  In quantizing magnetic fields, graphene superlattices exhibit a complex
fractal spectrum often referred to as the Hofstadter butterfly. It can be
viewed as a collection of Landau levels that arise from quantization of
Brown-Zak minibands recurring at rational ($p/q$) fractions of the magnetic
flux quantum per superlattice unit cell. Here we show that, in
graphene-on-boron-nitride superlattices, Brown-Zak fermions can exhibit
mobilities above 10$^6$ cm$^2$V$^{-1}$s$^{-1}$ and the mean free path exceeding
several micrometers. The exceptional quality of our devices allows us to show
that Brown-Zak minibands are $4q$ times degenerate and all the degeneracies
(spin, valley and mini-valley) can be lifted by exchange interactions below 1K.
We also found negative bend resistance at $1/q$ fractions for electrical probes
placed as far as several micrometers apart. The latter observation highlights
the fact that Brown-Zak fermions are Bloch quasiparticles propagating in high
fields along straight trajectories, just like electrons in zero field.
