arXiv subjects

Stefan Blügel

Publications and source records attributed to Stefan Blügel.

At least 19 recordsLinked to original sources

arXiv2026

Unbiased first-principles construction of complete tensorial spin Hamiltonians

Magnetic ground states are commonly predicted using spin Hamiltonians whose interaction terms are selected a priori, potentially overlooking the microscopic interactions that govern complex magnetic order. Here, we introduce a general framework for the unbiased first-principles construction of symmetry-complete tensorial spin Hamiltonians and its automated implementation in AMATIS. The framework constructs the Hamiltonian directly from density-functional theory while rigorously enforcing quantum spin algebra and crystallographic symmetry. Applied to representative two-dimensional van der Waals magnets, the framework reproduces established magnetic interactions and uncovers hidden physics beyond conventional spin models, including chiral interactions that stabilize metastable skyrmions, higher-rank tensorial interactions that reconstruct the magnetic phase diagram and establish stabilizing competing multi-Q phases, and an emergent p-wave altermagnetic electronic structure. Our results demonstrate that unbiased tensorial Hamiltonian construction provides a predictive alternative to the conventional practice of manually selecting spin-model interactions, enabling first-principles discovery of unconventional magnetic phases.

cond-mat.mtrl-sci↗

arXiv2026

Automated Generation of Commensurate Magnetic Structures based on Spin Space Groups and Graph Theory

Magnetic structures with symmetry constraints are candidates for energetically favorable configurations. Enumerating these structures is essential for identifying experimental observations and provides unbiased, linearly stable, and optimally sampled reference configurations for energy fitting when extracting spin interactions. We present SpinGraph, an automated workflow generating symmetry-distinct magnetic configurations. SpinGraph is not only compatible with the more general spin space groups, but also allows precise control of the prescribed single- or multi-Q states superposition. For finite groups, we enumerate compatible subgroups directly. For spin space groups with a continuous or special spin-only part, the finite component is enumerated first and then combined with the compatible exact spin-only constraints. These actions are translated into graph constraints in real or Fourier space. The symmetry of every generated structure is re-evaluated after construction. Finally, we integrate SpinGraph with the magnetic analysis code AMATIS to perform a thorough calculation for the spin interactions of the insulating monolayer CrI3. SpinGraph complements the last part of AMATIS and realizes the fully automated workflow for the spin Hamiltonian construction, which is essential for studying phase transitions, spin textures, magnons, and spin dynamics.

cond-mat.mtrl-sci↗

arXiv2026

CNN-Based Classifier for Automated Identification of Magnetic States in Spin Dynamics Simulations

The identification and classification of different magnetic states are essential for understanding the complex behavior of magnetic systems. Traditional approaches that rely on handcrafted features or manual inspection often fall short, particularly when dealing with subtle or topologically complex spin textures. In this study, we present an automated deep learning model that employs an EfficientNetV1B0 Convolutional Neural Network to classify nine distinct magnetic states, including both ferromagnetic (FM) and antiferromagnetic (AFM) spin textures such as AFM skyrmions and AFM stripe domains. The spin configurations are generated through atomistic spin dynamics simulations using the Spirit code, then visualized with VFRendering to produce RGB images, which serve as inputs to the classification model.

cond-mat.mtrl-sci↗

arXiv2026

Coexistence and tunability of orbital and spin Hall effects in RuO$_2$

Altermagnetic materials, especially RuO$_2$, have recently attracted considerable attention for their unique magnetic properties and energy-efficient spintronic applications. However, recent experimental studies have reported highly conflicting signatures regarding altermagnetic spin splitting and charge--spin interconversion (CSI) in RuO$_2$. While some experiments link efficient CSI to non-relativistic altermagnetic spin-splitting effects, others observe large CSI signals in non-spin-splitting RuO$_2$, which are instead explained by relativistic inverse spin Hall effects. In this work, based on first-principles calculations, we reveal that these controversial experimental results originate from a phase-dependent coexistence and relative dominance of the orbital Hall effect (OHE) and spin Hall effect (SHE) in RuO$_2$. We systematically investigate the OHE and SHE in both altermagnetic and nonmagnetic phases of RuO$_2$. Our results show that the altermagnetic state hosts a giant OHE that exceeds the SHE by two orders of magnitude and carries an opposite sign. This dominant OHE can generate experimentally observed "SHE-like" voltages through orbital-to-spin conversion, explaining previously reported altermagnetic CSI signals. In contrast, OHE of nonmagnetic RuO$_2$ is suppressed and a large relativistic SHE emerges, in agreement with recent angle-resolved photoemission and spin-pumping experiments. Finally, we demonstrate that the coexistence of OHE and SHE is tunable via chemical doping, enabling on-demand modulation of CSI in in RuO$_2$. Our work provides a new physical mechanism for understanding CSI in RuO$_2$ and highlights the central role of orbital transport.

cond-mat.mtrl-sci↗

arXiv2026

Emergence of a non-bulk hexagonal Fe$_2$S$_2$ single layer via phase transformation

Two-dimensional materials can stabilize crystal structures that are absent from their bulk counterparts, offering opportunities for materials design. Here, we report the synthesis of a previously unknown hexagonal Fe$_2$S$_2$ single layer with $β$-CuI structure, a buckled layer of two vertically stacked FeS honeycomb lattices, realized by thermally induced transformation of single layer mackinawite grown on graphene/Ir(111). In situ scanning tunneling microscopy and low-energy electron diffraction reveal a transition from a tetragonal to a hexagonal lattice accompanied by distinct morphological and electronic signatures. The hexagonal Fe$_2$S$_2$ forms reproducibly upon annealing and represents a new structural motif within the Fe-S material family. First-principles calculations identify the $β$-CuI structure as most consistent with experiment. The calculations suggest that on-site Coulomb interactions and magnetic order are relevant to understanding the stability of the new 2D Fe-S compound. The preferred nucleation of single-layer mackinawite, despite being energetically disfavored, is speculated to result from its low edge energy, analogous to the 3D case. Our results establish Fe$_2$S$_2$ as a platform for exploring structural polymorphism in two dimensions and demonstrate that reduced dimensionality can stabilize crystal structures not accessible in bulk materials.

cond-mat.mtrl-sci↗

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

Efficient all-electron Bethe-Salpeter implementation using crystal symmetries

We describe an all-electron implementation of the Bethe-Salpeter equation (BSE) for the calculation of optical absorption spectra in the full-potential linearized augmented-plane-wave (FLAPW) method. So far, FLAPW implementations have resorted to a simple plane-wave basis for the bare and screened Coulomb potentials, thereby forgoing the all-electron description to some extent. In contrast, we expand the interaction potentials in the all-electron mixed basis. As in most implementations, the BSE is solved by the diagonalization of a two-particle Hamiltonian matrix, whose dimension is proportional to the number of $\mathbf{k}$ points. Due to the large number of $\mathbf{k}$ points required to converge the BSE, the resulting matrix becomes large even for small unit cells. We describe a method that exploits the crystal symmetries to accelerate the construction and diagonalization of the two-particle Hamiltonian. In particular, we employ group theoretical tools to bring the Hamiltonian into block-diagonal form. Furthermore, it is shown that often only one of the blocks needs to be taken into account for the optical absorption spectrum leading to a considerable speedup of the diagonalization step. The code allows for the inclusion of spin-orbit coupling and is parallelized with the possibility of storing the Hamiltonian in distributed memory over many nodes, keeping the memory demands low. To validate our implementation, we show optical absorption spectra and report exciton binding energies for bulk Si, LiF, and MoS$_2$. By exploiting the crystal symmetries, we can reduce the dimension of the Hamiltonian matrix of Si by a factor of five, resulting in a 125-fold speedup in its diagonalization. The calculated exciton binding energies of 22~meV and 76~meV for Si and MoS$_2$ are closer to experimental values than in previous BSE studies.

cond-mat.mtrl-sci↗

arXiv2026

Implementation and application of a DFT$+U$$+V$ approach within the all-electron FLAPW method

We present an implementation of the density-functional theory DFT$+U$$+V$ formalism within the all-electron full-potential linearized augmented-plane-wave (FLAPW) method as implemented in the FLEUR code. The DFT$+U$$+V$ formalism extends DFT, supplemented by the onsite Coulomb interaction $U$, to address local correlation effects in localized states by incorporating intersite Coulomb interaction terms $V$. It holds promise for improving charge and bond disproportionation, charge and orbital ordering, charge density wave formation, charge transfer, and the intersite correlation resulting from hybridization between states of neighboring sites in a solid. $U$ and $V$ parameters are obtained from first principles using the constrained random-phase approximation (cRPA) employing two different atom basis representations to project the screened Coulomb interaction: the Wannier and the muffin-tin basis functions. We investigate in detail the impact of the $V$ term for typical covalently bonded materials like graphene, for bulk semiconductors such as silicon and germanium, and for charge-transfer insulators like NiO. Our results demonstrate an improvement in accuracy of specific properties across these systems, providing a framework for describing materials with different interaction regimes. We compare our DFT$+U$$+V$ results using our cRPA parameter sets with (i) previous DFT$+U$$+V$ calculation employing pseudopotential approximations, (ii) with experimental results and (iii) with our $GW$ results.

cond-mat.mtrl-sci↗

arXiv2026

Atomically-sharp magnetic soliton in the square-net lattice EuRhAl$_{4}$Si$_{2}$

Topological spin textures are hallmark manifestations of competing interactions in magnetic matter. Their effective description by nonlinear field theories reflects an energetic frustration that destabilizes uniform order while selecting finite-size, topologically nontrivial configurations as stationary states. Among the most extreme realizations are atomically-sharp domain wall excitations, namely one-dimensional (1D) magnetic solitons, which represent the ultimate scaling limit of magnetic textures. Such solitons may emerge in magnetic systems where effective exchange interactions compete directly with uniaxial magnetic anisotropy. Here we show that the square-net rare earth compound EuRhAl$_{4}$Si$_{2}$ realizes a very susceptible regime where the magnetic anisotropy competes with highly frustrated exchange interactions stabilizing a rare ferrimagnetic $\uparrow\uparrow\downarrow$ state that, under applied magnetic field, supports the formation of atomically-sharp soliton defects. We confirm the bulk response of the 1D magnetic solitons via magnetization and electrical transport measurements. We establish both the zero- and in-field $\uparrow\uparrow\downarrow$ order via neutron diffraction, while magnetic force microscopy visualizes its real-space evolution into a stripe-like array. To elucidate the microscopic origin of the soliton, we relate the Ruderman-Kittel-Kasuya-Yosida (RKKY)-driven exchange interactions and the magnetic anisotropy through density functional theory, and we construct an effective 1D $J_{1}$-$J_{2}$-$K$ model whose atomistic spin dynamics simulations reproduce the observed soliton states as a function of external field. Our results demonstrate that EuRhAl$_{4}$Si$_{2}$ hosts atomically-sharp, field-driven 1D magnetic solitons, providing a new platform for studying 1D topological excitations at the atomic length scale.

cond-mat.str-el↗

arXiv2026

The 2026 Skyrmionics Roadmap

Magnetic skyrmions and related topological spin textures have emerged as a central topic in condensed-matter physics, combining fundamental significance with potential for transformative applications in spintronics, magnonics, and beyond. Over the past decade, advances in material platforms, imaging techniques, theoretical modeling, and device concepts have established skyrmionics as a rapidly expanding field. At the same time, challenges remain in stabilizing, controlling, and integrating such textures into functional architectures, while novel phenomena such as antiskyrmions, higher-order skyrmions, hopfions, and antiferromagnetic textures arise. The 2026 Skyrmionics Roadmap represents a collective effort of many authors, providing a comprehensive perspective on the current state-of-the-art and the outlook for the coming years. In 33 focused sections, each co-authored by two researchers, we chart progress in theory and modeling, material systems, skyrmion dynamics, and skyrmion technologies. By offering a consolidated vision, this Roadmap aims to guide both fundamental research and application-driven efforts, accelerating the transition of skyrmionics from conceptual breakthroughs toward practical technologies.

cond-mat.mes-hall↗

arXiv2026

Linked skyrmions in shifted magnetic bilayer

Magnetic solitons have recently attracted significant attention due to their intricate physical properties and potential applications in information processing. The majority of the studies in this field, however, are focused on a particular type of solitons known as skyrmions, characterised by a unit topological charge. Here, we present a shifted magnetic bilayer that can demonstrate magnetic solitons with arbitrary large topological charges. These configurations, which we call linked skyrmions, consist of multiple skyrmions linked together with topological point defects. These topological point defects, termed as anti-aligned points, originate from the mutually orthogonal Dzyaloshinskii-Moriya interaction in two different layers. By tuning the interlayer exchange coupling and the external magnetic field, one can also achieve different ground states in this bilayer. Additionally, the system also demonstrates conventional skyrmion-bags and $kπ$-skyrmions. Finally we propose a suitable material candidate where these magnetic configurations can be realised. Our findings, thus, can provide a way to achieve solitons with large topological charge and realise them in realistic systems.

cond-mat.mes-hall↗

arXiv2025

Chiral split magnons in metallic g-wave altermagnets: Insights from many-body perturbation theory

Altermagnets are a novel class of magnetic materials that bridge the gap between ferromagnets (FMs) and antiferromagnets (AFMs). A key feature is the non-degeneracy of magnon modes where spin splitting occurs, leading to chirality and direction-dependent magnon dispersions governed by symmetry. We explore this in metallic g-wave altermagnets ($TPn$, where $T$= V, Cr; $Pn$= As, Sb, Bi) using density functional and many-body perturbation theories. We analyze the influence of pnictogen substitution on spin splitting and magnon behavior. We uncover anisotropic magnon band splitting aligned with electronic structure, and wavevector- and chirality-dependent damping due to Stoner excitations. We identify regions in the Brillouin zone where the chiral magnon splitting overcomes the damping. These findings suggest altermagnets are promising for spintronic and magnonic technologies, where direction-dependent magnon lifetimes and nonreciprocal magno transport may enable chiral magnon propagation, while wavevector-selective damping could be harnessed for fast and controllable magnetization switching.

cond-mat.str-el↗

arXiv2025

Hydrogen toggling between Yoshimori spin spirals and elliptical Dzyaloshinskii-Moriya skyrmions in Fe on Ir(110)

Skyrmions are particle-like spin textures that arise from spin spiral states in the presence of an external magnetic field. These spirals can originate from either frustrated Heisenberg exchange interactions or the interplay between exchange interactions and the relativistic Dzyaloshinskii-Moriya interaction, leading to atomic- and mesoscale textures, respectively. However, the conversion of exchange-stabilized spin spirals into skyrmions typically requires magnetic fields that exceed practical laboratory limits. Here, we demonstrate a strategy leveraging hydrogen adsorption to expand the range of magnetic films capable of hosting stable or metastable skyrmions. In a structurally open and anisotropic system of two pseudomorphic Fe layers on Ir(110), spin-polarized scanning tunneling microscopy combined with ab initio calculations reveals that a right-handed, exchange-stabilized Néel-type spin spiral propagating along the [$\overline{1}10$] direction with a $1.3$~nm period transitions upon hydrogen adsorption to a Dzyaloshinskii-Moriya type spiral with a sevenfold longer period of $8.5$~nm. This transition enables elliptical skyrmions to form at moderate magnetic fields. Hydrogenation thus provides a non-volatile mechanism to toggle between distinct magnetic states, offering a versatile platform for controlling spin textures.

cond-mat.mes-hall↗

arXiv2025

Rare-earth atoms on Nb(110) as a platform to engineer topological superconductivity

Helical spin textures in one-dimensional magnetic chains on superconductors can enable topological superconductivity and host Majorana zero modes, independent of the presence of intrinsic spin-orbit coupling. Here, we show that gadolinium (Gd) adatoms, possessing large 4f magnetic moments when placed on a Nb(110) surface, establish indirect exchange interactions mediated by valence electrons, manifesting as Yu-Shiba-Rusinov states. By combining scanning tunneling microscopy and spectroscopy with density functional theory, we analyze the emergence of the Yu-Shiba-Rusinov states in single Gd atoms and Gd dimers and uncover the underlying magnetic interaction mechanisms, on the basis of which we predict by means of spin-dynamics simulations the formation of stable chiral Néel-type spin-spiral configurations in Gd chains. These findings highlight rare-earth magnets as a promising platform for precisely tuning spin-spiral ground states, an essential prerequisite for the realization of topological superconductivity.

cond-mat.supr-con↗

arXiv2025

Quasiparticle interference on the surface of Bi$_{\mathbf{2}}$Se$_{\mathbf{3}}$ terminated (PbSe)$_{\mathbf 5}$(Bi$_{\mathbf 2}$Se$_{\mathbf 3}$)$_{\mathbf 6}$

Among the family of topological superconductors derived from {\BiSe}, $\mathrm{Cu}_x(\mathrm{PbSe})_{5}(\mathrm{Bi}_{2}\mathrm{Se}_{3})_{6}$ is unique in its surface termination of a single quintuple layer (QL) of the topological insulator (TI) \BiSe{} on an ordinary insulator PbSe. Here, we report a combined scanning tunneling microscopy (STM) and density functional theory (DFT) characterization of the cleaved surface of the parent compound $(\mathrm{PbSe})_{5}(\mathrm{Bi}_{2}\mathrm{Se}_{3})_{6}$ (PSBS). Interestingly, the potential disorder due to the random distribution of native defects is only $Γ\sim 4~\mathrm{meV}$, comparable to the smallest reported for TIs. Performing high-resolution quasiparticle interference imaging (QPI) near the Fermi energy ($E-E_\mathrm{F} = -1~\mathrm{eV}~\mathrm{to}~0.6~\mathrm{eV}$) we reconstruct the dispersion relation of the dominant spectral feature and our ab initio calculations show that this surface feature originates from two bands with Rashba-like splitting due to strong spin-orbit coupling and inversion symmetry breaking. Moreover, only a small hexagonal distortion of the calculated Fermi surface is seen in the full momentum space distribution of the measured scattering data. Nevertheless, the scattering pattern at lower energies transforms into a flower-like shape with suppressed intensity along the $\overline{Γ\mathrm{K}}$ direction. We show that this effect is not due to the forbidden backscattering in the spin-momentum locked surface state in Bi$_2$Se$_3$ but reflects the threefold symmetry of the scattering potential.

cond-mat.mes-hall↗

arXiv2025

Ab initio investigation of the topological Hall effect caused by magnetic skyrmions in Pd/Fe/Ir(111)

We present an ab-initio computational analysis of the topological Hall effect arising from stable magnetic skyrmions in the Pd/Fe/Ir(111) film using non-collinear spin density functional calculations within the Korringa-Kohn-Rostoker (KKR) Green function method. The semiclassical Boltzmann transport equation is employed for the resistivity and the Hall angle of the system. We explore the influence of the skyrmion size and the impact of disorder on the topological Hall angle.

cond-mat.mtrl-sci↗

arXiv2025

Orbital torques and orbital pumping in two-dimensional rare-earth dichalcogenides

The design of spin-orbit torque properties in two-dimensional (2D) materials presents one of the challenges of modern spintronics. In this context, 2D layers involving rare-earth ions $-$ which give rise to robust magnetism, exhibit pronounced orbital polarization of the states, and carry strong spin-orbit interaction $-$ hold particular promise. Here, we investigate ferromagnetic Janus H-phase monolayers of 4$f$-Eu rare-earth dichalcogenides EuSP, EuSSe, and EuSCl using first-principles calculations. We demonstrate that all compounds exhibit significant spin-orbit torques which originate predominantly in the colossal current-induced orbital response on the Eu $f$-electrons. Moreover, we demonstrate that the corresponding orbital torques can be used to drive strong in-plane currents of orbital angular momentum with non-trivial direction of orbital polarization. Our findings promote $f$-orbital-based 2D materials as a promising platform for in-plane orbital pumping and spin-orbit torque applications, and motivate further research on educated design of orbital properties for orbitronics with 2D materials.

cond-mat.mes-hall↗

arXiv2024

Magnetic excitations and interactions in the Weyl ferrimagnet NdAlSi

Weyl fermions can arise from time-reversal symmetry-breaking magnetism, but their impact on magnetic order is a source of ongoing research. Using high-precision neutron diffraction and spectroscopy, we present a comprehensive exploration of the magnetic structure and excitation spectrum of Weyl semimetal and helical magnet NdAlSi. We use Luttinger-Tisza, classical mean-field, and random-phase approximation techniques to model the dispersive crystal field excitons. We find extended-ranged and sign-changing interactions, suggesting a coupling between conduction electrons and the local moments. We demonstrate that low-symmetry anisotropic Dzyaloshinskii-Moriya interactions, in contrast with higher-symmetry interactions enabled by Weyl fermions, play an important role in stabilizing the complex spin spiral ground state of NdAlSi. Our work provides a first detailed view of microscopic interactions in a Weyl magnet, and constrains the role of Weyl electrons and their chirality on the spiral magnetism.

cond-mat.str-el↗