### Browsing by Author "van den Brink, Jeroen"

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- ItemAbsence of induced magnetic monopoles in Maxwellian magnetoelectrics(College Park, MD : APS, 2022) Nogueira, Flavio S.; van den Brink, JeroenThe electromagnetic response of topological insulators is governed by axion electrodynamics, which features a topological magnetoelectric term in the Maxwell equations. As a consequence magnetic fields become the source of electric fields and vice versa, a phenomenon that is general for any material exhibiting a linear magnetoelectric effect. Axion electrodynamics has been associated with the possibility to create magnetic monopoles, in particular, by an electrical charge that is screened above the surface of a magnetoelectric material. Here we explicitly solve for the electromagnetic fields in this geometry and show that while vortexlike magnetic screening fields are generated by the electrical charge their divergence is identically zero at every point in space, which implies an absence of induced magnetic monopoles. Nevertheless magnetic image charges can be made explicit in the problem, and even if no bound state with electric charges yielding a dyon arises, a dyonlike angular momentum follows from our analysis. Because of its dependence on the dielectric constant this angular momentum is not quantized, which is consistent with a general argument that precludes magnetic monopoles to be generated in Maxwell magnetoelectrics. We also solve for topologically protected zero modes in the Dirac equation induced by the point charge. Since the induced topological defect on the topological insulator's surface carries an electric charge as a result of the axion term, these zero modes are not self-conjugated.
- ItemAnomalous and anisotropic nonlinear susceptibility in the proximate Kitaev magnet α-RuCl3([London] : Nature Publishing Group, 2021) Holleis, Ludwig; Prestigiacomo, Joseph C.; Fan, Zhijie; Nishimoto, Satoshi; Osofsky, Michael; Chern, Gia-Wei; van den Brink, Jeroen; Shivaram, B.S.The leading order nonlinear (NL) susceptibility, χ3, in a paramagnet is negative and diverges as T → 0. This divergence is destroyed when spins correlate and the NL response provides unique insights into magnetic order. Dimensionality, exchange interaction, and preponderance of quantum effects all imprint their signatures in the NL magnetic response. Here, we study the NL susceptibilities in the proximate Kitaev magnet α-RuCl3, which differs from the expected antiferromagnetic behavior. For T < Tc = 7.5 K and field B in the ab-plane, we obtain contrasting NL responses in low (<2 T) and high field regions. For low fields, the NL behavior is dominated by a quadratic response (positive χ2), which shows a rapid rise below Tc. This large χ2 > 0 implies a broken sublattice symmetry of magnetic order at low temperatures. Classical Monte Carlo (CMC) simulations in the standard K − H − Γ model secure such a quadratic B dependence of M, only for T ≈ Tc with χ2 being zero as T → 0. It is also zero for all temperatures in exact diagonalization calculations. On the other hand, we find an exclusive cubic term (χ3) that describes the high field NL behavior well. χ3 is large and positive both below and above Tc crossing zero only for T > 50 K. In contrast, for B ∥ c-axis, no separate low/high field behaviors are measured and only a much smaller χ3 is apparent.
- ItemAxion Mie theory of electron energy loss spectroscopy in topological insulators(Amsterdam : SciPost Foundation, 2021) Schultz, Johannes; Nogueira, Flavio S.; Büchner, Bernd; van den Brink, Jeroen; Lubk, AxelElectronic topological states of matter exhibit novel types of responses to electromagnetic fields. The response of strong topological insulators, for instance, is characterized by a so-called axion term in the electromagnetic Lagrangian which is ultimately due to the presence of topological surface states. Here we develop the axion Mie theory for the electromagnetic response of spherical particles including arbitrary sources of fields, i.e., charge and current distributions. We derive an axion induced mixing of transverse magnetic and transverse electric modes which are experimentally detectable through small induced rotations of the field vectors. Our results extend upon previous analyses of the problem. Our main focus is on the experimentally relevant problem of electron energy loss spectroscopy in topological insulators, a technique that has so far not yet been used to detect the axion electromagnetic response in these materials.
- ItemChirality flip of Weyl nodes and its manifestation in strained MoTe2(College Park, MD : APS, 2021) Könye, Viktor; Bouhon, Adrien; Fulga, Ion Cosma; Slager, Robert-Jan; van den Brink, Jeroen; Facio, Jorge I.Due to their topological charge, or chirality, the Weyl cones present in topological semimetals are considered robust against arbitrary perturbations. One well-understood exception to this robustness is the pairwise creation or annihilation of Weyl cones, which involves the overlap of two oppositely charged nodes in energy and momentum. Here we show that their topological charge can in fact change sign, in a process that involves the merging of not two, but three Weyl nodes. This is facilitated by the presence of rotation and time-reversal symmetries, which constrain the relative positions of Weyl cones in momentum space. We analyze the chirality flip process, showing that transport properties distinguish it from the conventional, double Weyl merging. Moreover, we predict that the chirality flip occurs in MoTe$_2$, where experimentally accessible strain leads to the merging of three Weyl cones close to the Fermi level. Our work sets the stage to further investigate and observe such chirality flipping processes in different topological materials.
- ItemCorrelation between topological band character and chemical bonding in a Bi14Rh3I9-based family of insulators(London : Nature Publishing Group, 2016) Rasche, Bertold; Isaeva, Anna; Ruck, Michael; Koepernik, Klaus; Richter, Manuel; van den Brink, JeroenRecently the presence of topologically protected edge-states in Bi14Rh3I9 was confirmed by scanning tunnelling microscopy consolidating this compound as a weak 3D topological insulator (TI). Here, we present a density-functional-theory-based study on a family of TIs derived from the Bi14Rh3I9 parent structure via substitution of Ru, Pd, Os, Ir and Pt for Rh. Comparative analysis of the band-structures throughout the entire series is done by means of a unified minimalistic tight-binding model that evinces strong similarity between the quantum-spin-Hall (QSH) layer in Bi14Rh3I9 and graphene in terms of -molecular orbitals. Topologically non-trivial energy gaps are found for the Ir-, Rh-, Pt- and Pd-based systems, whereas the Os- and Ru-systems remain trivial. Furthermore, the energy position of the metal -band centre is identified as the parameter which governs the evolution of the topological character of the band structure through the whole family of TIs. The -band position is shown to correlate with the chemical bonding within the QSH layers, thus revealing how the chemical nature of the constituents affects the topological band character.
- ItemCoupled multiple-mode theory for s± pairing mechanism in iron based superconductors(London : Nature Publishing Group, 2016) Kiselev, M.N.; Efremov, D.V.; Drechsler, S.L.; van den Brink, Jeroen; Kikoin, K.We investigate the interplay between the magnetic and the superconducting degrees of freedom in unconventional multi-band superconductors such as iron pnictides. For this purpose a dynamical mode-mode coupling theory is developed based on the coupled Bethe-Salpeter equations. In order to investigate the region of the phase diagram not too far from the tetracritical point where the magnetic spin density wave, (SDW) and superconducting (SC) transition temperatures coincide, we also construct a Ginzburg-Landau functional including both SC and SDW fluctuations in a critical region above the transition temperatures. The fluctuation corrections tend to suppress the magnetic transition, but in the superconducting channel the intraband and interband contribution of the fluctuations nearly compensate each other.
- ItemCovalency and vibronic couplings make a nonmagnetic j=3/2 ion magnetic(London : Nature Publishing Group, 2016) Xu, Lei; Bogdanov, Nikolay A.; Princep, Andrew; Fulde, Peter; van den Brink, Jeroen; Hozoi, LiviuFor 4d1 and 5d1 spin–orbit-coupled electron configurations, the notion of nonmagnetic j=3/2 quartet ground state discussed in classical textbooks is at odds with the observed variety of magnetic properties. Here we throw fresh light on the electronic structure of 4d1 and 5d1 ions in molybdenum- and osmium-based double-perovskite systems and reveal different kinds of on-site many-body physics in the two families of compounds: although the sizable magnetic moments and g-factors measured experimentally are due to both metal d–ligand p hybridisation and dynamic Jahn–Teller interactions for 4d electrons, it is essentially d−p covalency for the 5d1 configuration. These results highlight the subtle interplay of spin–orbit interactions, covalency and electron–lattice couplings as the major factor in deciding the nature of the magnetic ground states of 4d and 5d quantum materials. Cation charge imbalance in the double-perovskite structure is further shown to allow a fine tuning of the gap between the t2g and eg levels, an effect of much potential in the context of orbital engineering in oxide electronics.
- ItemCreating Weyl nodes and controlling their energy by magnetization rotation(College Park, ML : American Physical Society, 2020) Ghimire, Madhav Prasad; Facio, Jorge I.; You, Jhih-Shih; Ye, Linda; Checkelsky, Joseph G.; Fang, Shiang; Kaxiras, Efthimios; Richter, Manuel; van den Brink, JeroenAs they do not rely on the presence of any crystal symmetry, Weyl nodes are robust topological features of an electronic structure that can occur at any momentum and energy. Acting as sinks and sources of Berry curvature, Weyl nodes have been predicted to strongly affect the transverse electronic response, like in the anomalous Hall or Nernst effects. However, to observe large anomalous effects the Weyl nodes need to be close to or at the Fermi level, which implies the band structure must be tuned by an external parameter, e.g., chemical doping. Here we show that in a ferromagnetic metal tuning of the Weyl node energy and momentum can be achieved by rotation of the magnetization. First, taking as example the elementary magnet hcp-Co, we use electronic structure calculations based on density-functional theory to show that by canting the magnetization away from the easy axis, Weyl nodes can be driven exactly to the Fermi surface. Second, we show that the same phenomenology applies to the kagome ferromagnet Co3Sn2S2, in which we additionally show how the dynamics in energy and momentum of the Weyl nodes affects the calculated anomalous Hall and Nernst conductivities. Our results highlight how the intrinsic magnetic anisotropy can be used to engineer Weyl physics.
- ItemCurvature induced magnonic crystal in nanowires(Amsterdam : SciPost Foundation, 2019) Korniienko, Anastasiia; Kravchuk, Volodymyr P.; Pylypovskyi, Oleksandr V.; Sheka, Denis D.; van den Brink, Jeroen; Gaididei, YuriA new type of magnonic crystals, curvature induced ones, is realized in ferromagnetic nanowires with periodically deformed shape. A magnon band structure of such crystal is fully determined by its curvature: the developed theory is well confirmed by simulations. An application to nanoscale spintronic devices with the geometrically tunable parameters is proposed, namely, to filter elements.
- ItemDifferent types of spin currents in the comprehensive materials database of nonmagnetic spin Hall effect(London : Nature Publ. Group, 2021) Zhang, Yang; Xu, Qiunan; Koepernik, Klaus; Rezaev, Roman; Janson, Oleg; Železný, Jakub; Jungwirth, Tomáš; Felser, Claudia; van den Brink, Jeroen; Sun, YanSpin Hall effect (SHE) has its special position in spintronics. To gain new insight into SHE and to identify materials with substantial spin Hall conductivity (SHC), we performed high-precision high-throughput ab initio calculations of the intrinsic SHC for over 20,000 nonmagnetic crystals. The calculations revealed a strong relationship between the magnitude of the SHC and the crystalline symmetry, where a large SHC is typically associated with mirror symmetry-protected nodal line band structures. This database includes 11 materials with an SHC comparable to or even larger than that of Pt. Materials with different types of spin currents were additionally identified. Furthermore, we found that different types of spin current can be obtained by rotating applied electrical fields. This improves our understanding and is expected to facilitate the design of new types of spin-orbitronic devices.
- ItemDisorder effects in the Kitaev-Heisenberg model(College Park, MD : APS, 2023) Singhania, Ayushi; van den Brink, Jeroen; Nishimoto, SatoshiWe study the interplay of disorder and Heisenberg interactions in the Kitaev model on a honeycomb lattice. The effect of disorder on the transition between Kitaev spin liquid and magnetic ordered states as well as the stability of magnetic ordering is investigated. Using Lanczos exact diagonalization we discuss the consequences of two types of disorder: (i) random-coupling disorder and (ii) singular-coupling disorder. They exhibit qualitatively similar effects in the pure Kitaev-Heisenberg model without long-range interactions. The range of spin-liquid phases is reduced and the transition to magnetic ordered phases becomes more crossoverlike. Furthermore, the long-range zigzag and stripy orderings in the clean system are replaced by their three domains with different ordering direction. Especially in the crossover range the coexistence of magnetically ordered and Kitaev spin-liquid domains is possible. With increasing the disorder strength the area of domains becomes smaller and the system goes into a spin-glass state. However, the disorder effect is different in magnetically ordered phases caused by long-range interactions. The stability of such magnetic ordering is diminished by singular-coupling disorder and, accordingly, the range of the spin-liquid regime is extended. This mechanism may be relevant to materials like α−RuCl3 and H3LiIr2O6 where the zigzag ground state is stabilized by weak long-range interactions. We also find that the flux gap closes at a critical disorder strength and vortices appears in the flux arrangement. Interestingly, the vortices tend to form kinds of commensurate ordering.
- ItemElectron-lattice interactions strongly renormalize the charge-transfer energy in the spin-chain cuprate Li2 CuO2(London : Nature Publishing Group, 2016) Johnston, Steve; Monney, Claude; Bisogni, Valentina; Zhou, Ke-Jin; Kraus, Roberto; Behr, Günter; Strocov, Vladimir N.; Málek, Jiři; Drechsler, Stefan-Ludwig; Geck, Jochen; Schmitt, Thorsten; van den Brink, JeroenStrongly correlated insulators are broadly divided into two classes: Mott–Hubbard insulators, where the insulating gap is driven by the Coulomb repulsion U on the transition-metal cation, and charge-transfer insulators, where the gap is driven by the charge-transfer energy Δ between the cation and the ligand anions. The relative magnitudes of U and Δ determine which class a material belongs to, and subsequently the nature of its low-energy excitations. These energy scales are typically understood through the local chemistry of the active ions. Here we show that the situation is more complex in the low-dimensional charge-transfer insulator Li2CuO2, where Δ has a large non-electronic component. Combining resonant inelastic X-ray scattering with detailed modelling, we determine how the elementary lattice, charge, spin and orbital excitations are entangled in this material. This results in a large lattice-driven renormalization of Δ, which significantly reshapes the fundamental electronic properties of Li2CuO2.
- ItemEngineering a pure Dirac regime in ZrTe5(Amsterdam : SciPost Foundation, 2023) Facio, Jorge I.; Nocerino, Elisabetta; Fulga, Ion Cosma; Wawrzynczak, Rafal; Brown, Joanna; Gu, Genda; Li, Qiang; Mansson, Martin; Sassa, Yasmine; Ivashko, Oleh; von Zimmermann, Martin; Mende, Felix; Gooth, Johannes; Galeski, Stanislaw; van den Brink, Jeroen; Meng, TobiasReal-world topological semimetals typically exhibit Dirac and Weyl nodes that coexist with trivial Fermi pockets. This tends to mask the physics of the relativistic quasiparticles. Using the example of ZrTe5, we show that strain provides a powerful tool for in-situ tuning of the band structure such that all trivial pockets are pushed far away from the Fermi energy, but only for a certain range of Van der Waals gaps. Our results naturally reconcile contradicting reports on the presence or absence of additional pockets in ZrTe5, and provide a clear map of where to find a pure three-dimensional Dirac semimetallic phase in the structural parameter space of the material.
- ItemEngineering spectral properties of non-interacting lattice Hamiltonians(Amsterdam : SciPost Foundation, 2021) Moghaddam, Ali G.; Chernyavsky, Dmitry; Morice, Corentin; van Wezel, Jasper; van den Brink, JeroenWe investigate the spectral properties of one-dimensional lattices with position-dependent hopping amplitudes and on-site potentials that are smooth bounded functions of the position. We find an exact integral form for the density of states (DOS) in the limit of an infinite number of sites, which we derive using a mixed Bloch-Wannier basis consisting of piecewise Wannier functions. Next, we provide an exact solution for the inverse problem of constructing the position-dependence of hopping in a lattice model yielding a given DOS. We confirm analytic results by comparing them to numerics obtained by exact diagonalization for various incarnations of position-dependent hoppings and on-site potentials. Finally, we generalize the DOS integral form to multi-orbital tight-binding models with longer-range hoppings and in higher dimensions.
- ItemEvidence for a percolative Mott insulator-metal transition in doped Sr2IrO4(College Park, MD : APS, 2021) Sun, Zhixiang; Guevara, Jose M.; Sykora, Steffen; Pärschke, Ekaterina M.; Manna, Kaustuv; Maljuk, Andrey; Wurmehl, Sabine; van den Brink, Jeroen; Büchner, Bernd; Hess, ChristianDespite many efforts to rationalize the strongly correlated electronic ground states in doped Mott insulators, the nature of the doping-induced insulator-to-metal transition is still a subject under intensive investigation. Here, we probe the nanoscale electronic structure of the Mott insulator Sr2IrO4−δ with low-temperature scanning tunneling microscopy and find an enhanced local density of states (LDOS) inside the Mott gap at the location of individual defects which we interpret as defects at apical oxygen sites. A chiral behavior in the topography for those defects has been observed. We also visualize the local enhanced conductance arising from the overlapping of defect states which induces finite LDOS inside of the Mott gap. By combining these findings with the typical spatial extension of isolated defects of about 2 nm, our results indicate that the insulator-to-metal transition in Sr2IrO4−δ could be percolative in nature.
- ItemExtended high-harmonic spectra through a cascade resonance in confined quantum systems(College Park, MD : APS, 2022) Zhang, Xiao; Zhu, Tao; Du, Hongchuan; Luo, Hong-Gang; van den Brink, Jeroen; Ray, RajyavardhanThe study of high-harmonic generation in confined quantum systems is vital to establishing a complete physical picture of harmonic generation from atoms and molecules to bulk solids. Based on a multilevel approach, we demonstrate how intraband resonances significantly influence the harmonic spectra via charge pumping to the higher subbands and thus redefine the cutoff laws. As a proof of principle, we consider the interaction of graphene nanoribbons, with zigzag as well as armchair terminations, and resonant fields polarized along the cross-ribbon direction. Here, this effect is particularly prominent due to many nearly equiseparated energy levels. In such a scenario, a cascade resonance effect can take place in high-harmonic generation when the field strength is above a critical threshold, which is completely different from the harmonic generation mechanism of atoms, molecules, and bulk solids. We further discuss the implications not only for other systems in a nanoribbon geometry, but also systems where only a few subbands (energy levels) meet this frequency-matching condition by considering a generalized multilevel Hamiltonian. Our study highlights that cascade resonance has a fundamentally distinct influence on the laws of harmonic generation, specifically the cutoff laws based on laser duration, field strength, and wavelength, thus unraveling additional insights in solid-state high-harmonic generation.
- ItemFinite temperature fluctuation-induced order and responses in magnetic topological insulators(College Park, MD : APS, 2021) Scholten, Marius; Facio, Jorge I.; Ray, Rajyavardhan; Eremin, Ilya M.; van den Brink, Jeroen; Nogueira, Flavio S.We derive an effective field theory model for magnetic topological insulators and predict that a magnetic electronic gap persists on the surface for temperatures above the ordering temperature of the bulk. Our analysis also applies to interfaces of heterostructures consisting of a ferromagnetic and a topological insulator. In order to make quantitative predictions for MnBi2Te4 and for EuS-Bi2Se3 heterostructures, we combine the effective field theory method with density functional theory and Monte Carlo simulations. For MnBi2Te4 we predict an upwards Néel temperature shift at the surface up to 15%, while the EuS-Bi2Se3 interface exhibits a smaller relative shift. The effective theory also predicts induced Dzyaloshinskii-Moriya interactions and a topological magnetoelectric effect, both of which feature a finite temperature and chemical potential dependence.
- ItemHorizon physics of quasi-one-dimensional tilted Weyl cones on a lattice(College Park, MD : APS, 2022) Könye, Viktor; Morice, Corentin; Chernyavsky, Dmitry; Moghaddam, Ali G.; van den Brink, Jeroen; van Wezel, JasperTo simulate the dynamics of massless Dirac fermions in curved space-times with one, two, and three spatial dimensions, we construct tight-binding Hamiltonians with spatially varying hoppings. These models represent tilted Weyl semimetals where the tilting varies with position, in a manner similar to the light cones near the horizon of a black hole. We illustrate the gravitational analogies in these models by numerically evaluating the propagation of wave packets on the lattice and then comparing them to the geodesics of the corresponding curved space-time. We also show that the motion of electrons in these spatially varying systems can be understood through the conservation of energy and the quasiconservation of quasimomentum. This picture is confirmed by calculations of the scattering matrix, which indicate an exponential suppression of any noncontinuous change in the quasimomentum. Finally, we show that horizons in the lattice models can be constructed also at finite energies using specially designed tilting profiles.
- ItemIntercalant-mediated Kitaev exchange in Ag3LiIr2O6(College Park, MD : APS, 2022) Yadav, Ravi; Reja, Sahinur; Ray, Rajyavardhan; van den Brink, Jeroen; Nishimoto, Satoshi; Yazyev, Oleg V.The recently synthesized Ag3LiIr2O6 has been proposed as a Kitaev magnet in proximity to the quantum spin liquid phase. We explore its microscopic Hamiltonian and magnetic ground state using many-body quantum chemistry methods and exact diagonalization techniques. Our calculations establish a dominant bond dependent ferromagnetic Kitaev exchange between Ir sites and find that the inclusion of Ag 4d orbitals in the configuration interaction calculations strikingly enhances the Kitaev exchange. Furthermore, using exact diagonalization of the nearest-neighbor fully anisotropic J−K−Γ Hamiltonian, we obtain the magnetic phase diagram as a function of further neighbor couplings. We find that the antiferromagnetic off-diagonal coupling stabilizes long range order, but the structure factor calculations suggest that the material is very close to the quantum spin liquid phase and the ordered state can easily collapse into a liquid by small perturbations such as structural distortion or bond disorder.
- ItemIntermixing-Driven Surface and Bulk Ferromagnetism in the Quantum Anomalous Hall Candidate MnBi6Te10(Weinheim : Wiley-VCH, 2023) Tcakaev, Abdul‐Vakhab; Rubrecht, Bastian; Facio, Jorge I.; Zabolotnyy, Volodymyr B.; Corredor, Laura T.; Folkers, Laura C.; Kochetkova, Ekaterina; Peixoto, Thiago R. F.; Kagerer, Philipp; Heinze, Simon; Bentmann, Hendrik; Green, Robert J.; Gargiani, Pierluigi; Valvidares, Manuel; Weschke, Eugen; Haverkort, Maurits W.; Reinert, Friedrich; van den Brink, Jeroen; Büchner, Bernd; Wolter, Anja U. B.; Isaeva, Anna; Hinkov, VladimirThe recent realizations of the quantum anomalous Hall effect (QAHE) in MnBi2Te4 and MnBi4Te7 benchmark the (MnBi2Te4)(Bi2Te3)n family as a promising hotbed for further QAHE improvements. The family owes its potential to its ferromagnetically (FM) ordered MnBi2Te4 septuple layers (SLs). However, the QAHE realization is complicated in MnBi2Te4 and MnBi4Te7 due to the substantial antiferromagnetic (AFM) coupling between the SLs. An FM state, advantageous for the QAHE, can be stabilized by interlacing the SLs with an increasing number n of Bi2Te3 quintuple layers (QLs). However, the mechanisms driving the FM state and the number of necessary QLs are not understood, and the surface magnetism remains obscure. Here, robust FM properties in MnBi6Te10 (n = 2) with Tc ≈ 12 K are demonstrated and their origin is established in the Mn/Bi intermixing phenomenon by a combined experimental and theoretical study. The measurements reveal a magnetically intact surface with a large magnetic moment, and with FM properties similar to the bulk. This investigation thus consolidates the MnBi6Te10 system as perspective for the QAHE at elevated temperatures.