Graphene Characterization and Technological Applications with Julien Barrier

By Pranoti Kshirsagar /

Julien Barrier works in mesoscopic transport, a branch of physics at the intersection of quantum mechanics and classical physics. He builds graphene devices, cools them to tens of millikelvin, and measures their electrical properties, looking for phenomena that can’t be found in larger or smaller devices. He does it in Manchester, where graphene was discovered.

About the guest

Julien Barrier is an experimental physicist who trained in chemical physics at ESPCI Paris and did his master’s project at the SLAC National Accelerator Laboratory. He moved to the University of Manchester for his PhD in 2018, and at the time of recording in 2022 he held an EPSRC postdoctoral fellowship there. He was first author of the 2020 Nature Communications paper on Brown-Zak fermions in graphene superlattices.

Julien Barrier curated the Real Scientists Nano Twitter account (@RealSci_Nano) from August 8 to August 15 2022. Read all of Julien’s tweets from that week.

Key highlights in this episode

  • 02:29 Mesoscopic transport, explained. Systems with too many particles for pure quantum mechanics, but small enough that quantum effects still show up. Graphene is the ideal test bed.
  • 04:17 Huge fridge, tiny chip. Devices are cooled to tens of millikelvin in a cryostat two metres high to measure a chip about a micron wide. Measuring resistance is easy; explaining how it changes is the hard part.
  • 08:00 From Paris to Manchester. Hands-on physics at ESPCI Paris, phase transitions at SLAC, then a PhD at the birthplace of graphene, where a failed device project turned him into a condensed matter physicist.
  • 13:50 Brown-Zak fermions. Aligning graphene with hexagonal boron nitride creates a moiré pattern. In strong magnetic fields this revealed new quasiparticles, named after two 1960s theorists and published in 2020.
  • 19:18 Three wishes and the tenure debate. More time to think, longer and less precarious contracts, and less admin. Then a candid discussion on whether permanent positions help or hurt science.
Graphene Characterization and Technological Applications with Julien Barrier
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Why it matters

Julien’s field looks for phenomena that can’t be found in devices that are larger or smaller, and graphene is the best system to study them. Brown-Zak fermions are a good example: theorists described the mathematics in the 1960s, but it was completely inaccessible until graphene superlattices made it possible to see. Julien is also clear about what this kind of work needs. Most of the time you don’t find what you are looking for, and you can’t find something groundbreaking in three months. In his view, research would be better if scientists had more time to think and longer, less precarious contracts.

Terms explained

In the guest’s own words, with a link to the moment they explain it.

  • Mesoscopic transport: a branch of physics at the intersection of quantum mechanics and classical physics: systems with many particles, but small enough that quantisation effects appear. (Julien at 02:29)
  • Hexagonal boron nitride: sometimes called white graphene. It has a hexagonal structure like graphene but is 1.8% larger, and it is insulating. (Julien at 14:17)
  • Moiré pattern: an interference pattern, like the one you see when two slightly misaligned fabrics overlap. Aligning graphene with hexagonal boron nitride creates a periodic modulation of the electronic potential. (Julien at 14:41)
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