Zero-bias photocurrent and valley current in a circular graphene quantum dot with noncollinear leads

We investigate theoretically the charge and valley current in a circular graphene quantum dot (GQD) illuminated by a linearly polarized light. The two leads connected to the GQD have an angle γ. The device in the noncolinear situation (γ≠0) lacks inversion symmetry, allowing a finite zero-bias photocurrent Ī. We find that as γ changes from 0^∘ to 90^∘, the maximum amplitude of the photocurrent |Ī| changes nomonotonously and depends on the boundary type. The valley current ĪV, calculated from the Floquet scattering theory, exhibits a weak dependence on γ and is much larger than the photocurrent |Ī|. A crude energy-valley locking in zigzag-edged graphene nanoribbons is vital for ĪV. The photocurrent Ī can be greatly tuned by the light intensity, photon energy, and polarization direction and varies nomonotonically with the device size. The valley current ĪV shows a minor change with the light parameters and a remarkable increase with the device size. These findings could be helpful for the generation and optimization of photocurrents and valley currents in graphene-based devices.