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

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  • Julien Barrier
  • Piranavan Kumaravadivel
  • Roshan Krishna Kumar
  • Leonid Ponomarenko
  • Na Xin
  • Matthew Holwill
  • Ciaran Mullan
  • Minsoo Kim
  • R. V. Gorbachev
  • Michael Thompson
  • Jonathan Prance
  • T. Taniguchi
  • K. Watanabe
  • I. V. Grigorieva
  • K. S. Novoselov
  • Artem Mishchenko
  • V. I. Fal'ko
  • A. K. Geim
  • A. I. Berdyugin
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Article number5756
<mark>Journal publication date</mark>13/11/2020
<mark>Journal</mark>Nature Communications
Volume11
Number of pages7
Publication StatusPublished
<mark>Original language</mark>English

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 106 cm2 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 1 K. 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.