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Frequency-resolved multifold fermions in the chiral topological semimetal CoSi | | B. Xu
; Z. Fang
; M. A. Sánchez-Martínez
; J. W. F. Venderbos
; Z. Ni
; T. Qiu
; K. Manna
; K. Wang
; J. Paglione
; C. Bernhard
; C. Felser
; E. J. Mele
; A. G. Grushin
; A. M. Rappe
; Liang Wu
; | | Date: |
4 May 2020 | | Abstract: | We report the optical conductivity in the linear-response regime of the
chiral topological semimetal CoSi, predicted to host elusive topological
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 response is
captured by a narrow Drude peak that broadens strongly with increasing
temperature from 10 K to 300 K and is dominated by a high-mobility electron
pocket near a double Weyl fermion at the $R$ point. At high frequencies, we
observe a sharp peak at 0.56 eV. Using tight-binding calculations, we link this
peak to inter-band transitions around the $M$ point that are dominant due to
the presence of a saddle point in the band structure. By subtracting the
low-frequency sharp Drude and phonon peaks at low temperatures, we reveal two
intermediate quasi-linear inter-band contributions separated by a kink at 0.2
eV. Using tight-binding models, we link the optical conductivity above and
below 0.2 eV to inter-band transitions near the double Weyl fermion at the $R$
point and a three-fold fermion at $Gamma$, respectively. To do so, we find the
chemical potential to be slightly below the latter node, activating transitions
between a linearly dispersing band and a flat band, for frequencies below 0.2
eV. 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 spin-3/2 node and a
Weyl node. Our results highlight that different types of multifold fermions in
CoSi are activated at different frequencies, providing a way to resolve them in
energy. Our results provide the necessary basis to interpret the burgeoning set
of optical and transport experiments in chiral multifold semimetals. | | Source: | arXiv, 2005.1581 | | Services: | Forum | Review | PDF | Favorites | |
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