Luxembourg National Research Fund

BACK TO RESEARCH WITH IMPACT: FNR HIGHLIGHTS

What made you decide to study physics?

“I was always good at maths and physics, but it was thanks to one of my high school teachers that I went on to study physics at university.”

How so?

“He ran an after school class in physics for students who were interested, where we did experiments and took part in physics tournaments. This class went beyond the physics in our school curriculum and I really enjoyed it, also taking part in all the competitions.

“I like physics because I want to understand how things work. What fascinates me about physics in particular is that all the effects you observe really boil down to very few fundamental principles – I do not observe this in too many other sciences.”

Are you involved in any science outreach activities yourself?

“I am involved in the Luxembourgish physics Olympiad – some teachers reached out for a University contact in physics and I was happy to do it. It brought me back to how much it meant to me when I was doing after school physics myself.”

Would you say your own experience of having a teacher who fostered your interest in physics has inspired you to do the same?

“Absolutely. My teacher noticed I was good at physics, and so encouraged me to join his physics class. I think that as a student you need certain ‘promoters’ who can encourage you to dive deeper into something you are good at.”

Would you say activities to encourage school students to be interested in science are important?

“For us working in physics at the University, outreach activities are extremely important, as we would like to encourage more students in Luxembourg to study physics. I would say two approaches are needed: First, you need something accessible to everyone to get people interested in science in the first place.

“The University, for example, runs the Scienteens Lab, where entire school classes can go and do experiments. I also recently went to the Luxembourg Science Center for the first time with my family. My soon five-year-old son really enjoyed it, he was fascinated – I think it is great to have something like this. However, these activities are not very selective, therefore you also need more targeted activities – such as the Physics Olympiad — for people who show an aptitude for science and foster their interest in it.”

Your scientific field is mesoscopic physics – can you elaborate?

“Mesoscopic physics is anything that happens between the small scale – governed by quantum mechanics – and larger scale systems, which are mostly governed by classical physics and can be seen with the bare eye. Mesoscopic physics is therefore between microscopic and macroscopic physics.

“If I throw a ball, then I can use classical physics to describe where the ball will be and which velocity it will have at any given time. In quantum physics, this is not possible. An electron does not behave this way because of the uncertainty principle – for example the more precisely you measure the position of a particle, the less sure you are about its velocity. I find mesoscopic physics interesting because it allows for the study of the crossover between classical and quantum physics.”

Is your field new?

“Physicists have investigated mesoscopic systems for more than 20 years, so the field as such is already quite mature. Nevertheless, experimental, and also theoretical, techniques have strongly evolved over that time so the systems we are studying nowadays are quite different from those studied 20 years ago. Most of the things I study are systems on a scale between 1 nanometre and 1 micrometre, such as quantum dots or nanowires, where technological progress has made it possible to study entirely new questions.

“Moreover, novel materials have also had a drastic impact on the field of mesoscopic physics. One of my main research interests is topological insulators. They are a very novel class of materials that were discovered about 10 years ago, and which have created quite a hype in solid state research and materials physics, because they have a wide range of potential technological applications.

You are a theoretical physicist. Can you elaborate on what this means?

“The main goal in theoretical physics is to obtain a mathematical modelling of a complex physical effect. There are two ways to do it: you can use an analytical or a numerical approach. Analytical work means you operate on mathematical equations and can, in principle, do everything with pen and paper. Numerical work requires some programming, but essentially you use a computer to solve the equations.”

Do you have a preference between using the pen and paper (analytical) versus the computer (numerical) approach?

“Both have advantages and disadvantages: I usually solve a problem, for instance describing a complicated experimental system, by first modelling it, but then I obtain some equations which I am not able to solve. Then comes the part where you need some physical intuition and you need to ask yourself where you can apply approximations.

“For example, in school you learn how to mathematically describe an apple falling from a tree, but air friction is usually neglected. If there is air friction, it becomes a rather complicated problem, if there is none then it is simple. If I have experimental data on nanowires, I have to look at the data and develop a model, also looking at which parts of the model I can ‘ignore’, which approximations I can make, and then simplify my system enough so that I can solve it mathematically.

“A disadvantage in the numerical approach is mainly that you get no insight into the physical mechanisms at play, meaning you learn less. Going back to the apple falling from the tree – you can solve the equations of motion numerically; the computer then tells you it takes one second for the apple to reach the ground. What have you learned? You don’t find out why it takes one second.

“Moreover, if you want to know what happens if the apple is twice as heavy, you have to re-run the entire simulation with new parameters.  In that respect, we often learn more from analytical calculations than we do from numerical calculations and they are more general. On the other hand, if we really want to compare our results quantitatively and not just qualitatively with experiments, we usually do need numerical simulations.

“My research is mainly analytical – we try to do as much as possible by just solving mathematical equations. However, numerics and analytics work hand in hand: I can approximate my equations to solve them exactly, but then I would also like to know how good the approximations I make are, so ultimately I often compare them to numerical results.”

What is the purpose of your work as a theorist?

“The purpose of a theorist – besides consuming coffee – is to predict experiments that might be interesting, and to help with the interpretation of experimental data.

“In theoretical physics, we operate on timescales where one project sometimes takes less than half a year. We propose potential experiments, analyse them mathematically, and see what should be found by experimental physicists. If our predictions are interesting experimentalists will try to verify them and build the device we proposed, which can then take a much longer time, sometimes several years. But it also works the other way: they measure something that they don’t fully understand, then they go to a theorist for help with the interpretation. I would say an advantage of materials physics and condensed matter physics is that there is a close interaction between theory and experiment.”

You mentioned your projects often do not take longer than 6 months – can you elaborate on your research process?

“As a theorist, you can work pretty much anywhere. One thing I enjoy is how fast we can get to work on interesting new projects: If I read a paper about something interesting, I invest a few days on reading the available literature, and then we can get on with the calculations, as we do not need to get any machines, but more or less just need a pen and paper. It’s nice because it allows us more freedom to work on new ideas.

“The obvious downside is that you often have to act quickly: someone publishes something really novel and interesting, and you’ve got thousands of theorists in the world who want to work on it. Therefore, if you want to be the first to publish on it, you have to be quick. The timelines are short, especially in the field of topological systems which is a hot field right now.”

Why did you choose topological physics?

“I work on it because it is interesting and still promises many exciting discoveries. The field is certainly one of the major advances of condensed-matter physics in the past decade. This is why the forefathers of topological physics were awarded the Nobel Prize in 2016. Topological physics is fundamentally interesting and has a huge potential for technological applications for instance in nanoelectronics – some of which are maybe 10-15 years away. However, for me as a physicist the fundamental questions are what fascinates me most.”

Do you have any overarching goals, particular questions you want to answer, or is your focus on being reactive to new developments in your field?

“In the moment, I tend rather towards the latter – I have the feeling I am still a bit too young to concentrate all my attention on a particular problem. I am still at the level where I am eager to learn new things, and I like it that my focus is quite broad within my research domain. Learning something new every day is what makes life as a researcher nice and enjoyable for me.

“Of course, you have to become a specialist in certain topics, because otherwise you will not have visibility in the world of science. I have certain niches where my expertise is strongest, but I try to stay open and I like to experiment with new topics.”

Tell me about your group and how it works – is it what you had in mind when you first created it?

“My group currently has four PhD students and three postdocs, along with temporary members such as Bachelor or Masters students and sometimes interns. As I do not require a lot of equipment for my research, I have been able to put my full ATTRACT funding towards my group, with some members funded by other FNR programmes.

“My ideal group size is closer to three than thirty – I want to be close to the work and I want to know all the nuts and bolts of the calculations. That would not be possible if I had a much larger group.

“I do not want to become a science manager – I am not only interested in the results, but also in how we come to that result. It is thus important to strike a balance – if the group is too small, you have no critical mass to pursue your ideas, but if it is too large then I think you slip more into the role of a manager and lose touch with the actual research.”

How do you work?

“Everybody has a project – one member may be working on nanomechanics, another on nanowires, and yet another on topological insulators. There is also a close collaboration among my group members, with many members being involved in each other’s projects. We discuss, develop ideas, and the project responsible has to carry it out mathematically.”

How do you supervise your PhD students?

“I have the ultimate responsibility for them, but every PhD project in my group also involves another postdoc or another professor. I think it’s important for my postdocs to get some supervision experience, just as I think it is important that my PhD students have more than one person they can speak to.”

Do you also teach?

“Yes. While I have the ATTRACT grant, I have a reduced teaching load. I teach one class per year, rather than per semester. Teaching is part of the job and I enjoy it. It is also important for me, as I want to attract Bachelor and Master’s students to my research group.”

You were a Junior Group leader at the University of Basel before taking up your ATTRACT Fellowship – why did you come to Luxembourg?

“The position in Switzerland was not permanent, so I was looking for opportunities. They were advertising a professorship at the University of Luxembourg. I applied and the research unit replied that they would like to propose me for an FNR ATTRACT Fellowship. Then I wrote a research proposal and everything fell into place. I did not seek out Luxembourg specifically, but I enjoy life here.”

What difference has it made? Do you feel you will stay?

“Yes, my plan it to stay. The University here is very good, in recent rankings it is at 12^th place worldwide among all universities aged 50 years or less. This is an excellent achievement for a university of this size and I am proud to be part of it. Moreover, I like life here; my children are growing up trilingual, and my wife and I are learning to speak French and Luxembourgish. I appreciate the many cultures in Luxembourg and I would certainly miss it if I went elsewhere.”

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THOMAS SCHMIDT’S ROAD TO ATTRACT