Split Kondo resonances in quantum dots at finite magnetic fields

Kondo resonances have recently been observed in transport measurements through quantum dots. In certain Coulomb blockade regions a peak in the differential conductance at zero source-drain voltage develops with lowering the temperature. The Anderson impurity model, used to explain these observations, requires a degeneracy. For a slightly lifted degeneracy, the Kondo resonances are expected to split. Here we present measurements in magnetic fields showing split Kondo resonances which occur in a regular pattern as a function of gate voltage and magnetic field. This pattern is related to the electronic spectrum of the quantum dot and it is found that differences in the tunnel coupling are essential to understand these experimental results. (C) 2001 Elsevier Science B.V. All rights reserved.