Singlet–triplet transition in lateral quantum dots: A numerical renormalization group study

We discuss transport through a lateral quantum dot in the vicinity of a singlet-triplet spin transition in its ground state. Extracting the scattering phase shifts from the numerical renormalization group spectra, we determine the linear conductance at zero temperature as a function of a Zeeman field and the splitting of the singlet and triplet states. We find reduced low-energy transport, and a nonmonotonic magnetic-field dependence both in the singlet and the triplet regime. For a generic set of dot parameters and no Zeeman splitting, the singlet–triplet transition may be identified with the conductance maximum. The conductance is least sensitive to the magnetic field in the region of the transition, where it decreases upon application of a magnetic field. Our results are in good agreement with recent experimental data.