Researchers in Italy have studied the ferromagnetic Kondo effect and theoretically predicted how the phenomenon might be observed, describing its behaviour in detail. The team has also suggested how both the ferromagnetic and the ordinary Kondo effects might be observed in a simple circuit made up of three quantum dots. The researchers hope that their study will provide experimental physicists with clear indications on how to verify the theory in the future.

In the early 20th century, scientists surprisingly found that the electrical resistance of extremely cold samples of some metals increased rapidly with falling temperature. In 1964 Japanese physicist Jun Kondo was the first to suggest that a conduction electron in a metal such as gold can pair up with an electron of opposite spin associated with a magnetic impurity, for example iron, at low temperatures. Such an interaction curtails the electron's ability to conduct current and "screens" the magnetic moment of the impurity, making it undetectable. This low-temperature effect, which comes into play as a result of electron–electron interactions that are not present in pure bulk materials, is now known as the "Kondo effect". Although Kondo came up with his model for the effect, there were some anomalies that could not be resolved and it was physicist Kenneth Wilson who, as part of his work with complex systems and critical phenomena that led to his 1982 Nobel prize, finally came up with a numerical solution for the model in 1974.

Spinning screens

"Each electron features a moment, both of rotation and magnetic, called spin. Kondo is a phenomenon linked to the spin of metal electrons," says Erio Tosatti, at the International School for Advanced Studies (SISSA) in Trieste, Italy, who is one of the authors of the paper recently published in Physical Review Letters. "The free metal electrons surround the impurity like a cloud and arrange themselves into a spin that screens out the impurity, to a point that it is not detectable any longer, at least as long the temperature is sufficiently low," explains Tosatti. He goes on to say that the screening affects specific properties of the materials, such as an increase in resistivity and in the resistance to the flow of electrons in the metal.

Now, Tosatti, along with Michele Fabrizio and Ryan Requist also of SISSA, and Paolo Baruselli, now at Dresden University of Technology in Germany, have specifically studied a particular variant of the Kondo effect – the "ferromagnetic Kondo model" (FKM) – where the impurity and metal electrons are ferromagnetically coupled. In the case of the FKM, the metal electrons align their spins with those of the iron atoms in such a way that it does not screen them out – in fact, the researchers say that it "anti-screens" impurity electrons, preserving their magnetism instead of removing it. Tosatti likens the FKM to "the dark side of the Moon", saying that "The ferromagnetic Kondo state is theoretically expected, but the circumstances in which you might experience it have not been clarified, and the effect has not yet been observed." Compared to the traditional Kondo effect, the ferromagnetic variant has its own effects on the resistivity properties of the material.

Triple dot experiments

Having identified the conditions, in principle, that allow for the ferromagnetic Kondo effect, the team has proposed a gedanken experiment (thought experiment) that uses a circuit consisting of three laterally coupled quantum dots – "puddles" of electrons trapped in a semiconductor – where by simply adjusting a parameter, both the ferromagnetic and the ordinary Kondo effects can be observed, distinguished by their different and opposed electrical conduction anomalies and extreme sensitivity to a magnetic field. Tuning the gate voltages of the lateral dots could allow experimentalists to study the transition from the ferromagnetic to antiferromagnetic Kondo effect. The scientists claim that their proposed experiment is entirely feasible and simple to achieve.

The researchers hope that the effect may now be experimentally verified. "We expect that our experimental colleagues will now try to reproduce the same conditions we have indicated in order to carry out what could be the first observation of a phenomenon that has been theorized for a long time but never verified so far," says Tosatti.

The research is published in Physical Review Letters.