Tunable surface plasmon polaritons in a Weyl semimetal waveguide.

Weyl semimetals have recently attracted extensive attention due to
 their anomalous band structure manifested by topological properties that lead to
 some unusual and unique physical properties. We investigate novel features of
 surface plasmon polaritons in a slot waveguide comprised from two semi-infinite Weyl
 semimetals. We consider symmetric Voigt-Voigt and Faraday-Faraday configurations
 for plasmon polaritons in two interfaces of waveguide and show that the resulting
 dispersion is symmetric and the propagation of surface plasmon polaritons is
 bidirectional. We introduce exotic and novel asymmetric structures making use of
 difference in magnitude or orientation of chiral anomalies in two Weyl semimetals
 in both Voigt and Faraday configurations. These structures show a tremendous
 nonreciprocal dispersion and unidirectional propagation of surface plasmon polaritons.
 Moreover, we study an hybrid configuration of Voigt-Faraday for surface plasmon
 polartions in two interfaces of the waveguide. We find that this structure possesses
 unique futures. It shows surface plasmon polariton modes with unidirectional
 propagation above the bulk plasmon frequency. Furthermore, we find a surface
 plasmon polariton band which admits the Voigt and Faraday features simultaneously.
 Also, we show that the waveguide thickness and the chemical potential of the Weyl
 semimetals can be used as a fine-tuning parameters in these structures. Our findings
 may be employed in optical devices which exploit the unidirectional surface plasmon
 propagation features.