Realizing the Kirchhoff's superposition law of light-irradiated pure spin currents in paralleled graphene nanoribbons

Light-irradiated pure spin current has been achieved via several methods. In this work, we aim to realize its superposition law by reducing the quantum interference effect between parallel circuits via first-principles. As a example, a single-layer zigzag graphene nanoribbon (ZGNR) and a two-layer ZGNR constituted two-probe structures are chosen as our model. It is found that the lowest total energy of the system constructed by bilayer ZGNR occurs at a distance of about d=4{\AA}, which indicates that the quantum interference between the two layer ZGNRs can be neglected when the interlayer distance exceeds 4{\AA}. In our anticipation, the total light-irradiated pure spin current in the two-layer ZGNR constituted two-probe structures is twice of that in the single-layer ZGNR constituted two-probe structure when the interlayer distance is set to 6{\AA}, satisfying the superposition law. While when the interlayer distance is 2{\AA}, the total light-irradiated pure spin current in the two-layer ZGNR constituted two-probe structures is not bigger but smaller than that in the single-layer ZGNR constituted two-probe structure, which may be induced by the quantum coherent cancellation. So the superposition law of photogalvanic pure spin currents can be realized in ZGNR-based devices via manipulating the layer distances. In addition, the combination type of light irradiation area won't affect the realization of the superposition law of photogalvanic pure spin currents. The results provides a reliable method for strengthening the pure spin currents in two-dimensional graphene materials using photogalvanic effect and also for the integration of optoelectronic devices.