Puzzle of negative magnetoresistance in Blatter radical molecular junctions solved

PGI Colloquium-initiated Collaboration

The Blatter radical is highly interesting for spintronics and thus for faster and more energy-efficient information technology. A team of experimental and theoretical scientists in Physics and Chemistry from the University of Konstanz, Forschungszentrum Jülich, the University of Hamburg, Xiamen University, China and Columbia University, USA, have now solved the long-standing physics puzzle of how large positive or negative magnetoresistance in Blatter radical molecular junctions correlates with zero-bias anomalies, combining an interdisciplinary approach. The collaboration began at a Peter Grünberg Colloquium in Jülich, and has resulted in a publication in the renowned journal Chem (Cell Press).

Blatter radical molecules host unpaired electrons, resulting in a spin magnetic moment S=1/2. Upon contact with gold leads, the spin of the Blatter molecule survives and its interaction with the lead’s electrons gives rise to the well understood S=1/2 Kondo effect, whose hallmark is a zero-bias Kondo peak. The application of a magnetic field gradually destroys this peak, resulting in a positive magnetoresistance. Experiments conducted by Prof. Elke Scheer’s group at the University of Konstanz indeed confirmed this hypothesis, but in addition the experimentalists also measured a wholly distinct behaviour in a number of Blatter radical molecular junctions: namely, a negative magnetoresistance correlating with the absence of a zero-bias Kondo peak. This unexpected behaviour remained a puzzle until Prof. Scheer presented the group’s latest results at a PGI Colloquium in 2021.

After the Colloquium, Dr. Theodoulos Costi from the Peter Grünberg Institute (PGI) at Forschungszentrum Jülich was asked for his comments on possible theoretical explanations for the negative magnetoresistance measured, which at first appeared to contradict current understanding of the Kondo effect. It eventually became clear that the samples exhibiting negative magnetoresistance could be understood in terms of dimer configurations in the junctions, as two spin S=1/2 molecules would bind antiferromagnetically to yield a singlet (S=0) ground state with a triplet (S=1) excited state. This model fully accounts for the observation of both a negative magnetoresistance and the absence of a zero-bias Kondo peak, and is known as the “singlet-triplet Kondo effect”.

In order to confirm the viability of a singlet-triplet scenario, it was necessary to conduct quantum chemical calculations, carried out by Prof. Carmen Herrmann´s group at the University of Hamburg. The results obtained indeed demonstrated the possibility of stable configurations of Blatter radical dimers with singlet ground states, thereby underpinning the singlet-triplet Kondo model calculations of the magnetoresistance. According to Dr. Costi, “understanding such correlations in molecular junctions is important for future applications of molecular spintronics, particularly in quantum information and information storage”.

Original publication:
Gautam Mitra et al.;
Conventional versus singlet-triplet Kondo effect in Blatter radical molecular junctions: Zero-bias anomalies and magnetoresistance;
Chem, 2025, 11, 102500, DOI: 10.1016/j.chempr.2025.102500

News of the University of Konstanz from 26.03.2025:
"Radical" spintronics: Why the Blatter radical is highly interesting for spintronics