  The density-dependent mobility of n-type HgTe quantum wells with inverted
band ordering has been studied both experimentally and theoretically. While
semiconductor heterostructures with a parabolic dispersion exhibit an increase
in mobility with carrier density, high quality HgTe quantum wells exhibit a
distinct mobility maximum. We show that this mobility anomaly is due to
backscattering of Dirac fermions from random fluctuations of the band gap
(Dirac mass). Our findings open new avenues for the study of Dirac fermion
transport with finite and random mass, which so far has been hard to access.
