# Chiral topological semimetal with multifold band crossings and long Fermi arcs

## Abstract

Topological semimetals in crystals with a chiral structure (which possess a handedness due to a lack of mirror and inversion symmetries) are expected to display numerous exotic physical phenomena, including fermionic excitations with large topological charge1, long Fermi arc surface states2,3, unusual magnetotransport4 and lattice dynamics5, as well as a quantized response to circularly polarized light6. So far, all experimentally confirmed topological semimetals exist in crystals that contain mirror operations, meaning that these properties do not appear. Here, we show that AlPt is a structurally chiral topological semimetal that hosts new four-fold and six-fold fermions, which can be viewed as a higher spin generalization of Weyl fermions without equivalence in elementary particle physics. These multifold fermions are located at high symmetry points and have Chern numbers larger than those in Weyl semimetals, thus resulting in multiple Fermi arcs that span the full diagonal of the surface Brillouin zone. By imaging these long Fermi arcs, we experimentally determine the magnitude and sign of their Chern number, allowing us to relate their dispersion to the handedness of their host crystal.

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## Data availability

The data that support the plots within this paper and other findings of this study are available from the corresponding authors on reasonable request.

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## Acknowledgements

The authors are grateful for excellent technical support from L. Nue and A. Pfister. The authors acknowledge Diamond Light Source for access to beamline I05 (proposal nos. SI19883 and SI21400) and the Paul Scherrer Institut, Villigen, Switzerland for provision of synchrotron radiation beam time at beamline ADRESS of the SLS. N.B.M.S. acknowledges partial financial support from Microsoft. Y.L.C. acknowledges support from the Engineering and Physical Sciences Research Council (grant no. EP/M020517/1). D.P. acknowledges support from the China Scholarship Council. F.J. acknowledges funding from the European Union’s Horizon 2020 research and innovation programme under Marie-Sklodowska Curie grant agreement no. 705968. J.A.K. acknowledges the Swiss National Science Foundation (grant no. 200021_165910). Part of the work of B.B. and M.G.V. was carried out at the Aspen Center for Physics, which is supported by National Science Foundation grant PHY-1607611. M.G.V. was supported by the IS2016-75862-P national project of the Spanish MINECO. K.M. and C.F. acknowledge financial support from the ERC through grant no. 742068 ‘TOP-MAT’.

## Author information

Authors

### Contributions

N.B.M.S. conducted the SX-ARPES experiments with the support of J.A.K. and V.N.S. and the VUV-ARPES experiments with the support of D.P. and P.D. The experimental data were analysed by N.B.M.S. and D.P. M.G.V. and Y.S. performed the VASP slab and bulk ab initio calculations, and N.B.M.S. performed the Wien2k bulk calculations with the support of D.P. F.J. and B.B. provided further theoretical support. K.M., V.S. and M.S. grew the samples and K.M. performed the powder X-ray diffraction refinement. P.D., T.K.K., T.S., C.C. and V.N.S. maintained the ARPES end stations. N.B.M.S. and F.J. wrote the manuscript with input and discussion from co-authors. V.N.S., C.F. and Y.C. supervised the research.

### Corresponding authors

Correspondence to Niels B. M. Schröter or Yulin Chen.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

Journal peer review information: Nature Physics thanks David Carpentier and the other anonymous reviewer(s) for their contribution to the peer review of this work.

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## Supplementary information

### Supplementary Information

Supplementary Figures 1–6.

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Schröter, N.B.M., Pei, D., Vergniory, M.G. et al. Chiral topological semimetal with multifold band crossings and long Fermi arcs. Nat. Phys. 15, 759–765 (2019). https://doi.org/10.1038/s41567-019-0511-y

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