  Non-Hermitian quasicrystals possess PT and metal-insulator transitions
induced by gain and loss or nonreciprocal effects. In this work, we uncover the
nature of localization transitions in a generalized Aubry-Andre-Harper model
with dimerized hopping amplitudes and complex onsite potential. By
investigating the spectrum, adjacent gap ratios and inverse participation
ratios, we find an extended phase, a localized phase and a mobility edge phase,
which are originated from the interplay between hopping dimerizations and
non-Hermitian onsite potential. The lower and upper bounds of the mobility edge
are further characterized by a pair of topological winding numbers, which
undergo quantized jumps at the boundaries between different phases. Our
discoveries thus unveil the richness of topological and transport phenomena in
dimerized non-Hermitian quasicrystals.
