Localization of helical edge states in the absence of external magnetic field

Theoretically, the helical edge states of two-dimensional topological insulators are protected from coherent backscattering due to nonmagnetic disorder provided electron interactions are not too strong. Experimentally, the edges typically do not demonstrate systematic and robust quantization, but at the same time little is known about the sub-Kelvin temperature behavior. Here, we report the surprising localization of the edge states in an 8-nm HgTe quantum well in zero magnetic field at millikelvin temperatures. Additionally, the magnetoresistance data at 0.5 K for edges a few micrometers long suggest the field-dependent localization length l(B) proportional to B-a, with alpha ranging approximately from 1.6 to 2.8 at fields B less than or similar to 0.1 T and alpha approximate to 1.1 at higher fields up to 0.5 T. In the frame of the disordered interacting edge, these values of alpha correspond to the Luttinger liquid parameters K approximate to 0.9-1.1 and K approximate to 0.6, respectively. We discuss possible scenarios which could result in zero magnetic field localization.