Abnormal photon-assisted current in a two-level quantum dot system with the time-varying external field

We have studied the photon-assisted current properties in a two-level quantum dot (QD) system, where the QD and two normal metal leads are simultaneously irradiated by the time-varying external field. In contrast to the monotonous variety with the strength of the external field irradiated only on two leads or only on the QD, here the photon-assisted current presents some abnormal properties. The results show that, when the external field on the two leads is symmetrical and the strength is not changed, the current sideband peaks of the ground state become gradually weaker and weaker, whereas the main resonance becomes much stronger as WCWC is increased, where WCWC is the external field intensity irradiated on the central QD. The current properties of photon-induced excited state get more complicated. Its right sideband peak disappears in the presence of WCWC. While the external field on the two leads is completely asymmetrical, the accession of WCWC could control the photon–electron pumping effect, which is not only a non-monotonic variation but also changes current direction. Moreover, WCWC also has an inhibitory influence on the multi-photon process.