Valley caloritronics in a photodriven heterojunction of Dirac materials

Abstract

We consider a lateral hetero-junction where the left and right leads are made of monolayer graphene and the middle region is made of a gapped tilted Dirac material (borophene or quinoid graphene) illuminated with off-resonant circularly polarized radiation. The tilt parameter $v_t$ makes the band gap indirect and smaller in magnitude as compared to Dirac materials without tilt. Exposure to radiation makes the band gaps of the central region valley-dependent which show their signatures as valley polarized charge and thermal currents, thereby causing a valley Seebeck effect. We study the variation of the valley polarized electrical conductance, thermal conductance, thermopower and figure of merit of this junction with chemical potential $\mu$ and a tunable gap parameter $\eta$. For non-zero $\eta$, all the valley polarized quantities are peaked at certain values of chemical potential and then vanish asymptotically. Increase in gap parameter enhances the valley thermopower and valley figure of merit, whereas the valley conductances (electrical and thermal) show non monotonic behavior with $\eta$. We also compare the valley polarized quantities with their corresponding charge counterparts (effective contribution from both the valleys). The charge thermopower and the charge figure of merit behave non monotonically with $\eta$ and the charge conductances (electrical and thermal) depict a decreasing trend with $\eta$. Furthermore, the tilt parameter reduces the effective transmission of carriers through the junction, thereby diminishing all the charge and valley polarized quantities. As the gaps in the dispersion can be adjusted by varying the intensity of light as well as the Semenoff mass, the tunability of this junction with regard to its thermoelectric properties may be experimentally realizable.