Absence of Even-Odd Effect in Kondo Resonance in Single-Molecule Transistors

We have observed the Kondo effect in single-electron transistors incorporating individual cyclo[$n$]pyrrole molecules. In contrast to the usual spin-1/2 Kondo effect, the zero-bias Kondo resonance in these devices appears for both even and odd number of electrons. We suggest a model in which instead of alternate spin filling, two successive electrons are added to the molecule with parallel spins due to strong exchange interaction within the molecule.