Highlights 2021

Over the past decade, heterogeneous catalysis and asymmetry synthesis have gained considerable attention. Topological materials exhibit symmetrically protected metallic surface states and massless high-mobility electrons, making them ideal materials for designing heterogeneous catalysts. However, there is limited information available on how topological materials with specific properties interact with reaction intermediates. Therefore, understanding the role of electrons and surface structures in topological
materials is crucial for designing highly efficient heterogeneous catalysts for use in electrochemical water splitting and fuel cells. more

Many good thermoelectric semiconductors have been demonstrated to be topological insulators. The recently discovered class of topological semimetals provides a platform to search for new thermoelectric materials. In our work, we focus on the thermoelectric transport properties of the single crystals of topological semimetals and aim to find materials with advantageous band structure features for high-thermoelectric performance. more

Chirality is related to special symmetry. For crystal structures, it describes the absence of mirror planes and inversion centers and, besides translations, only rotations are allowed. We investigated various  compounds with the chiral B20 (FeSi) structure. more

Topology combined with broken symmetry gives rise to Weyl semimetals, considering they exist only if the inversion symmetry or time reversal symmetry (TRS) is broken. Although Weyl semimetals with broken inversion symmetry were discovered in the recent past, direct indicators of their broken TRS were elusive. Magnetic Weyl semimetals (WSMs) are expected to exhibit fascinating physical properties, such as the intrinsic and large anomalous Hall effect (AHE). Based on multiple theoretical  calculations, single crystal growth, and transport measurements, we proposed a novel topological family comprising the Shandite compounds that exhibit a Kagomé lattice, in which a magnetic WSM Co₃Sn₂S₂ was discovered. more

The interplay between topology and magnetism has sparked interest in frontier studies of magnetic topological materials. We investigate the synthesis as well as magnetic, electrical, and thermal transport properties of various topological magnetic Heusler compounds, in addition to their electronic band structures, with angle-resolved photoemission spectroscopy experiments.
We discovered a three-dimensional topological magnet, Co₂MnGa, for the first time. more

Angle-resolved photoemission spectroscopy (ARPES) is one of the foremost experimental techniques applied in cutting-edge condensed matter physics research, enabling the electronic structure of diverse systems to be studied directly. Here, we apply ARPES to analyze the structures of topological materials, such as topological superconductors, chiral materials, and iron-based superconductors. Moreover, we reveal the electronic structures of thermoelectric materials. more

A high anomalous Hall conductivity and large anomalous angle were observed in Co₃Sn₂S₂ owing to enhancements arising from Weyl point and nodal-line band structures. The topological state of Co₃Sn₂S₂ – as a magnetic Weyl semimetal – was verified via both ARPES and STM measurements. Antiferromagnets with zero net
magnetization entail substantially fast spin dynamics; consequently, it would be interesting to obtain strong anomalous transport signals along with a topological band structure and an antiferromagnetic magnetic structure. more

Using a recently developed formalism, titled topological quantum chemistry (TQC) and magnetic TQC, we carry out a high-throughput search of topological materials in well known databases, such as Inorganic Crystal Structure Database. We identified as many as 20000 materials that display topological features and another ~100 new topological magnetic materials. Herein, we review this discovery and provide insights for future material search directions. more

Starting in 2021, the Solid State Chemistry department now includes a new research direction investigating topology of covalently bonded oxides and chalcogenides with the aim of synthesizing traditional chemical understanding of bonding with the possibilities of topology. It is likely that topology and correlations are deeply intertwined due to complex interactions of orbital parities, structural symmetries, spin texture, and electron–electron interactions. Herein we review the recent findings of the Functional Oxides group group in light of this new direction and outline the envisioned new research to come. more

Magnetic skyrmions are topologically protected nanoscopic vortices of magnetization that can be stabilized in a magnetic field. Magnetocrystalline anisotropy (K1) in uniaxial crystal structures is required to be strong enough to resist self-demagnetization in the shape of a thin lamella or film. We discovered antiskyrmions in Mn_1.4PtSn with uniaxial anisotropy and spin-reorientation transition at low temperature. Inspired by Mn_1.4PtSn, we further discovered new skyrmion bubble materials with similar features of perpendicular anisotropy and spin-reorientation in traditional hard magnetic materials, MnBi and Nd₂Fe₁₄B, which exhibit a large topological Hall effect at room temperature. However, spin-reorientation transition is unfavourable in hard magnets. To avoid this, new rare-earth-free hard magnetic materials with a strong K1 at room temperature, including Rh₂CoSb and (Fe,Co)₂(P,Si), were discovered at Felser’s department. more

Our work focuses on the growth of thin-film topological materials, lithographic structuring of devices, and characterization of the resulting properties. We aim to push forward the frontiers of the field of topology, enhancing the fundamental insights into physical phenomena such as topological magnetism, and making progress towards the utilization of such materials in devices. Several new phenomena and devices are being studied, including materials that display unique magnetic textures, like noncollinear structures and antiskyrmions. more

The application of an external pressure to drive topological quantum-phase transitions is an effective method for obtaining a better  understanding of topologically nontrivial phases by elucidating the interaction between different ground states. Achieving a superconducting state in topological materials is of particular interest as an important step toward topological superconductors. more

An exhilarating new line of research in our  department is the experimental study of light-matter interactions. In August 2020, we established a new team, led by Dr. Fabian Menges, to advance our understanding of the fundamental electromagnetic properties and functionalities of material systems with quantum phases and topological surface states. more