Julien Barrier

Writing software is a big part of experimental physics. Even a moderately complex experiment involves several instruments that need to talk to each other and stay synchronised, so sooner or later we all have to decide how to control them all.

For a long time, LabVIEW was the answer. It is easy to pick up, but is awkward to version-control, and clumsy when you just want to write a quick scripts. Fewer and fewer students learn as part of the community has moved to Python. Written in Python, QTLab was popular for a while, with a simple interface, but it is no longer supported. QCoDeS and LabRAD have since become common in mesoscopic physics and quantum transport. They are really powerful, but they are not easy to learn for a new graduate student.

When I was a PhD student in Manchester, I looked at several of these options. With my friend Servet Ozdemir we tried a few different things to try to replace the aging LabView interface. Eventually I settled on QCoDeS, and started writing my own functions, drivers for the instruments we needed, station files and scripts.

I carried that set of scripts, commands and station files with me from Manchester to ICFO during my postdoc, and it showed. I ended up with scripts nobody else could run, overnight queues that were lost when something crashed, and Jupyter notebooks so large they could no longer be opened on a modest laptop. It was also difficult to train students this way. Everyone has their own idea of what good code looks like, and most people do not read the QCoDeS documentation. Some students end up writing fixes for problems that existing functions already solve, which complicates the analysis, because datasets measured by different people have their own nomenclature and conventions.

In the end, nothing was as easy as the plain old LabVIEW interface. It got me started quickly, and let me learn the physics before learning how to write advanced functions. Now that I am starting my own group and have PhD students to train, I wanted a tool that makes the learning curve gentle without giving up the functionality that makes QCoDeS useful.

For this reason, I have released mesoscopy, an open-source graphical interface for running experiments in quantum transport and mesoscopic physics. It is built on top of QCoDeS, so it works with the instrument drivers and databases many transport labs already use.

• Code: github.com/mpiLDE/mesoscopy
• Documentation: mpilde.github.io/mesoscopy

What it does

• Sweeps. Linear, logarithmic and “together” sweeps (several parameters moving in lockstep), with live plotting while the measurement runs. Takes advantage of the dond function of QCoDeS and its modularity.
• Safety first. Configurable limits and maximum ramp rates on every parameter, so a typo cannot blow up a device.
• A queue that survives crashes. Queue up measurements and leave them running overnight. If something goes wrong, the queue can be resumed.
• Monitoring. Alarms and persistent logs for the quantities you care about (temperature, leakage current, and so on).
• Reproducibility Set experiment parameters based on those recorded for a previous run.
• Standard data format. Everything is written to regular QCoDeS databases, so your existing analysis code keeps working.

Getting started

I wrote it and tested it using Python 3.14+ and QCoDeS 0.60.0+. Installation instructions and a walkthrough are in the documentation. The test suite runs on simulated instruments, so you can try the software without any hardware attached.

Open source

Mesoscopy is released under the MIT license. Bug reports, feature requests and pull requests are welcome on GitHub. If you use it in your lab, I would be glad to hear about it.

This is a press release for my recent paper on Coulomb screening of superconductivity in magic-angle twisted bilayer graphene. Original on the University of Manchester website.

New research shows superconductivity in magic-angle graphene can be switched off by weakening electron interactions, helping clarify what drives the phenomenon.

Scientists from the National Graphene Institute at The University of Manchester have demonstrated that superconductivity in magic-angle graphene can be completely switched off by screening interactions between electrons. The finding provides strong evidence that electron interactions play a central role in the phenomenon and helps address a key question that has remained unresolved since superconductivity was first discovered in the material.

In the new study, published in Physical Review X, researchers developed a graphene device that allowed them to test this question directly. The device consisted of two twisted graphene bilayers separated by less than a nanometre but kept electronically separate. This design enabled the team to weaken interactions between electrons in the magic-angle graphene layer and observe how superconductivity responded. The international collaboration involved researchers from the National Graphene Institute, the Henry Royce Institute, Washington University in St Louis, the University of Pennsylvania, the University of Antwerp, Japan’s National Institute for Materials Science and the National University of Singapore.

Magic-angle twisted bilayer graphene, created by stacking two sheets of graphene with a rotational offset of approximately 1.1 degrees, has become one of the most intensely studied quantum materials over the past decade. However, researchers have continued to debate what causes its superconductivity. While some theories propose that electrons themselves drive the pairing responsible for superconductivity, others suggest a more conventional mechanism involving vibrations of the atomic lattice.

Dr Julien Barrier, the lead author of the study, explained: “To make a difference, we had to solve two issues. First, to build a device in which the screening layer sits extremely close, a fraction of a nanometre, to the superconducting graphene while remaining electronically separate. Second, we had to make that screening layer tuneable. To this effect, we used a twisted graphene bilayer in atomic contact to the magic-angle graphene”.

Professor Alexey Berdyugin from the National University of Singapore, the corresponding author of this study, added: “When we switched on the screening, we were surprised to find that superconductivity was completely suppressed. This provides clear experimental evidence that superconductivity in this system originates from strong electron-electron interactions. This behaviour offers a new opportunity to better understand the mechanisms underlying superconductivity in other materials with strong electronic interactions, including high-temperature superconductors.”

Professor Sir Andre Geim, the corresponding author of this work, said: “Personally, I am interested only in high-temperature superconductivity – preferably at room temperature or above. This study was done at temperatures so low that even helium turns liquid. But unless we understand what makes superconductivity work, we are unlikely ever to reach room-temperature superconductivity, let alone make this remarkable phenomenon commercially useful. Our study takes only a tiny step - but still a step - in that direction, helping to nail down the mechanism of exotic superconductivity in graphene. Rome was not built in a day.”

The team found that increasing the carrier density in the neighbouring graphene bilayer progressively weakened superconductivity in the adjacent magic-angle graphene. At sufficiently high carrier densities, superconductivity was completely suppressed.

The researchers also observed that correlated insulating states, another characteristic feature of magic-angle graphene, disappeared under the same conditions. Measurements showed that the superconducting critical temperature could be reduced by more than an order of magnitude through screening.

The effect was substantially stronger than reported in earlier screening experiments. According to the researchers, the enhanced response resulted from the exceptionally small separation between the superconducting layer and the screening layer, allowing Coulomb interactions to be modified much more effectively.

To understand the observations, the team compared the experimental results with theoretical modelling. Conventional phonon-mediated superconductivity would be expected to remain largely unchanged or increase slightly when Coulomb interactions are screened. Instead, the researchers observed the opposite behaviour, indicating that conventional phonon pairing does not explain the superconductivity in this class of materials.

While the authors emphasise that the work does not identify a single definitive pairing mechanism, several unconventional theories remain consistent with the results, including mechanisms based on collective electronic interactions. However, the findings place much tighter constraints on future theories seeking to explain superconductivity in magic-angle graphene.

Professor Berdyugin concludes: “In this study, we introduced a method for screening electron-electron interactions over scales as short as 0.3 nm, which turned out to be crucial for controlling superconductivity in magic-angle graphene. We anticipate that this unprecedented level of short-range screening could also help clarify many other debated phenomena.”

A bit late announcing this formally. I will be joining the Max Planck Institute for Solid State Research next Spring as an Independent Research Group Leader.

Our research group will focus on exploring low-dimensional quantum materials using a combination of electronic transport and nano-photonics techniques. I am looking forward to working with a talented team of scientists on this journey.

PhD and postdoctoral positions will be available starting next summer − feel free to reach out for more details! Prospective PhD candidates should apply through the Max Planck Graduate Center for Quantum Materials (deadline for this call is 10 December 2025). Prospective postdocs can contact me by e-mail with a CV. I am also able to support potential applications to Humboldt or MSCA-PF fellowships.

This is a press release for my recent paper of 1D proximity superconductivity along graphene domain walls

Manchester Scientists Found Novel One-Dimensional Superconductor In a significant development in the field of superconductivity, researchers at The University of Manchester have successfully achieved robust superconductivity in high magnetic fields using a newly created one-dimensional (1D) system. This breakthrough offers a promising pathway to achieving superconductivity in the quantum Hall regime, a longstanding challenge in condensed matter physics. Superconductivity, the ability of certain materials to conduct electricity with zero resistance, holds profound potential for advancements of quantum technologies. However, achieving superconductivity in the quantum Hall regime, characterised by quantised electrical conductance, has proven to be a mighty challenge.

The research, published this week (25 April 2024) in Nature, details extensive work of the Manchester team led by Professor Andre Geim, Dr Julien Barrier and Dr Na Xin to achieve superconductivity in the quantum Hall regime. Their initial efforts followed the conventional route where counterpropagating edge states were brought into close proximity of each other. However, this approach proved to be limited.

“Our initial experiments were primarily motivated by the strong persistent interest in proximity superconductivity induced along quantum Hall edge states,” explains Dr Barrier, the paper’s lead author. “This possibility has led to numerous theoretical predictions regarding the emergence of new particles known as non-abelian anyons.”

The team then explored a new strategy inspired by their earlier work demonstrating that boundaries between domains in graphene could be highly conductive. By placing such domain walls between two superconductors, they achieved the desired ultimate proximity between counterpropagating edge states while minimising effects of disorder.

“We were encouraged to observe large supercurrents at relatively ‘balmy’ temperatures up to one Kelvin in every device we fabricated,” Dr Barrier recalls.

Further investigation revealed that the proximity superconductivity originated not from the quantum Hall edge states propagating along domain walls, but rather from strictly 1D electronic states existing within the domain walls themselves. These 1D states, proven to exist by the theory group of Professor Vladimir Fal’ko’s at the National Graphene Institute, exhibited a greater ability to hybridise with superconductivity as compared to quantum Hall edge states. The inherent one-dimensional nature of the interior states is believed to be responsible for the observed robust supercurrents at high magnetic fields. This discovery of single-mode 1D superconductivity shows exciting avenues for further research. “In our devices, electrons propagate in two opposite directions within the same nanoscale space and without scattering”, Dr Barrier elaborates. “Such 1D systems are exceptionally rare and hold promise for addressing a wide range of problems in fundamental physics.” The team has already demonstrated the ability to manipulate these electronic states using gate voltage and observe standing electron waves that modulated the superconducting properties.

“It is fascinating to think what this novel system can bring us in the future. The 1D superconductivity presents an alternative path towards realising topological quasiparticles combining the quantum Hall effect and superconductivity,” concludes Dr Xin. This is just one example of the vast potential our findings holds.”

20 years after the advent of the first 2D material graphene, this research by The University of Manchester represents another step forward in the field of superconductivity. The development of this novel 1D superconductor is expected to open doors for advancements in quantum technologies and pave the way for further exploration of new physics, attracting interest from various scientific communities.

Bibliographic reference: J. Barrier et al., “One-dimensional proximity superconductivity in the quantum Hall regime” Nature 628, 741-745, (2024)

I had the chance to present my research at the American Physical Society’s (APS) March meeting 2024. The event brought together over 13,000 physicists from across the globe. All talks were recorded, which allowed me to post mine on Youtube. If you missed it, you may hear about my recent research about inducing proximity superconductivity along graphene domain walls in the quantum Hall effect regime:

A preprint is available on arXiv.