Suppression of ballistic helical transport by isotropic dynamical magnetic impurities

Dynamical magnetic impurities (MI) are considered as a possible origin for suppression of the ballistic helical transport on edges of two-dimensional (2D) topological insulators. The MIs provide a spin-flip backscattering of itinerant helical electrons. Such a backscattering reduces the ballistic conductance if the exchange interaction between the MI and the electrons is anisotropic and the Kondo screening is unimportant. It is well known that the isotropic MIs do not suppress the helical transport in systems with axial spin symmetry of the electrons. We show that, if this symmetry is broken, the isotropic MI acquires an effective anisotropy and suppresses the helical conductance. The peculiar underlying mechanism is a successive backscattering of the electrons which propagate in the same direction and have different energies. The respective correction to the linear conductance is determined by the allowed phase space of the electrons and scales with temperature as T-4. Hence, it disappears at small temperatures. This qualitatively distinguishes effects governed by the MIs with the induced and bare anisotropy; the latter is temperature independent. If T is smaller than the applied bias, finite eV, the allowed phase space is provided by the bias and the differential conductance scales as (eV)(4). We point out regimes where the combined effect of the MI and the broken spin symmetry dominates over that governed by electron interactions.

Automated summary

Dynamical magnetic impurities can suppress the helical transport in two-dimensional topological insulators when the spin symmetry is broken. This suppression is caused by successive backscattering of electrons with different energies in the same direction, leading to a decrease in conductance scaling as T-4 with temperature. The combined effect of magnetic impurities and broken spin symmetry can dominate over electron interactions in certain regimes.