Abstract
We investigate two serially aligned quantum dots in the molecular regime of large tunnel couplings . A Zeeman field is used to tune the energy difference of singlet and triplet spin configurations. Attaching this geometry to BCS source and drain leads with gap and phase difference gives rise to an equilibrium supercurrent . To compute in the presence of Coulomb interactions between the dot electrons, we employ the functional renormalization group (FRG). For , where the singlet and (one out of a) triplet spin states are equal in energy, the current exhibits characteristics of a 0- transition similar to a single impurity. Its magnitude in the phase, however, jumps discontinuously at , being smaller on the triplet side. By exploiting the flexibility of the FRG, we demonstrate that this effect is generic and calculate for realistic experimental parameters , , and gate voltages . To obtain a more thorough understanding of the discontinuity, we analytically treat the limit , where one can access the exact many-particle states. Finally, carrying out perturbation theory in the dot-lead couplings substantiates the intuitive picture that Cooper-pair tunneling is favored by a singlet spin configuration while inhibited by a triplet one.
1 More- Received 28 July 2011
DOI:https://doi.org/10.1103/PhysRevB.84.134512
©2011 American Physical Society