  Under the action of a weak constant force a wavepacket in periodic potential
undergoes periodic oscillations in space, returning to the initial position
after one oscillation cycle. This wave phenomenon, known as Bloch oscillations
(BOs), pertains to many physical systems. Can BOs also occur in topological
insulators with topologically protected edge states? This question is highly
nontrivial, because in topological insulators with broken time-reversal
symmetry, the edge states propagate unidirectionally without backscattering,
hence BOs that typically involve stages, where wavepacket moves along and
against the direction of the force, seem to be impossible in such systems when
force acts parallel to the edge of the insulator. Here we reveal that BOs still
occur with topological edge states, but in a nonconventional way: they are
accompanied not only by oscillations along the edge in the direction of force,
but also by oscillations in the direction transverse to that of the force. Full
BO cycle involves switching between edge states at the opposite edges through
delocalized bulk modes. Bloch oscillations of the topological edge states
require to scan the first Brillouin zone twice to complete one cycle, thus they
have a period that is two times larger than the period of usual BOs. All these
unusual properties are in contrast to BOs in non-topological systems.
