  We address the increase of electron drift velocity that arises in
semiconductor superlattices (SLs) subjected to constant electric and magnetic
fields. It occurs if the magnetic field possesses nonzero components both along
and perpendicular to the SL axis and the Bloch oscillations along the SL axis
become resonant with cyclotron rotation in the transverse plane. It is a
phenomenon of considerable interest, so that it is important to understand the
underlying mechanism. In an earlier Letter (Phys. Rev. Lett. 114, 166802
(2015)) we showed that, contrary to a general belief that drift enhancement
occurs through chaotic diffusion along a stochastic web (SW) within
semiclassical collisionless dynamics, the phenomenon actually arises through a
non-chaotic mechanism. In fact, any chaos that occurs tends to reduce the
drift. We now provide fuller details, elucidating the mechanism in physical
terms, and extending the investigation. In particular, we: (i) demonstrate that
pronounced drift enhancement can still occur even in the complete absence of an
SW; (ii) show that, where an SW does exist and its characteristic slow dynamics
comes into play, it suppresses the drift enhancement even before strong chaos
is manifested; (iii) generalize our theory for non-small temperature, showing
that heating does not affect the enhancement mechanism and accounting for some
earlier numerical observations; (iv) demonstrate that certain analytic results
reported previously are incorrect; (v) provide an extended critical review of
the subject and closely related issues; and (vi) discuss some challenging
problems for the future.
