Quantum dot in a two-dimensional topological insulator: The two-channel Kondo fixed point

Abstract

A quantum dot coupled to two helical edge states of a two-dimensional topological insulator through electron tunnelings is studied. We show that if the electron interactions on the edge states are repulsive, with Luttinger liquid parameter K<1, the system reaches a stable two-channel Kondo fixed point at low temperatures. This is in contrast to the Luttinger liquid leads case in which K<1/2 is needed. This two-channel fixed point is described by a boundary sine-Gordon Hamiltonian with a K dependent boundary term. The impurity entropy, the impurity specific heat and the conductance are calculated.