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. 2021 Jan 8;12:154. doi: 10.1038/s41467-020-20408-5

Fig. 1. Schematic diagrams of the crystal and band structure of topological chiral semimetals, and the second harmonic generation on CoSi.

Fig. 1

a A chiral Weyl semimetal has two Weyl nodes with opposite monopole charges  ±1 at different energies. b Without spin-orbit coupling CoSi features a threefold node and a double Weyl node located at different energies and momenta. The three bands in the threefold fermions have Chern numbers +2, 0 and −2, respectively. When spin degeneracy is accounted for, the total charges at the two modes are  ±4. When circularly polarized light is incident on the sample, excitations around the right Weyl fermion (a) or the double Weyl fermion (b) are Pauli blocked, but excitations around the left Weyl fermion (a) or the threefold fermions (b) are allowed, generating a CPGE. c Schematic top view of CoSi (111) surface. The different transparency of Si and Co atoms indicates the different depth of each atom from the top plane. The green anticlockwise/clockwise circles indicate the chirality of the Co/Si atoms. d Second harmonic generation signal generated from CoSi (111) natural facet under normal incident laser pulses with a photon energy of 1.55 eV. Dots are experimental data from parallel and crossed configurations between incident and detecting linear polarization. Solid lines are the best fit.