Edge resistance in 2D topological insulators introduced by charge puddles

Time-reversal symmetry prohibits elastic backscattering of electrons propagating within a helical edge of a 2D topological insulator. However, the small band gaps in these systems make them sensitive to doping disorder, which may lead to the formation of electron and hole puddles. Such a puddle can be thought of as a quantum dot tunnel-coupled to the edge, and may significantly enhance the inelastic backscattering rate, due to the long dwelling time of an electron in the dot. The added resistance is especially strong for dots carrying an odd number of electrons, due to the Kondo effect. For the same reason, the temperature dependence of the added resistance becomes rather weak. We have developed a detailed theory of the quantum dot effect on the helical edge resistance, which provides a qualitative explanation of the resistance fluctuations observed in short HgTe quantum wells. In addition to the single-dot theory, we have formulated a statistical description of the helical edge resistivity introduced by random charge puddles in a long heterostructure carrying helical edge states. The presence of charge puddles in long samples may explain the observed coexistence of a high sample resistance with the propagation of electrons along the sample edges.