The modern version of the KKR (Korringa-Kohn-Rostoker) method represents the electronic structure of a system directly and efficiently in terms of its single-particle Green's function (GF). This is ...in contrast to its original version and many other traditional wave-function-based all-electron band structure methods dealing with periodically ordered solids. Direct access to the GF results in several appealing features. In addition, a wide applicability of the method is achieved by employing multiple scattering theory. The basic ideas behind the resulting KKR-GF method are outlined and the different techniques to deal with the underlying multiple scattering problem are reviewed. Furthermore, various applications of the KKR-GF method are reviewed in some detail to demonstrate the remarkable flexibility of the approach. Special attention is devoted to the numerous developments of the KKR-GF method, that have been contributed in recent years by a number of work groups, in particular in the following fields: embedding schemes for atoms, clusters and surfaces, magnetic response functions and anisotropy, electronic and spin-dependent transport, dynamical mean field theory, various kinds of spectroscopies, as well as first-principles determination of model parameters.
The 2020 skyrmionics roadmap Back, C; Cros, V; Ebert, H ...
Journal of physics. D, Applied physics,
09/2020, Letnik:
53, Številka:
36
Journal Article
Recenzirano
Odprti dostop
The notion of non-trivial topological winding in condensed matter systems represents a major area of present-day theoretical and experimental research. Magnetic materials offer a versatile platform ...that is particularly amenable for the exploration of topological spin solitons in real space such as skyrmions. First identified in non-centrosymmetric bulk materials, the rapidly growing zoology of materials systems hosting skyrmions and related topological spin solitons includes bulk compounds, surfaces, thin films, heterostructures, nano-wires and nano-dots. This underscores an exceptional potential for major breakthroughs ranging from fundamental questions to applications as driven by an interdisciplinary exchange of ideas between areas in magnetism which traditionally have been pursued rather independently. The skyrmionics Roadmap provides a review of the present state of the art and the wide range of research directions and strategies currently under way. These are, for instance, motivated by the identification of the fundamental structural properties of skyrmions and related textures, processes of nucleation and annihilation in the presence of non-trivial topological winding, an exceptionally efficient coupling to spin currents generating spin transfer torques at tiny current densities, as well as the capability to purpose-design broad-band spin dynamic and logic devices.
Magnetically doped topological insulators enable the quantum anomalous Hall effect (QAHE), which provides quantized edge states for lossless charge-transport applications1-8. The edge states are ...hosted by a magnetic energy gap at the Dirac point2, but hitherto all attempts to observe this gap directly have been unsuccessful. Observing the gap is considered to be essential to overcoming the limitations of the QAHE, which so far occurs only at temperatures that are one to two orders of magnitude below the ferromagnetic Curie temperature, TC (ref. 8). Here we use lowtemperature photoelectron spectroscopy to unambiguously reveal the magnetic gap of Mn-doped Bi2Te3, which displays ferromagnetic out-of-plane spin texture and opens up only below TC. Surprisingly, our analysis reveals large gap sizes at 1 kelvin of up to 90 millielectronvolts, which is five times larger than theoretically predicted9. Using multiscale analysis we show that this enhancement is due to a remarkable structure modification induced by Mn doping: instead of a disordered impurity system, a self-organized alternating sequence of MnBi2Te4 septuple and Bi2Te3 quintuple layers is formed. This enhances the wavefunction overlap and size of the magnetic gap10. Mn-doped Bi2Se3 (ref. 11) and Mn-doped Sb2Te3 form similar heterostructures, but for Bi2Se3 only a nonmagnetic gap is formed and the magnetization is in the surface plane. This is explained by the smaller spin-orbit interaction by comparison with Mn-doped Bi2Te3. Our findings provide insights that will be crucial in pushing lossless transport in topological insulators towards roomtemperature applications.
Porous material systems are commonly used for thermal insulation due to their low total effective thermal conductivities. However, the prediction of this physical property is difficult for some ...porous materials, since there is no straightforward relation to describe the impact of coupling between solid and gaseous thermal conduction. Thus, the aim of this work was to get a better general understanding of the coupling effect. For this purpose, different carbon and organic (resorcinol-formaldehyde) aerogels were used as model system with high and low intrinsic backbone conductivity and were investigated by means of gas-pressure dependent hot-wire measurements in different gas atmospheres (Ar, N2, He and CO2) up to 10MPa. The thermal conductivity data received for all samples analyzed show a significant amount of thermal coupling between gas and solid phase: The experimental gas-pressure dependent thermal conductivities are up to a factor of 3.3 higher than the expected pure gaseous thermal conductivities.
A simple theoretical approach (“scaling model”) is introduced providing a good description of the experimental gas-pressure dependent thermal conductivity data. Thus, this model is useful for easily estimating the mean pore sizes of porous samples. Moreover, applying the scaling model to the helium data reveals that the accommodation coefficient of helium must be close to one for all aerogel samples investigated.
Furthermore, the coupling term for a simple layer system is analytically described by a series connection of the thermal resistances of the two phases, in order to analyze the impact of different parameters on the coupling term: coupling increases with the ratio of solid phase thickness to mean pore size and with the ratio of solid to gaseous thermal conductivity. Overall, the theoretical results are consistent with the experimental results.
A scheme is presented that is based on the alloy analogy model and allows one to account for thermal lattice vibrations as well as spin fluctuations when calculating response quantities in solids. ...Various models to deal with spin fluctuations are discussed concerning their impact on the resulting temperature-dependent magnetic moment, longitudinal conductivity, and Gilbert damping parameter. It is demonstrated that, by using the Monte Carlo (MC) spin configuration as input, the alloy analogy model is capable of reproducing the results of MC simulations on the average magnetic moment within all spin fluctuation models under discussion. On the other hand, the response quantities are much more sensitive to the spin fluctuation model. Separate calculations accounting for the thermal effect due to either lattice vibrations or spin fluctuations show that they give comparable contributions to the electrical conductivity and Gilbert damping. However, comparison to results accounting for both thermal effects demonstrates violation of Matthiessen's rule, showing the nonadditive effect of lattice vibrations and spin fluctuations. The results obtained for bcc Fe and fcc Ni are compared with the experimental data, showing rather good agreement for the temperature-dependent electrical conductivity and the Gilbert damping parameter.
Ferromagnetic thin films of Heusler compounds are highly relevant for spintronic applications owing to their predicted half-metallicity, that is, 100% spin polarization at the Fermi energy. However, ...experimental evidence for this property is scarce. Here we investigate epitaxial thin films of the compound Co2MnSi in situ by ultraviolet-photoemission spectroscopy, taking advantage of a novel multi-channel spin filter. By this surface sensitive method, an exceptionally large spin polarization of (93(-11)(+7)) % at room temperature is observed directly. As a more bulk sensitive method, additional ex situ spin-integrated high energy X-ray photoemission spectroscopy experiments are performed. All experimental results are compared with advanced band structure and photoemission calculations which include surface effects. Excellent agreement is obtained with calculations, which show a highly spin polarized bulk-like surface resonance ingrained in a half metallic bulk band structure.
Autism spectrum disorders such as Rett syndrome (RTT) have been hypothesized to arise from defects in experience-dependent synapse maturation. RTT is caused by mutations in MECP2, a nuclear protein ...that becomes phosphorylated at S421 in response to neuronal activation. We show here that disruption of MeCP2 S421 phosphorylation in vivo results in defects in synapse development and behavior, implicating activity-dependent regulation of MeCP2 in brain development and RTT. We investigated the mechanism by which S421 phosphorylation regulates MeCP2 function and show by chromatin immunoprecipitation-sequencing that this modification occurs on MeCP2 bound across the genome. The phosphorylation of MeCP2 S421 appears not to regulate the expression of specific genes; rather, MeCP2 functions as a histone-like factor whose phosphorylation may facilitate a genome-wide response of chromatin to neuronal activity during nervous system development. We propose that RTT results in part from a loss of this experience-dependent chromatin remodeling.
► Loss of activity-dependent MeCP2 phospho-S421 in vivo alters synaptic development ► MeCP2 S421A knockin mice display abnormal behavioral responses to novelty ► MeCP2 binding across the neuronal genome is not detectably altered by stimulation ► Activity-dependent pS421 MeCP2 occurs genome-wide suggesting it has a global function
As a fundamental parameter in magnetism, the phenomenological Gilbert damping constant α determines the performance of many spintronic devices. For most magnetic materials, α is treated as an ...isotropic parameter entering the Landau–Lifshitz–Gilbert equation. However, could the Gilbert damping be anisotropic? Although several theoretical approaches have suggested that anisotropic α could appear in single-crystalline bulk systems, experimental evidence of its existence is scarce. Here, we report the emergence of anisotropic magnetic damping by exploring a quasi-two-dimensional single-crystalline ferromagnetic metal/semiconductor interface—that is, a Fe/GaAs(001) heterojunction. The observed anisotropic damping shows twofold C2v symmetry, which is expected from the interplay of interfacial Rashba and Dresselhaus spin–orbit interaction, and is manifested by the anisotropic density of states at the Fe/GaAs (001) interface. This discovery of anisotropic damping will enrich the understanding of magnetization relaxation mechanisms and can provide a route towards the search for anisotropic damping at other ferromagnetic metal/semiconductor interfaces.
An extension of the Heisenberg Hamiltonian is discussed that allows us to go beyond the standard bilinear spin Hamiltonian taking into account various contributions due to multispin interactions ...having both chiral and nonchiral character. The parameters of the extended Hamiltonian are calculated from first principles within the framework of the multiple-scattering Green's function formalism giving access to an explicit representation of these parameters in real space. The discussions are focused on the chiral interactions, i.e., biquadratic and three-spin Dzyaloshinskii-Moriya-like vector interactions D⃗ijij (BDMI) and D⃗ijkj (TDMI), respectively, as well as the three-spin chiral interaction Jijk. Although all parameters are driven by spin-orbit coupling (SOC), some differences in their properties are demonstrated by calculations for real materials. In particular it is shown that the three-spin chiral interactions Jijk may be topology as well as SOC induced, while the TDMI is associated only with the SOC. As the magnitude of the chiral interactions can be quite sizable, they can lead to a stabilization of a noncollinear magnetic texture in some materials that is absent when these interactions are neglected.
It has been shown that structural properties of open porous solids, such as the mean pore size, can be derived from gas pressure-dependent thermal conductivity data. However, a reliable prediction of ...the total thermal conductivity of a porous sample with complex backbone structure from structural data is not possible, because the degree of coupling of gaseous and solid thermal conduction is hard to estimate. To explore the impact of structural effects, the thermal performance of different model structures, generally characteristic for porous solids (necks, dead ends, tortuosity), was theoretically evaluated by means of finite-difference calculations. As a result, we find that dead ends cause the highest amount of thermal coupling. On the other hand, independent experimental investigations were performed to support the theoretical findings. That means, the gas pressure-dependent thermal conductivities of two sample systems in a nitrogen atmosphere were analyzed: At first, thermal conductivity data for three organic, resorcinol–formaldehyde based aerogels with different structural properties were received from hot-wire measurements. Secondly, the regular cell structure of melamine resin foam was systematically changed by uniaxial compression within a guarded hot plate apparatus prior to determining the resulting thermal conductivity in the direction of compression. Overall, the measured gas pressure-dependent thermal conductivities of both systems indicate that the connectivity of the solid network significantly affects the solid–gas coupling term in porous solids. Both the experimental and theoretical results show that the coupling term decreases with increasing connectivity of the backbone material of a porous solid.
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