Multi-stage Kondo effect - A long standing problem unveiled

A multi-orbital quantum impurity with effective spin S>1/2 connected
to several reservoirs of conduction electrons can lead to a Kondo
effect exhibiting multi-stage screening. In a prototypical
nano-scale device consisting of an S=1 quantum impurity connected to
two conducting reservoirs, the first-stage screening process
constitutes an under-screened Kondo effect where the impurity spin is
effectively reduced from S=1 to S=1/2. Subsequently, second-stage
screening leads to complete screening of the impurity spin and the
formation of a spin singlet. Due to this two-stage screening
mechanism, transport coefficients such as the differential conductance
exhibit a non-monotonic dependence on external magnetic field, applied
source-drain voltage and temperature. Theoretically understanding the
equilibrium and non-equilibrium properties of the strong-coupling
regime resulting from this two-stage screening mechanism has been an
open problem for more than two decades, because, in contrast
to standard impurity models such as the Kondo model or the
single-impurity Anderson model, not one but two non-trivial phase shifts
are involved. In this contribution,  Deepak Karki and Mikhail Kiselev from CMSP ICTP, 

in collaboration with Christophe Mora from ENS Paris and Jan von Delft from LMU Munich, 

offer a detailed description of this strong-coupling regime in terms of an effective local two-color
Fermi liquid theory. The work has been recently selected as an Editor's Suggestion in
Physical Review B.