Long-range ballistic transport of Brown-Zak fermions in graphene superlattices

TL;DR

This paper demonstrates that Brown-Zak fermions in graphene superlattices can exhibit high mobility and long mean free paths, behaving as Bloch quasiparticles capable of long-range ballistic transport in high magnetic fields.

Contribution

It reveals that Brown-Zak minibands are highly degenerate and can be manipulated by exchange interactions, providing new insights into quasiparticle behavior in graphene superlattices.

Findings

Brown-Zak fermions show mobilities above 10^6 cm^2V^{-1}s^{-1}

Mean free path exceeds several micrometers

Negative bend resistance observed at fractional flux quanta

Abstract

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.