A quantitative theory of backscattering in quantum spin Hall materials

It is demonstrated quantitatively and with no adjustable parameters that the presence of paramagnetic acceptor dopants elucidates a variety of hitherto puzzling properties of two-dimensional topological semiconductors, such as HgTe quantum wells, at the topological phase transition and in the regime of the quantum spin Hall effect. The concepts of charge correlation, Coulomb gap, anisotropic exchange interaction between edge electrons and holes localized on acceptors, strong coupling limit of the Kondo effect, and the influence of the Luttinger effects on the s-p exchange anisotropy explain: (i) high hole mobilities compared with electron mobilities, (ii) a short topological protection length, and (iii) the temperature dependence of conductance in the edge transport regime in HgTe quantum wells [1,2]. The extension of the theory to (Hg,Mn)Te quantum wells brings two new ingredients: (i) precessional spin dephasing leading to Mn-induced appreciable backscattering between the helical states despite the absence, in the case of sp-d exchange interaction between edge electrons and Mn spins, of the Kondo effect and exchange anisotropy; (ii) the formation of acceptor bound magnetic polarons that destroy the Kondo effect associated with acceptor holes. These phenomena explain the opposite temperature dependence of edge conductance in HgTe and (Hg,Mn)Te quantum wells.

[1] T. Dietl, arXiv:2206.01613. [2] T. Dietl, arXiv:2209.03283.