Julien Barrier

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.

I have recently been asked by a journalist to comment my work on Brown-Zak fermions (see my previous blog here ). As part of the discussion, I was asked to introduce the concept of a quasiparticle.

In condensed matter physics, simple equations are usually used to describe complex behaviours. For example, it is very simple to consider the equations of an electron not interacting with its surroundings, because the number of atoms or any other particle is usually in the order of $N_A \approx 10^{23}$, which would make analytical resolutions impossible to solve. When one tries to make the model a tad more realistic, in semiconductors for example, it is possible to treat the effect of surrounding electrons as “perturbations” on the free electron. By adding a different mass to that free electron, its behaviour will be different, slowed down or accelerated (basic understanding with newton’s law: $\Sigma F = ma$). That way, electrons in semiconductors, for example, can be modelled as a “quasiparticle”, that is, move like a heavy or light free particle.

The idea for first proposed by the Russian physicist Lev Landau in the early 1950s. He was studying the properties of liquid helium, a superfluid that can flow without any friction. He found that the behaviour of liquid helium could be explained by treating it as if it were made up of not He atoms, but of a different kind of particles that were not actually present in the liquids. These particles, which he called quasiparticles, were excitations of the underlying quantum system that could be described by their own equations of motion. This was a major breakthrough in our understanding of condensed matter physics. It has been used to explain the behaviour of a wide variety of materials, including superconductors, semiconductors, insulators. Quasiparticles are now an essential part of the theoretical framework for understanding condensed matter physics.   Let us give important examples of how the idea of quasiparticles has been used to explain the behaviour of materials:

• Superconductors. In a superconductor, the electrons are able to move without any resistance. This is because they are paired up into what are called Cooper pairs. These Cooper pairs can be thought of as quasiparticles that are different from the electrons that make up the normal state of the material.
• Semiconductors and metals. In semiconductors and metals electrons can move through the ideal crystal without any resistivity, and in real crystals all the resistivity appears due to scattering of electrons on defects, and due to the oscillations of the crystal lattice (when the position of atoms is deflected from their equilibrium).

In this picture, electrons just ignore the atoms of the crystal, while one would think that those atoms also should be an obstacle for electrons. This is happening because electrons in solid state conductors are not the same electrons which we usually discuss in vacuum. Electrons in crystals are quasiparticles, which have a very different mass from the mass of the free electron, and in some exotic cases the even have a different charge (e.g. fractional quantum Hall effect).

The idea of quasiparticles is a powerful tool that has helped us to understand a wide variety of phenomena in condensed matter physics. It is likely to continue to play a major role in our understanding of this fascinating field of science.

Since I defended my PhD, I have been applying to multiple research fellowships, with a relatively high success rate. I notably secured a prestigious Marie-Skłodowska Curie Fellowship from the European comission, applying 2 days after my PhD viva. For this reason my friend Alexey Berdyugin (now Assistant Professor at NUS Singapore) asked me to give him advice on his proposal for Singapore’s NRF Fellowship. I spent quit a bit of time thinking about my past strategies and writing down my advice, so I thought I could share this to a broader audience instead.

Here I attached the text I sent to Alexey, althought I edited it to keep his project confidential. He is not proposing to work on non-centrosymmetric materials as written below.

About the readership problem

It is important to understand how your proposal will be judged, and how to convince the panel that it is worth funding. I think the scheme is similar for many funding agencies. Here I describe how it works for all grants from the EPSRC, or other private funders in the UK like Leverhulme, and for the Marie-Curie scheme.

First, there will be a referee. Typically, they are doing research in relation to your grant application, but it is not always the case. It is likely that they have a high level of knowledge about the general field. The referee will spend several hours reading and judging your proposal. They will receive one or two applications to judge. They will not be involved in the decision of whether to fund you or not. They will, however, write a report and a recommendation, which will be sent to the second group of readers: members of the committee.

This is step 2. The job of the committee is to produce an ordered list of rankings, with a score assigned to each application. The grant that is the highest in the ranking list gets funded, then the second one, etc. until the budget runs out. Here you want to get a high mark. You need to convince the referee to convince the committee that your project is what they are looking for.

The committee will be composed of several people, each of them having read several proposals. They will probably spend 30min-1h max per proposal. This time will be split between reading the report from the referee and your own proposal. They will use both of this to present your project to all other members of the committee, who will not have read the report. The presenting member is usually from a different field. They will probably be physicists, but they might work on polymer elasticity, acoustic waves or whatever. Not condensed matter. They might even be electrical engineer or any other applied science. As a result, they will probably not recommend a mark of 100%. All other members of the committee will have other proposals to present, and they will not have read yours. It is very likely that they work in a very different field from yours, like chemistry or biology. They might skim through it while the presenting member talks about yours, but that’s all. However, they will still vote on your proposal in order to rank it. So you want the plan to be as clear as possible so that the presenting member gives an accurate description and will also recommend a good mark. If it is too complicated, you can be sure that the presenting member will keep your proposal for the end of the session, when there is no longer enough time to present it clearly, and he will not bother recommend funding.

For this Singapore grant, it looks quite similar in principle. In the application guide, they say that it is a 3 stage process:

1. shortlist by host institutions;
2. shortlist by NRF Fellowship evaluation panel (FEP), comprising eminent scientists (this last part is irrelevant)
3. interview.

So you need to convince one or more institutions that your project is worth being selected. As you already have a position at NUS, this is all politics. Identify who is on the shortlisting committee, and go out of your way to talk to them. Convince them that the project is worth being selected. After all, it would be good that they maximise the chances of their own academics. Otherwise, why would they hire you in the first place? For the evaluation panel, it is all on the proposal, based on recommendations from the school. However I don’t really understand if and at which stage it gets reviewed by an academic in your field.

The structure of your proposal.

Once you understand that your proposal will be judged by a lot of people who will not even read it properly, you understand that you should put all the relevant information (a) in the beginning of the proposal and (b) in the headers. Everything that grants you points should be there, because it might be the only thing the committee reads. Indeed, what I would recommend would be the following: your abstract or “executive summary” as they call it here will contain independent sentences that can just be picked by the presenting member of the committee. You want the sentences to be simple enough that they can be directly re-used by the referee and the presenting member of the committee. And these sentences will also be the first sentences of your main paragraphs. Don’t worry too much about this executive summary at the beginning of rewriting your proposal, you should write this at the end.

The Fellowship application guide gives you an example of the structure. In fact, it is not an example, it should be followed if you want to get funded. This is:

a) Title page
b) Table of contents
c) Executive Summary
Main body (should not exceed 8 pages) (this means you should write exactly between 7.5 and 8 pages)
d) Aims and objectives
e) Reasons for wanting to do your research in Singapore
f) Background and significance
g) Research design and methods
h) Milestones and deliverables
Annexes
i) References
j) Publication list
k) Patents and industry link.\

I think it is a very good structure, that can be reused in other proposals (unless they suggest something else!). In different proposals, point e will be changed to reasons to select a specific institution. Here I will only comment on c, d, e, f, g, h.

You should not necessary write them in order. You will be more productive if you write them in the following order: g1, f1, d, h, g2, f2, e and c.

1. Research plan (g1)

The first thing you want to do is to create a number of work packages. Work packages would be something like small projects. They are internally consistent. At that stage, you want to define broadly what you want to do, and define 3 to 4 tasks for each of these work packages. A task is, for example “fabricate the non-centrosymmetric material devices” or “set up the measurement platform including software control”. I find that 3 work packages each having 2 to 4 tasks is ideal for a project that lasts up to 4 years, and you might add a fourth work package for proposals that are for 5 or more years. Don’t go above that otherwise it becomes too complicated. Remember about the poor presenting member of the committee who will have to justify funding you. On top of that, you will add a few work packages for communications and training activities. Usually, I would have WP5: Training and management and WP6: Outreach, communication and dissemination of results, which are associated to their own research objectives (RO), milestones and deliverables. This way, you tell the committee that you have a good intention of doing all the administrative work related to managing the proposal, you won’t just be a scientist in their ivory tower. E.g. you will train the next generation of scientists as part of the funding, you will dedicate time to meet with the patent office of your institution to transform knowledge into innovation, you will deliver yearly reports to the funding agency, etc.

In the past, I have found that the work packages would be reorganised during the writing and editing. When I started working on my Marie Curie application I designed 3 work packages, but I noticed later that the third one necessitated different techniques and theoretical background. I decided to remove it and split the remaining two WPs into 4. This is quite a bit for a 2-year project, but it worked.

Keep in mind that you cannot have a work package that is “fabrication of devices” and another which is “measurement of devices”. In fact, you cannot have a work package that is dependent upon the completion of another, because the funder will always think of the worst outcome possible: you do not finish WP1, therefore not WP2. If you continue like this, you imagine no result at the end, therefore no need to fund the project. Whereas if you have 4 independent WPs, that could individually go wrong, but in total, the probability of success will be much higher.

2. Identify open questions (f1)

Your research objectives need to answer a question. For this, you would need to list the open questions in your field. You did it well already. You will need to write them as Q1, Q2, etc. and they will correspond to RO1, RO2, etc. For example Q4 could be “there were theoretical predictions of xxx in non-centrosymmetric materials, but so far no experimental proof of their existence and consequences”. Once you have your list of questions, keep them on the side, you will use them later.

3. Aims and objectives (d)

In this section, you should list the research objectives related to your project. In my opinion, it shouldn’t be a very long section, because it will be related to the open questions that you defined in (f). There will also be some background in (f) so any detail will not be necessary here. I find that it is a section that is more complicated that it seems because it has to find a common ground between two contradictory goals: it needs to be ambitious and show significance, but at the same time prove to be achievable. It is better to be relatively precise here. For example, RO4 could be: “I will study the existence and consequence of xxx in non-centrosymmetric materials”, where xxx is the exact same wording as in Q4.

4. Milestones and deliverables (h)

Now, you have a list of what you want to do (the work packages), open questions and research objectives, you can think about the milestones and deliverables. When will you say that you will have finished a work package? What will you need to do. It is usually not a paper, as the paper might come 1 or 2 years after you get the deliverable. For example you can have “M4.1: existence of xxx in non-centrosymmetric materials proven”, and “M4.2: consequences of xxx in non-centrosymmetric materials elucidated”. The deliverable will be something that the funder expects to get at that time. Usually it will take the form of a report on activities, including research, communication activities. For this, you should get advice from your research support officer, they would know that better, and provide information on what the funder seems to expect in terms of deliverables. Again, this will not be a long paragraph, but it should be precise. You need to prove that you will deliver the deliverables. A paper in Science cannot be a deliverable, because there is only a 10% possible success. However, a dataset placed on an open repository can be a deliverable, because you are practically sure that you will measure something.

5. Research design and methods (g2)

This is (almost) independent from the rest. Here you will list whatever you can do, how you do. You are the expert on this. Show it. I have had the intuition that these sorts of method section are not really here to describe all the details of the methodology, but are more like an opportunity to showcase any competitive advantage you might have. Write down exactly where and how your technique is unique and give details on this, the rest should be written very quickly. For example, in my Marie-Curie fellowship, I wrote a full paragraph on the cryo-SNOM techniques that is unique in the host lab, whereas all transport, cryogenic methods and sample fabrication, represented, in total, the same number of words. For those, I would recommend one or two sentences to describe quickly the technique. This is because there are too many groups that master these techniques. But it can be a technique you have and that is unique in the department or in Singapore: it is understood that you are at an early stage of your career and your portfolio is still expanding. Here again, you can write down which technique will be associated with each WP.

In the NRF fellowship application guidance they also ask you to determine the resources that you will need, like equipments, postdoctoral fellows or PhD students. It is good to estimate wisely. E.g. if you already supervise 1 PhD student and 1 postdoc, ask for 1 more student and one more postdoc. Don’t ask for 3 of each, because there is no proof yet that you will be able to manage something that big. I would write these request for resources as an independent section, placed at the end of the research design and method.

6. Write the background and significance section (f)

You already identified the open questions in the field. Now you need to write this so that it fits with the rest. It says you should split it in two subsections: 1. Background and 2. Significance. For the background subsection, add 1 or 2 sentence of context before your open question. This will be one paragraph. For the significance section, it will be a bit more complicated. You will have to pin down what consequence the research will have on other people lives, on the physics community. In summary, what are the reasons to fund your research? In my experience, you can write 3 paragraphs, and each will address the following question: what is the expected scientific impact, what is the expected social impact of the research (i.e. are there policies in Singapore that go into this direction? E.g. what is the government doing for quantum computation, how much investments, etc? how does your proposal match with the political strategy), and what is the economical impact (e.g. startups, patents, etc.). You will need precise examples and numbers.

7. Reasons for wanting to do research in Singapore (c)

Reasons to do your research in Singapore isn’t really a reason for you to choose as you are already submitting a proposal. In fact, I take this as a disguised question to justify what is the reason that the office of the NRF choses your project vs any other, on their own criteria. What I did for example in the Leverhulme application, at the stage of internal review, is asking the head of School and head of Department what the research priorities were, both within the School and in the Department. Then I picked 3 among all of them and justified that my research was aligned with these priorities. The consequence is that the University will hire many more people in this field in the future, and will have a shared interest in your success. There were other reasons that I justify Manchester for the project, but these have less weight, I could have written this in any other university that had cleanroom + helium regeneration, and there are many!

8. Finally, write the executive summary (e)

For this, the application guidelines are very explicit, they say:

i) Articulate the big problem your research wants to solve.

• The problem should be significant
• The problem and related ideas need to be understood by non-scientists^a
• The problem should be important
• People should care when you solve it successfully
• You should justify the significant difference between the proposed research beyond your postdoctoral work.

ii) Explain why you are qualified to do this research

• Give examples on how you have led a team to solve a scientific problem
• Give examples of previous successful proposals
• Showcase your technical skills necessary to solve the research problem
• Detail how you will use the funds

Each of these points will be one sentence (and no more!) of the executive summary. The details of this will be explained in all the other sections. Except point marked with (a) which is irrelevant at this stage. In addition, you will write sentences that will relate the open questions (Q), the research objectives (RO) and the work packages (WP). You can write the impact there. The construction will be very simple: “We need to understand XXX (Q1) in order to solve big problem 1(impact).” You write the 4 of them corresponding to your 4 research objectives. After that, you add “I will measure YYY (WP1) in order to understand XXX (Q1).” It is not a problem if “XXX” is repeated at several sentences of interval: it brings clarity to the presenting committee member. The details for the referee will be in section g: research design and method. In addition, the NRF guidance reads: “Your first sentence might start: ‘The broad objective of the research proposal is…’”. Here, ‘might’ should not be understood as ‘can’ but ‘must’. Below I gave an example of the abstract (called executive summary here) for my Marie-Curie proposal, with highlighted key points that appear clearly.

Now that you have written your executive summary, take every sentence from it and use them as the first sentence of every section or subsection. That way, the presenting committee member will be in a position where they can identify very quickly where to find more detailed information when discussing the proposal. Don’t rephrase and use synonyms for technical concepts, because they will not be an expert and it will just confuse them.

At the very end, check the formatting requirements. The application guide states you should have the following:

Page format	Font type	Font size	Line spacing	Alignment
A4	Arial	12	Single	Justify

You will get eliminated if this is not what you do. It means that all titles should be the same size. The trick here is to use combinations of bold, italic, underline and colours. However, they don’t give any detail on the margin, so it can virtually be 0 to fit more text. In practice they might not like it, so do between 0.5 and 1cm.

Example abstract of my Marie Curie fellowship.

A key ingredient governing the behaviour of electron dynamics in condensed matter systems is the valley degree of freedom. This fellowship aims at inducing topological (or valley-polarised) electronic phases in moiré systems and explore their tunability. Moiré quantum materials present a high degree of tunability as the ratio between interaction strength and electronic bandwidth can be optimised, resulting in flat bands and correlated electronic states. Control of such phases with circularly polarised light would allow to understand the valley-polarisation, topological protection, and the mechanisms underlying the emergence of flat-band superconductivity and correlated insulators. The project will use an interdisciplinary methodology combining electronic transport techniques with infrared far-field irradiation and near field imaging, to create, manipulate and explore novel phases with non-trivial topology (valley polarisation) in moiré materials. The researcher will conduct transport measurement with circularly polarised light to selectively induce inter-valley scattering in 1D channels and explore superconducting interferences in this regime. The project will also quantify the influence of circularly polarised light (CPL) on the superconducting phases and correlated insulators in flat band moiré systems, in order to comprehend the interplay of strongly correlated electronic states with CPL and understand their origin. The fellowship will control and explore novel phases with non-trivial topology in moiré materials, providing new understanding of valley-polarised phases. This objective is a steppingstone towards the creation of new optoelectronic devices such as photonic diodes, optical transistors and logic circuits, and could lead to new concepts as topological nanophotonics for optical information processing.

This abstract is composed of the following parts (colour-coded)

1. a quick introduction to situate the context. Note that this is not necessary. I also included a reminder of the goal of the project here to emphasise on this from the very beginning.
2. a summary of the background and significance section.
3. a summary of the research design and methods section.
4. a precise project description.. This is a key sentence that can be repeated as much as possible throughout the project, with the exact same wording.
5. a summary for the content of one work package
6. the aims and objectives corresponding to the described work package.
7. Finally, a sentence to explain the impact of the research and potential next steps. It will be related to the significance section.

Sentences 5 and 6 were written successively for each work package. Once these sentences are written,I repeated them as the header of each section. That way, I think that the committee doesn’t have the impression of being lost in the project. Using the same wording will ensure that even if they don’t understand the jargon, they understand the relation between the background, the aim, the work packages and the expected results.