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- ItemTunable positions of Weyl nodes via magnetism and pressure in the ferromagnetic Weyl semimetal CeAlSi([London] : Nature Publishing Group UK, 2024) Cheng, Erjian; Yan, Limin; Shi, Xianbiao; Lou, Rui; Fedorov, Alexander; Behnami, Mahdi; Yuan, Jian; Yang, Pengtao; Wang, Bosen; Cheng, Jin-Guang; Xu, Yuanji; Xu, Yang; Xia, Wei; Pavlovskii, Nikolai; Peets, Darren C.; Zhao, Weiwei; Wan, Yimin; Burkhardt, Ulrich; Guo, Yanfeng; Li, Shiyan; Felser, Claudia; Yang, Wenge; Büchner, BerndThe noncentrosymmetric ferromagnetic Weyl semimetal CeAlSi with simultaneous space-inversion and time-reversal symmetry breaking provides a unique platform for exploring novel topological states. Here, by employing multiple experimental techniques, we demonstrate that ferromagnetism and pressure can serve as efficient parameters to tune the positions of Weyl nodes in CeAlSi. At ambient pressure, a magnetism-facilitated anomalous Hall/Nernst effect (AHE/ANE) is uncovered. Angle-resolved photoemission spectroscopy (ARPES) measurements demonstrated that the Weyl nodes with opposite chirality are moving away from each other upon entering the ferromagnetic phase. Under pressure, by tracing the pressure evolution of AHE and band structure, we demonstrate that pressure could also serve as a pivotal knob to tune the positions of Weyl nodes. Moreover, multiple pressure-induced phase transitions are also revealed. These findings indicate that CeAlSi provides a unique and tunable platform for exploring exotic topological physics and electron correlations, as well as catering to potential applications, such as spintronics.
- ItemObservation of giant spin-split Fermi-arc with maximal Chern number in the chiral topological semimetal PtGa(London : Nature Publishing Group, 2020) Yao, M.; Manna, K.; Yang, Q.; Fedorov, A.; Voroshnin, V.; Valentin Schwarze, B.; Hornung, J.; Chattopadhyay, S.; Sun, Z.; Guin, S.N.; Wosnitza, J.; Borrmann, H.; Shekhar, C.; Kumar, N.; Fink, J.; Sun, Y.; Felser, C.Non-symmorphic chiral topological crystals host exotic multifold fermions, and their associated Fermi arcs helically wrap around and expand throughout the Brillouin zone between the high-symmetry center and surface-corner momenta. However, Fermi-arc splitting and realization of the theoretically proposed maximal Chern number rely heavily on the spin-orbit coupling (SOC) strength. In the present work, we investigate the topological states of a new chiral crystal, PtGa, which has the strongest SOC among all chiral crystals reported to date. With a comprehensive investigation using high-resolution angle-resolved photoemission spectroscopy, quantum-oscillation measurements, and state-of-the-art ab initio calculations, we report a giant SOC-induced splitting of both Fermi arcs and bulk states. Consequently, this study experimentally confirms the realization of a maximal Chern number equal to ±4 in multifold fermionic systems, thereby providing a platform to observe large-quantized photogalvanic currents in optical experiments.
- ItemObservation of fractional spin textures in a Heusler material([London] : Nature Publishing Group UK, 2022) Jena, Jagannath; Göbel, Börge; Hirosawa, Tomoki; Díaz, Sebastián A.; Wolf, Daniel; Hinokihara, Taichi; Kumar, Vivek; Mertig, Ingrid; Felser, Claudia; Lubk, Axel; Loss, Daniel; Parkin, Stuart S.P.Recently a zoology of non-collinear chiral spin textures has been discovered, most of which, such as skyrmions and antiskyrmions, have integer topological charges. Here we report the experimental real-space observation of the formation and stability of fractional antiskyrmions and fractional elliptical skyrmions in a Heusler material. These fractional objects appear, over a wide range of temperature and magnetic field, at the edges of a sample, whose interior is occupied by an array of nano-objects with integer topological charges, in agreement with our simulations. We explore the evolution of these objects in the presence of magnetic fields and show their interconversion to objects with integer topological charges. This means the topological charge can be varied continuously. These fractional spin textures are not just another type of skyrmion, but are essentially a new state of matter that emerges and lives only at the boundary of a magnetic system. The coexistence of both integer and fractionally charged spin textures in the same material makes the Heusler family of compounds unique for the manipulation of the real-space topology of spin textures and thus an exciting platform for spintronic and magnonic applications.
- ItemChirality coupling in topological magnetic textures with multiple magnetochiral parameters([London] : Nature Publishing Group UK, 2023) Volkov, Oleksii M.; Wolf, Daniel; Pylypovskyi, Oleksandr V.; Kákay, Attila; Sheka, Denis D.; Büchner, Bernd; Fassbender, Jürgen; Lubk, Axel; Makarov, DenysChiral effects originate from the lack of inversion symmetry within the lattice unit cell or sample’s shape. Being mapped onto magnetic ordering, chirality enables topologically non-trivial textures with a given handedness. Here, we demonstrate the existence of a static 3D texture characterized by two magnetochiral parameters being magnetic helicity of the vortex and geometrical chirality of the core string itself in geometrically curved asymmetric permalloy cap with a size of 80 nm and a vortex ground state. We experimentally validate the nonlocal chiral symmetry breaking effect in this object, which leads to the geometric deformation of the vortex string into a helix with curvature 3 μm−1 and torsion 11 μm−1. The geometric chirality of the vortex string is determined by the magnetic helicity of the vortex texture, constituting coupling of two chiral parameters within the same texture. Beyond the vortex state, we anticipate that complex curvilinear objects hosting 3D magnetic textures like curved skyrmion tubes and hopfions can be characterized by multiple coupled magnetochiral parameters, that influence their statics and field- or current-driven dynamics for spin-orbitronics and magnonics.