Quantum coherence and Kondo effect

How does a metal transport electrical current ? We have shown that a full understanding of electrical transport can only be achieved with the most modern tools of quantum mechanics and large computers. A metal can be viewed as a lattice of charged ions, through which the electrons flow freely. When an electric field is applied, by applying for example a voltage difference at the ends of a conductor, the electrons are accelerated: this is the electric current. One key question has been omitted: what limits the magnitude of the current ? The acceleration of the electron does not lead to an infinite increase of its speed and hence of the current. The basic ingredient is provided by collisions of the electrons with impurities or between themselves. First, electrons diffuse on atomic defects : this is crystallographic disorder. Second, collisions between electrons also slow down the current flow. At room temperature, the collisions between electrons are dominant, while impurities and lattice defects dominate at low temperature. Taking into account all these phenomena in transport equations, it is possible to account precisely for the resistance of metals.

But new experimental phenomena have been discovered : when a minute fraction of the lattice ions are replaced by magnetic ions (for example iron), the electrical resistance increases at low temperature, something which cannot be explained by standard diffusion processes ! Jun Kondo discovered the origin of this phenomenon which is known by his name: electrons carry not only an electric charge but also a magnetic moment. The interaction of this moment with magnetic impurities generates collisions of a novel type, which are more and more efficient as the temperature is lowered ! In our work, we investigated the influence of magnetic impurities and their coupling to conduction electrons on the quantum behaviour of the electrons and their phase coherence. This problem touches on the actual concept of the electron : since the coupling between the electrons and a magnetic impurity becomes so strong, does “a novel particle” replace the actual electron ?

In fact below a temperature TK (the Kondo temperature), the quantum state of electrons is strongly affected by collisions with magnetic impurities. This is what we have observed in a study of the quantum coherence of electrons as a function of temperature and applied magnetic field. In fact, as the temperature is lowered, electrons form a cloud around magnetic impurities which exactly screens their magnetic moment. In this “collective effect”, the magnetic moment of a large number of electrons compensates exactly the impurity’s magnetic moment. One can say that the impurities are “screened” and everything behaves as if ... the impurities have disappeared ! Using powerful computing algorithms we have been able to compare our measurements with exact numerical results and have shown that this scenario, based on the Kondo effect, is correct.
screening by the Kondo efffect
Top : electrons are diffused incoherently above the Kondo temperature, leading to an increase of the resistance. Bottom : below TK a cloud of electron spins screens the impurity, which becomes “transparent” to electronic transport.

The understanding of electrical transport in metals has thus reached a new level: the most modern theoretical models appear to be essential for a complete comprehension of phenomena which are as basic as the electronic transport governing the resistance of metals.

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