Interplay between Magnetoresistance and Kondo Resonance in Radical Single-Molecule Junctions

We report transport measurements on tunable single-molecule
junctions
of the organic perchlorotrityl radical molecule, contacted with gold
electrodes at low temperature. The current–voltage characteristics
of a subset of junctions shows zero-bias anomalies due to the Kondo
effect and in addition elevated magnetoresistance (MR). Junctions
without Kondo resonance reveal a much stronger MR. Furthermore, we
show that the amplitude of the MR can be tuned by mechanically stretching
the junction. On the basis of these findings, we attribute the high
MR to an interference effect involving spin-dependent scattering at
the metal–molecule interface and assign the Kondo effect to
the unpaired spin located in the center of the molecule in asymmetric
junctions.