A1: Tuning topological electronic states and topological magnons
Christine Kuntscher, Joachim Deisenhofer, István Kézsmárki
Within this project we will identify the spectral signatures of the electronic band topology, such as nodal points, lines, planes, and flat bands, in the diagonal and off-diagonal optical conductivity of magnetic semi-metals, with the focus on kagome layer-based materials. The magnetically-driven band reconstruction and the tunability of the topological fermionic states will be studied by optical spectroscopy over a broad frequency range in external magnetic fields and under hydrostatic pressure, respectively. Furthermore, we will search for the signatures of bulk topological magnons and of topologically protected magnon edge states in honeycomb materials, pyrochlore compounds, and kagome magnets by magneto-optical THz spectroscopy.
Publications
2024 |
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Ebad-Allah, J.; Jiang, M. -C.; Borkenhagen, R.; Meggle, F.; Prodan, L.; Tsurkan, V.; Schilberth, F.; Guo, G. -Y.; Arita, R.; Kézsmárki, I.; Kuntscher, C. A. Optical anisotropy of the kagome magnet FeSn: Dominant role of excitations between kagome and Sn layers Journal Article Phys. Rev. B 109, L201106, 2024. @article{ebad-allah_optical_2024,Antiferromagnetic FeSn is considered to be a close realization of the ideal two-dimensional (2D) kagome lattice, hosting Dirac cones, van Hove singularities, and flat bands, as it comprises Fe3Sn kagome layers well separated by Sn buffer layers. We observe a pronounced optical anisotropy, with the low-energy optical conductivity being surprisingly higher perpendicular to the kagome planes than along the layers. This finding contradicts the prevalent picture of dominantly 2D electronic structure for FeSn. Our material-specific theory reproduces the measured conductivity spectra remarkably well. A site-specific decomposition of the optical response to individual excitation channels shows that the optical conductivity for polarizations both parallel and perpendicular to the kagome plane is dominated by interlayer transitions between kagome layers and adjacent Sn-based layers. Moreover, the matrix elements corresponding to these transitions are highly anisotropic, leading to larger out-of-plane conductivity. Our results evidence the crucial role of interstitial layers in charge dynamics even in seemingly 2D systems. |  |
Kumar, H.; Köpf, M.; Telang, P.; Bura, N.; Jesche, A.; Gegenwart, P.; Kuntscher, C. A. Phys. Rev. B 110, 035140, 2024. @article{kumar_optical_2024,The synergy of strong spin-orbit coupling and electron-electron interactions gives rise to unconventional topological states, such as topological Mott insulator, Weyl semimetal, and quantum spin liquid. In this study, we have grown single crystals of the pyrochlore iridate Pr2Ir2O7 and explored its magnetic, lattice dynamical, and electronic properties. While Raman spectroscopy data reveal six phonon modes confirming the cubic Fd‾3m crystal symmetry, dc magnetic susceptibility data show no anomalies and hence indicate the absence of magnetic phase transitions down to 2 K. Both temperature-dependent electric transport and optical conductivity data reveal the metallic character of Pr2Ir2O7. The optical conductivity spectrum contains a midinfrared absorption band, which becomes more pronounced with decreasing temperature due to spectral weight transfer from high to low energies. The presence of the midinfrared band hints at the importance of correlation physics. The optical response furthermore suggests that Pr2Ir2O7 is close to the Weyl semimetal phase. |  |
Kunze, J.; Köpf, M.; Cao, Weizheng; Qi, Yanpeng; Kuntscher, C. A. Optical signatures of type-II Weyl fermions in the noncentrosymmetric semimetals RAlSi (R=La, Ce, Pr, Nd, Sm) Journal Article Phys. Rev. B 109, 195130, 2024. @article{kunze_optical_2024,Weyl semimetals with magnetic ordering provide a promising platform for the investigation of rare topological effects such as the anomalous Hall effect, resulting from the interplay of nontrivial bands with various spin configurations. The materials RAlSi, where R represents a rare-earth element, are prominent representatives of Weyl semimetals, where the Weyl states are induced by space inversion symmetry breaking, and in addition, for several rare-earth elements R, enhanced by time-reversal symmetry breaking through the formation of a magnetic order at low temperature. We report optical signatures of Weyl fermions in the magnetic compounds CeAlSi, PrAlSi, NdAlSi, and SmAlSi as well as the nonmagnetic family member LaAlSi by broad-frequency infrared spectroscopy at room temperature, i.e., in the paramagnetic phase. A similar profile of the optical conductivity spectrum and a metallic character are observed for all compounds, with LaAlSi showing the strongest free charge-carrier contribution. Furthermore, the linear-in-frequency behavior of the optical conductivity of all investigated compounds indicates the presence of Weyl nodes in close proximity to the Fermi energy, resulting from inversion symmetry breaking in noncentrosymmetric structures. According to the characteristics of these linear slopes, the RAlSi compounds are expected to host mainly type-II Weyl states with overtilted Weyl cones. The results are compared to the optical response of the closely related RAlGe materials, which are considered as potential hybridization-driven Weyl-Kondo systems. | |
2023 |
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Ghara, S.; Barts, E.; Vasin, K.; Kamenskyi, D.; Prodan, L.; Tsurkan, V.; Kézsmárki, I.; Mostovoy, M.; Deisenhofer, J. Magnetization reversal through an antiferromagnetic state Journal Article Nat. Commun. 14, 5174, 2023. @article{ghara_magnetization_2023,Magnetization reversal in ferro- and ferrimagnets is a well-known archetype of non-equilibrium processes, where the volume fractions of the oppositely magnetized domains vary and perfectly compensate each other at the coercive magnetic field. Here, we report on a fundamentally new pathway for magnetization reversal that is mediated by an antiferromagnetic state. Consequently, an atomic-scale compensation of the magnetization is realized at the coercive field, instead of the mesoscopic or macroscopic domain cancellation in canonical reversal processes. We demonstrate this unusual magnetization reversal on the Zn-doped polar magnet Fe2Mo3O8. Hidden behind the conventional ferrimagnetic hysteresis loop, the surprising emergence of the antiferromagnetic phase at the coercive fields is disclosed by a sharp peak in the field-dependence of the electric polarization. In addition, at the magnetization reversal our THz spectroscopy studies reveal the reappearance of the magnon mode that is only present in the pristine antiferromagnetic state. According to our microscopic calculations, this unusual process is governed by the dominant intralayer coupling, strong easy-axis anisotropy and spin fluctuations, which result in a complex interplay between the ferrimagnetic and antiferromagnetic phases. Such antiferro-state-mediated reversal processes offer novel concepts for magnetization control, and may also emerge for other ferroic orders. |  |
Schilberth, F.; Jiang, M. -C.; Minami, S.; Kassem, M. A.; Mayr, F.; Koretsune, T.; Tabata, Y.; Waki, T.; Nakamura, H.; Guo, G. -Y.; Arita, R.; Kézsmárki, I.; Bordacs, S. Nodal-line resonance generating the giant anomalous Hall effect of Co3Sn2S2 Journal Article Phys. Rev. B 107, 214441, 2023. @article{schilberth_nodal-line_2023,Giant anomalous Hall effect (AHE) and magneto-optical activity can emerge in magnets with topologically nontrivial degeneracies. However, identifying the specific band-structure features such as Weyl points, nodal lines, or planes which generate the anomalous response is a challenging issue. Since the low-energy interband transitions can govern the static AHE, we addressed this question in the prototypical magnetic Weyl semimetal Co3Sn2S2 also hosting nodal lines by broadband polarized reflectivity and magneto-optical Kerr effect spectroscopy with a focus on the far-infrared range. In the linear dichroism spectrum we observe a strong resonance at 40 meV, which also appears in the optical Hall conductivity and primarily determines the static AHE, and thus confirms its intrinsic origin. Our material-specific theory reproduces the experimental data remarkably well and shows that strongly tilted nodal-line segments around the Fermi energy generate the resonance. While the Weyl points only give vanishing contributions, these segments of the nodal lines gapped by the spin-orbit coupling dominate the low-energy optical response and generate the giant AHE. |  |