Kondo Resonance of a Carbon-Centered Radical in a Single-Molecule Junction

The low spin-orbit coupling and weak hyperfine interactions make organic radicals promising components used in molecular spintronics. Triphenylmethyl (TPM) is the first stable carbon-centered organic radical discovered more than a century ago. Here, we use scanning tunneling spectroscopy to study quantum transport through single TPM radicals that are attached to a Au(111) electrode via atomic contacts of Ni atoms. The transport exhibits a Kondo resonance, evidencing that the unpaired electron of the radical forms a spin singlet with the itinerant electrons of the electrode. Density functional theory calculations reveal an indirect Kondo screening mechanism: the itinerant electrons couple with the radical π electron via the d orbitals of the Ni atoms. These results envision a new way to regulate the spin transport of organic radicals using atomic contacts in solid-state spintronic devices.