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1. Nonsymmorphic symmetry-protected band crossings in a square-net metal PtPb4

   https://doi.org/10.1038/s41535-022-00441-x  

   Wu, Han; Hallas, Alannah M.; Cai, Xiaochan; Huang, Jianwei; Oh, Ji Seop; Loganathan, Vaideesh; Weiland, Ashley; McCandless, Gregory T.; Chan, Julia Y.; Mo, Sung-Kwan; et al (December 2022, npj Quantum Materials)

   Abstract Topological semimetals with symmetry-protected band crossings have emerged as a rich landscape to explore intriguing electronic phenomena. Nonsymmorphic symmetries in particular have been shown to play an important role in protecting the crossings along a line (rather than a point) in momentum space. Here we report experimental and theoretical evidence for Dirac nodal line crossings along the Brillouin zone boundaries in PtPb 4 , arising from the nonsymmorphic symmetry of its crystal structure. Interestingly, while the nodal lines would remain gapless in the absence of spin–orbit coupling (SOC), the SOC, in this case, plays a detrimental role to topology by lifting the band degeneracy everywhere except at a set of isolated points. Nevertheless, the nodal line is observed to have a bandwidth much smaller than that found in density functional theory (DFT). Our findings reveal PtPb 4 to be a material system with narrow crossings approximately protected by nonsymmorphic crystalline symmetries. 

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2. Weyl spin-momentum locking in a chiral topological semimetal

   https://doi.org/10.1038/s41467-024-47976-0  

   Krieger, Jonas A; Stolz, Samuel; Robredo, Iñigo; Manna, Kaustuv; McFarlane, Emily C; Date, Mihir; Pal, Banabir; Yang, Jiabao; B_Guedes, Eduardo; Dil, J Hugo; et al (December 2024, Nature Communications)

   Abstract Spin-orbit coupling in noncentrosymmetric crystals leads to spin-momentum locking – a directional relationship between an electron’s spin angular momentum and its linear momentum. Isotropic orthogonal Rashba spin-momentum locking has been studied for decades, while its counterpart, isotropic parallel Weyl spin-momentum locking has remained elusive in experiments. Theory predicts that Weyl spin-momentum locking can only be realized in structurally chiral cubic crystals in the vicinity of Kramers-Weyl or multifold fermions. Here, we use spin- and angle-resolved photoemission spectroscopy to evidence Weyl spin-momentum locking of multifold fermions in the chiral topological semimetal PtGa. We find that the electron spin of the Fermi arc surface states is orthogonal to their Fermi surface contour for momenta close to the projection of the bulk multifold fermion at the Γ point, which is consistent with Weyl spin-momentum locking of the latter. The direct measurement of the bulk spin texture of the multifold fermion at the R point also displays Weyl spin-momentum locking. The discovery of Weyl spin-momentum locking may lead to energy-efficient memory devices and Josephson diodes based on chiral topological semimetals. 

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3. Evidence for one-dimensional chiral edge states in a magnetic Weyl semimetal Co3Sn2S2

   https://doi.org/10.1038/s41467-021-24561-3  

   Howard, Sean; Jiao, Lin; Wang, Zhenyu; Morali, Noam; Batabyal, Rajib; Kumar-Nag, Pranab; Avraham, Nurit; Beidenkopf, Haim; Vir, Praveen; Liu, Enke; et al (December 2021, Nature Communications)

   Abstract The physical realization of Chern insulators is of fundamental and practical interest, as they are predicted to host the quantum anomalous Hall (QAH) effect and topologically protected chiral edge states which can carry dissipationless current. Current realizations of the QAH state often require complex heterostructures and sub-Kelvin temperatures, making the discovery of intrinsic, high temperature QAH systems of significant interest. In this work we show that time-reversal symmetry breaking Weyl semimetals, being essentially stacks of Chern insulators with inter-layer coupling, may provide a new platform for the higher temperature realization of robust chiral edge states. We present combined scanning tunneling spectroscopy and theoretical investigations of the magnetic Weyl semimetal, Co 3 Sn 2 S 2 . Using modeling and numerical simulations we find that depending on the strength of the interlayer coupling, chiral edge states can be localized on partially exposed kagome planes on the surfaces of a Weyl semimetal. Correspondingly, our d I /d V maps on the kagome Co 3 Sn terraces show topological states confined to the edges which display linear dispersion. This work provides a new paradigm for realizing chiral edge modes and provides a pathway for the realization of higher temperature QAH effect in magnetic Weyl systems in the two-dimensional limit. 

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4. Generation of helical topological exciton-polaritons

   https://doi.org/10.1126/science.abc4975  

   Liu, Wenjing; Ji, Zhurun; Wang, Yuhui; Modi, Gaurav; Hwang, Minsoo; Zheng, Biyuan; Sorger, Volker J.; Pan, Anlian; Agarwal, Ritesh (October 2020, Science)

   null (Ed.)

   Topological photonics in strongly coupled light-matter systems offer the possibility for fabricating tunable optical devices that are robust against disorder and defects. Topological polaritons, i.e., hybrid exciton-photon quasiparticles, have been proposed to demonstrate scatter-free chiral propagation, but their experimental realization to date has been at deep cryogenic temperatures and under strong magnetic fields. We demonstrate helical topological polaritons up to 200 kelvin without external magnetic field in monolayer WS 2 excitons coupled to a nontrivial photonic crystal protected by pseudo time-reversal symmetry. The helical nature of the topological polaritons, where polaritons with opposite helicities are transported to opposite directions, is verified. Topological helical polaritons provide a platform for developing robust and tunable polaritonic spintronic devices for classical and quantum information-processing applications. 

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5. Weyl nodal ring states and Landau quantization with very large magnetoresistance in square-net magnet EuGa4

   https://doi.org/10.1038/s41467-023-40767-z  

   Lei, Shiming; Allen, Kevin; Huang, Jianwei; Moya, Jaime M.; Wu, Tsz Chun; Casas, Brian; Zhang, Yichen; Oh, Ji Seop; Hashimoto, Makoto; Lu, Donghui; et al (December 2023, Nature Communications)

   Magnetic topological semimetals allow for an effective control of the topological electronic states by tuning the spin configuration. Among them, Weyl nodal line semimetals are thought to have the greatest tunability, yet they are the least studied experimentally due to the scarcity of material candidates. Here, using a combination of angle-resolved photoemission spectroscopy and quantum oscillation measurements, together with density functional theory calculations, we identify the square-net compound EuGa₄as a magnetic Weyl nodal ring semimetal, in which the line nodes form closed rings near the Fermi level. The Weyl nodal ring states show distinct Landau quantization with clear spin splitting upon application of a magnetic field. At 2 K in a field of 14 T, the transverse magnetoresistance of EuGa₄exceeds 200,000%, which is more than two orders of magnitude larger than that of other known magnetic topological semimetals. Our theoretical model suggests that the non-saturating magnetoresistance up to 40 T arises as a consequence of the nodal ring state. 

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