Optical signatures of multifold fermions in the chiral topological semimetal CoSi

We report the optical conductivity in high-quality crystals of the chiral topological semimetal CoSi, which hosts exotic quasiparticles known as multifold fermions. We find that the optical response is separated into several distinct regions as a function of frequency, each dominated by different ty...

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Bibliographic Details
Published in:Proceedings of the National Academy of Sciences - PNAS 2020-11, Vol.117 (44), p.27104-27110
Main Authors: Xu, Bing, Fang, Zhenyao, Sanchez-Mártınez, Miguel-Ángel, Venderbos, Jorn W. F., Ni, Zhuoliang, Qiu, Tian, Manna, Kaustuv, Wang, Kefeng, Paglione, Johnpierre, Bernhard, Christian, Felser, Claudia, Mele, Eugene J., Grushin, Adolfo G., Rappe, Andrew M., Wu, Liang
Format: Article
Language:English
Subjects:
Chemical potential
chiral topology, fermions, semimetal
CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSICS
CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY
Conductivity
Crystals
Elementary excitations
Fermions
First principles
INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
Low temperature
MATERIALS SCIENCE
Metalloids
multifold fermions
optical conductivity
Physical Sciences
Physics
science & technology
Spin-orbit interactions
Temperature dependence
topological semimetal
Topology
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Summary:We report the optical conductivity in high-quality crystals of the chiral topological semimetal CoSi, which hosts exotic quasiparticles known as multifold fermions. We find that the optical response is separated into several distinct regions as a function of frequency, each dominated by different types of quasiparticles. The low-frequency intraband response is captured by a narrow Drude peak from a high-mobility electron pocket of double Weyl quasiparticles, and the temperature dependence of the spectral weight is consistent with its Fermi velocity. By subtracting the low-frequency sharp Drude and phonon peaks at low temperatures, we reveal two intermediate quasilinear interband contributions separated by a kink at 0.2 eV. Using Wannier tightbinding models based on first-principle calculations, we link the optical conductivity above and below 0.2 eV to interband transitions near the double Weyl fermion and a threefold fermion, respectively. We analyze and determine the chemical potential relative to the energy of the threefold fermion, revealing the importance of transitions between a linearly dispersing band and a flat band. More strikingly, below 0.1 eV our data are best explained if spin-orbit coupling is included, suggesting that at these energies, the optical response is governed by transitions between a previously unobserved fourfold spin-3/2 node and a Weyl node. Our comprehensive combined experimental and theoretical study provides a way to resolve different types of multifold fermions in CoSi at different energy. More broadly, our results provide the necessary basis to interpret the burgeoning set of optical and transport experiments in chiral topological semimetals.
ISSN:0027-8424
1091-6490
DOI:10.1073/pnas.2010752117