shot noise in mesoscopic conductors - Low Temperature Laboratory

approximately e/C. Between the peaks (in the valleys), as we implicitly assumed, the zerotemperature
current is due to quantum tunneling, and is exponentially small. We concluded
therefore, that the Fano factor is 1 } the shot noise is Poissonian. In reality, however, there are
cotunneling processes } virtual transitions via the high-lying state in the dot } which are not
exponentially suppressed and thus give the main contribution to the current. Cotunneling is
a genuinely quantum phenomenon, and cannot be obtained by means of a classical approach. It
is clear that the cotunneling processes may modify the Fano factor in the valleys; moreover, it is
a good opportunity to study quantum e!ects in the shot noise. This problem, among many others,
remains unaddressed.
7.2. Anderson and Kondo impurities
Ya.M. Blanter, M. Bu( ttiker / Physics Reports 336 (2000) 1}166 141
7.2.1. Anderson impurity model
In the context of mesoscopic physics, the Anderson impurity model describes a resonant level
with a Hubbard repulsion. It is sometimes taken as a model of a quantum dot. Commonly the
entire system is described by the tight-binding model with non-interacting reservoirs and an
interaction ;n( s n( t on the site i"0 (resonant impurity), with n( s and n( t being the operators of the
number of electrons on this site with spins down and up, respectively. Tunneling into the dot is
described, as in the non-interacting case, by partial tunneling widths and , which may be
assumed to be energy independent. The on-site repulsion is important (in the linear regime) when
;