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 $\ensuremath{\gamma}$. The device in the noncolinear situation ($\ensuremath{\gamma}\ensuremath{\ne}0$) lacks inversion symmetry, allowing a finite zero-bias photocurrent $\overline{I}$. We find that as $\ensuremath{\gamma}$ changes from ${0}^{\ensuremath{\circ}}$ to ${90}^{\ensuremath{\circ}}$, the maximum amplitude of the photocurrent $|\overline{I}|$ changes nomonotonously and depends on the boundary type. The valley current ${\overline{I}}^{V}$, calculated from the Floquet scattering theory, exhibits a weak dependence on $\ensuremath{\gamma}$ and is much larger than the photocurrent $|\overline{I}|$. A crude energy-valley locking in zigzag-edged graphene nanoribbons is vital for ${\overline{I}}^{V}$. The photocurrent $\overline{I}$ can be greatly tuned by the light intensity, photon energy, and polarization direction and varies nomonotonically with the device size. The valley current ${\overline{I}}^{V}$ 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.