Fano effect of a strongly interacting quantum dot in contact with superconductor

The physics of a system consisting of an Aharonov Bohm (AB) interferometer containing a single level interacting quantum dot (QD) on one of its arms, and attached to normal (N) and superconducting (S) leads is studied and elucidated. Here the focus is directed mainly on N-AB-S junctions but the theory is capable of studying S-AB-S junctions as well. The interesting physics comes into play under the conditions that both the Kondo effect in the QD and the the Fano effect are equally important.It is found the conductance of the junction is suppressed as the Fano effect becomes more dominant.Comment: 4 pages, Talk to be given at the NATO Conference MQO, Bled, Slovenia 7-10 September 200

W=0 Pairing in (N,N)(N,N) Carbon Nanotubes away from Half Filling

We use the Hubbard Hamiltonian $H$ on the honeycomb lattice to represent the valence bands of carbon single-wall $(N,N)$ nanotubes. A detailed symmetry analysis shows that the model allows W=0 pairs which we define as two-body singlet eigenstates of $H$ with vanishing on-site repulsion. By means of a non-perturbative canonical transformation we calculate the effective interaction between the electrons of a W=0 pair added to the interacting ground state. We show that the dressed W=0 pair is a bound state for resonable parameter values away from half filling. Exact diagonalization results for the (1,1) nanotube confirm the expectations. For $(N,N)$ nanotubes of length $l$, the binding energy of the pair depends strongly on the filling and decreases towards a small but nonzero value as $l \to \infty$. We observe the existence of an optimal doping when the number of electrons per C atom is in the range 1.2$\div$1.3, and the binding energy is of the order of 0.1 $\div$ 1 meV.Comment: 16 pages, 6 figure