Magnetic field dependence of the spin- 1/2 and spin-1 Kondo effects in a quantum dot

We study the magnetic field evolution of the spin-1/2 and spin-1 Kondo effects in a quantum dot formed from a single-walled carbon nanotube. In the spin-1/2 case, the energy of spin-resolved Kondo conductance peaks is proportional to magnetic field at high fields, contrary to recent reports. At lower fields, the energy falls below this linear dependence, in qualitative agreement with theoretical expectations. For even electron occupancy, we observe a zero-bias Kondo peak due to the degeneracy of the spin-1 triplet ground states. Tuning gate voltage within the same Coulomb diamond drives a transition to a singlet ground state. We also independently tune the energy difference between singlet and triplet states with a magnetic field. The Zeeman splitting thus measured confirms the value of the g factor deduced from the high-field behavior of spin-1/2 Kondo.