ps/m in $\mathrm{BaMn}_2\mathrm{As}_2$). We also identify a strong correlation with the computed ME susceptibility on pnictogen substitution in the trigonal subfamily albeit with weaker amplitudes ($α\simeq$ 0.2-1.7 ps/m). Additionally, we provide the dependence of these predictions on the Hubbard +U correction, at the level of the local density approximation, which show large variations on the calculated ME coefficients in the tetragonal compounds highlighting the role of strong correlation in these compounds.

cond-mat.mtrl-sci↗
arXiv2024

Antiferromagnetism in two-dimensional materials: progress and computational challenges

We present a perspective on the status of antiferromagnetism in two-dimensional (2D) materials. Various types of spin-compensated orders are discussed and include non-collinear order, spin spirals and altermagnetism. Spin-orbit effects ultimately determine, whether compounds exhibit long range order, Kosterlitz-Thouless physics, or multiferroic properties and we discuss the basic magnetic prototypes that may arise in 2D materials depending on the magnetic anisotropy and ordering vector. A summary of 2D antiferromagnets that have been characterized experimentally is provided - with particular emphasis on magnetic anisotropies and Neel temperatures. We then outline the ingredients needed to describe the magnetic properties using density functional theory. In particular, the systematic determination of magnetic ground states from the generalized Bloch theorem and the magnetic force theorem, which may be used to calculate magnetic excitations from the Heisenberg model with parameters determined from first principles. The methods are exemplified by application to the monolayer helimagnet NiBr$_2$. Finally, we present a summary of predicted and prospective 2D antiferromagnets and discuss the challenges associated with the prediction of Néel temperatures from first principles.

cond-mat.mtrl-sci↗
arXiv2024

Two-dimensional altermagnets from high throughput computational screening: symmetry requirements, chiral magnons and spin-orbit effects

We present a high throughput computational search for altermagnetism in two-dimensional (2D) materials based on the Computational 2D Materials Database (C2DB). We start by showing that the symmetry requirements for altermagnetism in 2D are somewhat more strict compared to bulk materials and applying these yields a total of 7 altermagnets in the C2DB. The collinear ground state in these monolayers are verified by spin spiral calculations using the generalized Bloch theorem. We focus on four $d$-wave altermagnetic materials belonging to the $P2_1'/c'$ magnetic space group - RuF$_4$, VF$_4$, AgF$_2$ and OsF$_4$. The first three of these are known experimentally as van der Waals bonded bulk materials and are likely to be exfoliable from their bulk parent compounds. We perform a detailed analysis of the electronic structure and non-relativistic spin splitting in $k$-space exemplified by RuF$_4$. The magnon spectrum of RuF$_4$ is calculated from the magnetic force theorem and it is shown that the symmetries that enforce degenerate magnon bands in anti-ferromagnets are absent in altermagnets and give rise to the obtained non-degenerate magnon spectrum. We then include spin-orbit effects and show that these will dominate the splitting ofmagnons in RuF$_4$. Finally, we provide an example of $i$-wave altermagnetism in the 2H phase of FeBr$_3$.

cond-mat.mtrl-sci↗
arXiv2023

Selenium and the role of defects for photovoltaic applications

We present first principles calculations of the electronic properties of trigonal selenium with emphasis on photovoltaic applications. The band gap and optical absorption spectrum of pristine selenium is calculated from many-body perturbation theory yielding excellent agreement with experiments. We then investigate the role of intrinsic as well as extrinsic defects and estimate the equilibrium concentrations resulting from realistic synthesis conditions. The intrinsic defects are dominated by vacancies and we show that these do not result in significant non-radiative recombination. The charge balance remains dominated by vacancies when extrinsic defects are included, but these may give rise to sizable non-radiative recombination rates, which could severely limit the performance of selenium based solar cells. Our results thus imply that the pollution by external elements is a decisive factor for the photovoltaic efficiency, which will be of crucial importance when considering synthesis conditions for any type of device engineering.

cond-mat.mtrl-sci↗
arXiv2023

Magnetic order in the computational 2D materials database (C2DB) from high throughput spin spiral calculations

We report a detailed investigation of the magnetic order in 192 stable magnetic two-dimensional materials from the Computational 2D Materials Database having one magnetic atom in the unit cell. The calculations are based on a systematic workflow that employs spin spiral calculations and yields the magnetic order in terms of a two-dimensional ordering vector $\mathbf{Q}$. We then include spin-orbit coupling to extract the easy and hard axes for collinear structures and the orientation of spiral planes in non-collinear structures. Finally, for all predicted ferromagnets we compute the Dzyaloshinskii-Moriya interactions and determine whether or not these are strong enough to overcome the magnetic anisotropy and stabilise a chiral spin spiral ground state. These steps completely determines the ground state order within the spiralling ansatz. We find 58 ferromagnets, 21 collinear anti-ferromagnets, and 85 non-collinear ground states of which 15 are chiral spin spirals driven by Dzyaloshinskii-Moriya interactions. The results show that non-collinear order is in fact as common as collinear order in these materials and emphasise the need for detailed investigation of the magnetic ground state when reporting magnetic properties of new materials. Furthermore, non-collinear order typically breaks symmetries inherent to the lattice and may give rise to emergent properties such as multiferroicity, magnetoelectricity or second order optical effects that would be predicted as absent based on a collinear assumption.

cond-mat.mtrl-sci↗
arXiv2023

Imaging the Electric Field with X-Ray Diffraction Microscopy

The properties of semiconductors and functional dielectrics are defined by their response in electric fields, which may be perturbed by defects and the strain they generate. In this work, we demonstrate how diffraction-based X-ray microscopy techniques may be utilized to image the electric field in insulating crystalline materials. By analysing a prototypical ferro- and piezoelectric material, BaTiO$_{3}$, we identify trends that can guide experimental design towards imaging the electric field using any diffraction-based X-ray microscopy technique. We explain these trends in the context of dark-field X-ray microscopy, but the framework is also valid for Bragg scanning probe X-ray microscopy, Bragg coherent diffraction imaging and Bragg X-ray ptychography. The ability to quantify electric field distributions alongside the defects and strain already accessible via these techniques offers a more comprehensive picture of the often complex structure-property relationships that exist in many insulating and semiconducting materials.

cond-mat.mtrl-sci↗
arXiv2023

Emergent properties of van der Waals bilayers revealed by computational stacking

Stacking of two-dimensional (2D) materials has emerged as a facile strategy for realising exotic quantum states of matter and engineering electronic properties. Yet, developments beyond the proof-of-principle level are impeded by the vast size of the configuration space defined by layer combinations and stacking orders. Here we employ a density functional theory (DFT) workflow to calculate interlayer binding energies of 8451 homobilayers created by stacking 1052 different monolayers in various configurations. Analysis of the stacking orders in 247 experimentally known van der Waals crystals is used to validate the workflow and determine the criteria for realizable bilayers. For the 2586 most stable bilayer systems, we calculate a range of electronic, magnetic, and vibrational properties, and explore general trends and anomalies. We identify an abundance of bistable bilayers with stacking order-dependent magnetic or electrical polarisation states making them candidates for slidetronics applications.

cond-mat.mtrl-sci↗
arXiv2023

Type II multiferroic order in two-dimensional transition metal halides from first principles spin-spiral calculations

We present a computational search for spin spiral ground states in two-dimensional transition metal halides that are experimentally known as van der Waals bonded bulk materials. Such spin spirals break the rotational symmetry of the lattice and lead to polar ground states where the axis of polarization is strongly coupled to the magnetic order (type II multiferroics). We apply the generalized Bloch theorem in conjunction with non-collinear density functional theory calculations to find the spiralling vector that minimizes the energy and then include spin-orbit coupling to calculate the preferred orientation of the spin plane with respect to the spiral vector. We find a wide variety of magnetic orders ranging from ferromagnetic, stripy anti-ferromagnetic, 120$^\circ$ non-collinear structures and incommensurate spin spirals. The latter two introduce polar axes and are found in the majority of materials considered here. The spontaneous polarization is calculated for the incommensurate spin spirals by performing full supercell relaxation including spinorbit coupling and the induced polarization is shown to be strongly dependent on the orientation of the spiral planes. We also test the effect of Hubbard corrections on the results and find that for most materials LDA+U results agree qualitatively with LDA. An exception is the Mn halides, which are found to exhibit incommensurate spin spiral ground states if Hubbard corrections are included whereas bare LDA yields a 120$^\circ$ non-collinear ground state.

cond-mat.mtrl-sci↗
arXiv2023

Plane wave implementation of the magnetic force theorem for magnetic exchange constants: Application to bulk Fe, Co and Ni

We present a plane wave implementation of the magnetic force theorem, which provides a first principles framework for extracting exchange constants parameterizing a classical Heisenberg model description of magnetic materials. It is shown that the full microscopic exchange tensor may be expressed in terms of the static Kohn-Sham susceptibility tensor and the exchange-correlation magnetic field. This formulation allows one to define arbitrary magnetic sites localized to predefined spatial regions, hence rendering the problem of finding Heisenberg parameters independent of any orbital decomposition of the problem. The susceptibility is calculated in a plane wave basis, which allows for systematic convergence with respect to unoccupied bands and spatial representation. We then apply the method to the well-studied problem of calculating adiabatic spin wave spectra for bulk Fe, Co and Ni, finding good agreement with previous calculations. In particular, we utilize the freedom of defining magnetic sites to show that the calculated Heisenberg parameters are robust towards changes in the definition of magnetic sites. This demonstrates that the magnetic sites can be regarded as well-defined and thus asserts the relevance of the Heisenberg model description despite the itinerant nature of the magnetic state.

cond-mat.mtrl-sci↗
arXiv2022

Giant gate-tunable renormalization of spin-correlated flat-band states and bandgap in a 2D magnetic insulator

Emergent quantum phenomena in two-dimensional van der Waal (vdW) magnets are largely governed by the interplay between the exchange and Coulomb interactions. The ability to tune the Coulomb interaction in such strongly correlated materials enables the precise control of spin-correlated flat-band states, bandgap (Eg) and unconventional magnetism, all of which are crucial for next-generation spintronics and magnonics applications. Here, we demonstrate a giant gate-tunable renormalization of spin-correlated flat-band states and bandgap in magnetic chromium tribromide (CrBr3) monolayers grown on graphene. Our gate-dependent scanning tunneling spectroscopy (STS) studies reveal that the inter-flat-band spacing and bandgap of CrBr3 can be continuously tuned by 120 meV and 240 meV respectively via electrostatic injection of carriers into the hybrid CrBr3/graphene system, equivalent to the modulation of the Cr on-site Coulomb repulsion energy by 500 meV. This can be attributed to the self-screening of CrBr3 arising from the gate-induced carriers injected into CrBr3, which dominates over the opposite trend from the remote screening of the graphene substrate. Precise tuning of the spin-correlated flat-band states and bandgap in 2D magnets via electrostatic modulation of Coulomb interactions not only provides new strategies for optimizing the spin transport channels but also may exert a crucial influence on the exchange energy and spin-wave gap, which could raise the critical temperature for magnetic order.

cond-mat.mes-hall↗
arXiv2022

Abundance of second order topology in $C_3$ symmetric two-dimensional insulators

We have screened 71 two-dimensional (2D) materials with $C_3$ symmetry for non-trivial second order topological order and find that 28 compounds exhibit an obstructed atomic limit (OAL). In the case of $C_3$ symmetry, the second order topology can be calculated from bulk symmetry indicator invariants, which predict the value of fractional corner charges in symmetry conserving nanoflakes. The procedure is exemplified by MoS$_2$ in the H-phase, which constitutes a generic example of a 2D OAL material and the predicted fractional corner charges is verified by direct calculations of nanoflakes with armchair edges. We also determine the bulk topological polarization, which always lead to gapless states at zigzag edges and thus deteriorates the concept of fractional corner charges in nanoflakes with zigzag edges that are typically more stable that armchair flakes. We then consider the case of TiCl$_2$, which has vanishing polarization as well as an OAL and we verify that the edge states of nanoflakes with zigzag edges may indeed by passivated such that the edges remain insulating and the corner charges are well defined. For the 28 OAL materials we find that 16 have vanishing polarization and these materials thus constitute a promising starting point for experimental verification of second order topology in a 2D material.

cond-mat.mtrl-sci↗
arXiv2022

Polarization switching induced by domain wall sliding in two-dimensional ferroelectric monochalcogenides

The ability to switch between distinct states of polarization comprises the defining property of ferroelectrics. However, the microscopic mechanism responsible for switching is not well understood and theoretical estimates based on coherent monodomain switching typically overestimate experimentally determined coercive fields by orders of magnitude. In this work we present a detailed first principles characterization of domain walls (DWs) in two-dimensional ferroelectric GeS, GeSe, SnS and SnSe. In particular, we calculate the formation energies and migration barriers for 180 and 90 DWs, and then derive a general expression for the coercive field assuming that polarization switching is mediated by DW migration. We apply our approach to the materials studied and obtain good agreement with experimental coercive fields. The calculated coercive fields are up to two orders of magnitude smaller than those predicted from coherent monodomain switching in GeSe, SnS and SnSe. Finally, we study the optical properties of the compounds and find that the presence of 180 DWs leads to a significant red shift of the absorption spectrum, implying that the density of DWs may be determined by means of simple optical probes.

cond-mat.mtrl-sci↗
arXiv2022

Two-dimensional ferroelectrics from high throughput computational screening

We report a high throughput computational search for two-dimensional ferroelectric materials. The starting point is 252 pyroelectric materials from the computational 2D materials database (C2DB) and from these we identify 64 ferroelectric materials by explicitly constructing adiabatic paths connecting states of reversed polarization. In particular we find 49 materials with in-plane polarization, 8 materials with out-of-plane polarization and 6 materials with coupled in-plane and out-of-plane polarization. Most of the known 2D ferroelectrics are recovered by the screening and the far majority of the new predicted ferroelectrics are known as bulk van der Waals bonded compounds, which implies that these could be experimentally accessible by direct exfoliation. For roughly 25{\%} of the materials we find a metastable state in the non-polar structure, which could have important consequences for the thermodynamical properties and may imply a first order transition to the polar phase. Finally, we list the magnetic pyroelectrics extracted from the C2DB and focus on the case of VAgP$_2$Se$_6$, which exhibits a three-state switchable polarization vector that is strongly coupled to the magnetic excitation spectrum.

cond-mat.mtrl-sci↗
arXiv2022

Magnons in antiferromagnetic bcc-Cr and Cr$_2$O$_3$ from time-dependent density functional theory

We apply time-dependent density functional theory to calculate the transverse magnetic susceptibility of bcc-Cr and Cr$_2$O$_3$, which constitute prototypical examples of antiferromagnets with itinerant and localized magnetic moments respectively. The exchange-correlation kernel is rescaled in order to enforce the Goldstone condition and the magnon dispersion relations are extracted based on a symmetry analysis relying on the generalized Onsager relation. Doing so, our calculations yield the characteristic linear magnon dispersion of antiferromagnets in the long wavelength limit. In the case of Cr$_2$O$_3$, we find that the adiabatic local density approximation yields a good qualitative agreement with the measured dispersion, but overestimates the magnon velocity and bandwidth by a factor of two. Including a Hubbard correction improves the magnon velocity, but at the expense of the overall qualitative agreement with the experimental magnon dispersion. For bcc-Cr we find a sharp acoustic magnon mode at low energies with a velocity in agreement with previously reported values. At higher energies, the acoustic magnon mode becomes subject to strong Landau damping and rapidly vanishes once it enters the Stoner continuum. In addition to the acoustic magnon mode, we also observe an additional collective mode along the $Γ\rightarrow\mathrm{R}$ direction with an energy of $\sim$ 1 eV, which is located inside the Stoner continuum, but appears to elude the effect of Landau damping.

cond-mat.mtrl-sci↗
arXiv2022

Influence of static correlation on the magnon dynamics of an itinerant ferromagnet with competing exchange interactions -- a first principles study of MnBi

We present first principles calculations of the dynamic susceptibility in strained and doped ferromagnetic MnBi using time-dependent density functional theory. In spite of being a metal, MnBi exhibits signatures of strong correlation and a proper description in the framework of density functional theory requires Hubbard corrections to the Mn $d$-orbitals. To permit calculations of the dynamic susceptibility with Hubbard corrections applied to the ground state electronic structure, we use a consistent rescaling of the exchange-correlation kernel maintaining the delicate balance between the magnon dispersion and the Stoner continuum. We find excellent agreement with the experimentally observed magnon dispersion for pristine MnBi and show that the material undergoes a phase transition to helical order under application of either doping or strain. The presented methodology paves the way for future LR-TDDFT studies of magnetic phase transitions, also for the wide range of materials with pronounced static correlation effects that are not accounted for at the LDA level.

cond-mat.mtrl-sci↗
arXiv2021

Spread balanced Wannier functions: Robust and automatable orbital localization

We introduce a new type of Wannier functions (WFs) obtained by minimizing the conventional spread functional with a penalty term proportional to the variance of the spread distribution. This modified Wannierisation scheme is less prone to produce ineffective solutions featuring one or several poorly localized orbitals, making it well suited for complex systems or high-throughput applications. Furthermore, we propose an automatable protocol for selecting the initial guess and determine the optimal number of bands (or equivalently WFs) for the localization algorithm. The improved performance and robustness of the approach is demonstrated for a diverse set of test systems including the NV center in diamond, metal slabs with atomic adsorbates, spontaneous polarization of ferroelectrics and 30 inorganic monolayer materials comprising both metals and semiconductors. The methods are implemented in Python as part of the Atomic Simulation Environment (ASE).

cond-mat.other↗
arXiv2021

Unified Treatment of Magnons and Excitons in Monolayer CrI$_3$ from Many-Body Perturbation Theory

We present first principles calculations of the two-particle excitation spectrum of CrI$_3$ using many-body perturbation theory including spin-orbit coupling. Specifically, we solve the Bethe-Salpeter equation, which is equivalent to summing up all ladder diagrams with static screening and it is shown that excitons as well as magnons can be extracted seamlessly from the calculations. The resulting optical absorption spectrum as well as the magnon dispersion agree very well with recent measurements and we extract the amplitude for optical excitation of magnons resulting from spin-orbit interactions. Importantly, the results do not rely on any assumptions on the microscopic magnetic interactions such as Dzyaloshinskii-Moriya (DM), Kitaev or biquadratic interactions and we obtain a model independent estimate of the gap between acoustic and optical magnons of 0.3 meV. In addition, we resolve the magnon wavefunction in terms of band transitions and show that the magnon carries a spin that is significantly smaller than $\hbar$. This highlights the importance of terms that do not commute with $S^z$ in any Heisenberg model description.

cond-mat.mtrl-sci↗