  We investigate electronic transport through parallel double quantum dot(DQD)
system with strong on-site Coulomb interaction and capacitive interdot
coupling. By applying numerical renormalization group(NRG) method, the ground
state of the system and the transmission probability at zero temperature have
been obtained. For a system of quantum dots with degenerate energy levels and
small interdot tunnel coupling, the spin correlations between the DQDs is
ferromagnetic, and the ground state of the system is a spin 1 triplet state.
The linear conductance will reach the unitary limit ($2e^2/h$) due to the
underscreened Kondo effect at low temperature. As the interdot tunnel coupling
increases, there is a quantum phase transition from ferromagnetic to
anti-ferromagnetic spin correlation in DQDs and the linear conductance is
strongly suppressed.
