The Kondo effect is a key phenomenon in condensed matter physics because it is a paradigm for strongly electronic systems. In magnetic alloys, it appears as the increase of the resistance of an alloy such as Cu0.998Fe0.002 at low temperatures. The basic mechanism for it is the antiferromagnetic coupling of a magnetic impurity carrying a spin with the spin of the conduction electrons of the host matrix, as suggested by Kondo in 1964. This coupling has emerged since as a very generic property of a localized electronic state coupled to a continuum. It has been observed in many different systems ranging from an adatom adsorbed on a metallic surface to quantum dots fabricated in two dimensional electron gas, carbon nanotubes or molecules. The possibility to design artificial magnetic impurities in nanoscale conductors has opened an avenue to the study of the Kondo effect in unusual situations as compared to the original one, and, in particular, in out of equilibrium situations. So far though, only the average current, which flows through such artificial impurities has been studied. Current fluctuations on the other hand are a sensitive probe of correlation effects in electronic transport. In this work, the group of mesoscopic physics in collaboration with that of theory of the LPA, at the physics department of the Ecole Normale Supérieure, has studied the current noise in artificial Kondo impurities realized in carbon nanotube quantum dots. An enhancement of the noise has been found, in contradiction with the non-interacting theory. This enhancement is quantitatively reproduced by an interacting theory based on the slave boson technique. In addition, an invariant of the noise for the Kondo effect has been determined. It should provide a particularly useful test bench for the theory of the Kondo in out of equilibrium situations, which is one of the most important challenges in the theory of condensed matter in many-body systems.
Reference : T. Delattre et al. Nature Physics 5, 208 (2009)
News and Views by R. Egger Nature Physics 5, 175 (2009).
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