Two-dimensional topological Anderson insulator in a HgTe-based semimetal

We report the experimental observation of Anderson localization in two-dimensional (2D) electrons and holes in the bulk of HgTe quantum wells with a semimetallic spectrum and under strong disorder. Surprisingly, the one-dimensional (1D) edge channels, arising from the spectrum's inversion, demonstrate remarkable robustness against disorder due to topological protection. Strong disorder induces a mobility gap in the bulk, enabling access to the 1D edge states and thereby realizing the two-dimensional topological Anderson insulator (TAI) state. Nonlocal transport measurements confirm the emergence of topologically protected edge channels. The TAI state appears to be very sensitive to an external magnetic field applied perpendicular to the sample. The weak magnetic field of 30 mT breaks the topological protection of 1D edge channels, thus turning the system into an ordinary Anderson insulator. At a field of 0.5 T, the 2D bulk electrons become delocalized, and the system transforms into a quantum Hall liquid.