Removing spin blockade by photon-assisted tunneling in double quantum dots

One of the main properties of few electron quantum dots is the supression of charge current when the energy cost of introducing an extra electron in the system is large due to the charge repulsion inside the quantum dot (QD). In such a case, the system presents Coulomb blockade, showing characteristic curves where current only flows for certain values of the chemical potentials[1]. Between these peaks, current is allowed only through second order processes, which we do not treat here. However, by introducing an external AC field, the electrons obtain (from the interaction with the field) enough energy to fulfill the energy requirements and tunnel through the system[2]. In this way, a finite current may appear even in the zero bias configuration (pumping regime). These photon-assisted tunneling (PAT) processes through the contact barriers have been studied in AC driven single quantum dots[3] but are usually neglected in the theoretical study of double quantum dot (DQD) resonant pumps[4], where only interdot PAT is considered. If one takes into account the spin of the electron, another interesting effect takes part in two-site systems like, for example, a DQD having one level each. If an electron is trapped in one of the quantum dots, transport is only available through the two-electron singlet state of that QD, when an electron with the opposite spin tunnels from the other QD and afterwards to the electrode. But, if the other QD is occupied by a trapped electron with the same spin, Pauli exclusion principle does not allow the formation of the doubly occupied state and, therefore, the current is blocked. This phenomenon is known as spin blockade (SB)[5]. In this work, we study a concrete case in which spin blockade may appear in AC driven DQD spin pumps[6] and show that PAT processes through the contacts can be important, allowing the trapped spins to tunnel out of the system breaking the SB effect.