Noisy Kondo impurities

Abstract

Sensitive measurements of fluctuations in the current through carbon-nanotube-based quantum dots provide insight into the many-body physics of such systems. In the original discussion of the Kondo effect, the increase of the resistance in an alloy such as Cu0.998Fe0.002 at low temperature was explained by the antiferromagnetic coupling between a magnetic impurity and the spin of the host’s conduction electrons1. This coupling has since emerged as a very generic property of localized electronic states coupled to a continuum2,3,4,5,6,7. Recently, the possibility to design artificial magnetic impurities in nanoscale conductors has opened avenues to the study of this many-body phenomenon in a controlled way and, in particular, in out-of-equilibrium situations8,9,10. So far though, measurements have focused on the average current. Current fluctuations (noise) on the other hand are a sensitive probe that contains detailed information about electronic transport. Here, we report on noise measurements in artificial Kondo impurities realized in carbon-nanotube devices. We find a striking enhancement of the current noise within the Kondo resonance, in contradiction with simple non-interacting theories. Our findings provide a sensitive test bench for one of the most important many-body theories of condensed matter in out-of-equilibrium situations and shed light on the noise properties of highly conductive molecular devices.