Screening Clouds and Majorana Fermions

Ken Wilson developed the Numerical Renormalization Group technique whichgreatly enhanced our understanding of the Kondo effect and other quantum impurity problems.Wilson’s NRG also inspired Philippe Nozières to propose the idea of a large “Kondoscreening cloud”. While much theoretical evidence has accumulated for this idea it hasremained somewhat controversial and has not yet been confirmed experimentally. Recentlya new possibility for observing an analogous crossover length scale has emerged, involving aMajorana fermion localized at the interface between a topological superconductor quantumwire and a normal wire.We give an overview of this topic both with and without interactionsincluded in the normal wire.