When exactly did your curiosity for theoretical physics start?
“I have always been fascinated by the technology around me, which was solidified in high school by great math, chemistry, and physics teachers. Especially math and physics caught my attention. During my studies at the University of Bremen (Germany), this enthusiasm for mathematics and physics focused towards theoretical physics, although (or since) it was most challenging for me. For my Bachelor project at the Institute for Theoretical Physics, I needed to numerically solve the so-called Hubbard model, a famous model to describe correlation effects in solid-state materials. This combination of mathematics, programming, theoretical physics, and the personal challenge finally hooked me up for the fascinating field of theoretical condensed matter physics.”
What has been your career path so far?
“My 'career' started at the University of Bremen, where I got my Bachelor, Master, and PhD. For the latter, I joined the group of Prof. Tim Wehling, who was recently appointed professor at the Institute for Theoretical Physics. The corresponding PhD project was about interactions in two-dimensional materials. Towards the end of my PhD, I realized that, while I loved living in Bremen, it was time for a change. So I applied for a Feodor Lynen Research Fellowship of the Alexander-von-Humboldt Foundation which allowed me to join the group of Prof. Stephan Haas as a postdoc at the University of South California in Los Angeles.”
How did you experience being abroad?
“Scientifically and institute-wise it was a great and a most-valuable experience. I got in touch with different research environments, with new topics, and with a variety of people with completely different backgrounds. I especially enjoyed meeting people with different mindsets and different approaches to their scientific problems but also to their daily life. Socially, it was also quite an experience for my wife and me. While we had an outstandingly enjoyable time with our new friends in Los Angeles, it was especially hard for my wife to get “a foot in the door” of a potential employer, so that, after about 1.5 years, we eventually decided to return to Europe … one or the other way.”
How did you end up at Radboud University?
“After we realized that Los Angeles will not become our new home, both of us applied for several positions. My wife found a new job at a university in northern Germany and I got offers from the Center for Computational Quantum (CCQ) Physics at the newly founded Flatiron Institute in New York and also from Institute for Molecules and Materials at Radboud University. I still remember how happy I was when I read Herma Cuppen’s email offering me the position on my way back from the interview in Nijmegen. Both positions were extremely appealing to me so I had a very tough time deciding. In the end, I spend a few months at CCQ before finally joining Radboud.”
What is your research focus now?
“I am a theoretical physicist with a focus on computational condensed matter, or in other words: I am doing fundamental material research with the help of computers. We try to understand and predict the properties of novel materials that have either been just recently discovered or which do not even exist yet. My special focus lies thereby on so-called many-body effects in atomically thin materials, which I try to understand and explain with the help of theoretical models solved on large computers.”
Let’s go into the depth of material research...
“Sure. Every solid state material consists of lots and lots (and lots ...) of atoms with corresponding atomic nuclei surrounded by even more electrons. All of these parts interact with each other. This means every electron interacts with all other electrons and also with the nuclei and so do the nuclei among themselves. This creates an overwhelmingly complicated many-body problem, which we cannot fully solve. It is, however, important to understand these many-body effects since they are the origin of many technologically relevant material properties such as magnetism or superconductivity. To get a handle on these complicated many-body problems we design and apply approximate numerical methods. In my case, I do so for specific materials so that the results of my numerical simulations are “material-specific” allowing for direct comparison with measurements by our experimental colleagues.”
What kind of studies have you done so far?
"In the past few years, I mostly concentrated on electronic and optical properties of semiconducting layered materials. With great success, we developed the most accurate models to explain and predict substrate- and excitation-induced changes to the band gaps and optical absorption spectra of so-called transition metal dichalcogenides. Seeing our predictions become “real” in the form of experimental data gathered by external colleagues was overwhelming. At the IMM I started to strongly focus on superconducting and magnetic properties of novel layered materials and also on optical properties of correlated nanostructures."
Your research crosses departmental borders within IMM. Could you explain?
"Parts of our research closely align with the efforts from several experimental groups within IMM and HFML-FELIX. This includes, for example, the experiments by Nadine Hauptmann and Alex Khajetoorians from the Scanning Probe Microscopy department, as well as the experimental studies in high magnetic fields by Steffen Wiedmann and Uli Zeitler from the Semiconductors & Nanostructures group and Nigel Hussey’s Correlated Electron Systems group at HFML-FELIX. They are investigating these many-body properties in materials we are also very interested in. My expertise can help a lot to bridge between theory and experiments and can help to create strong synergy effects: puzzling features from experimental measurements often inspire us as theoreticians to have a closer look into specific aspects of our simulations while our theoretical results can also point towards interesting further experiments which have not been performed yet. Thus, in the best case, we have a closed mutual “inspiration” loop between theory and experiment. Nevertheless, as theoreticians, our holy grail will always be to make predictions which we ask our experimental colleagues to check for in their labs. If they can verify our predictions we know that our theory is correct … or at least not too bad.”
You really like to perform interdisciplinary research, don’t you?
“Yes, indeed! The variety of experimental facilities at IMM and HFML-FELIX is incredible. Excellent groups can measure astonishing and sometimes totally unpredictable features. When they reach out to us and ask what we think the actual fun starts. Synergy effects kick in and the IMM becomes more than just the sum of all departments.”
What exactly drives you in your work?
"It’s a combination of what I do and how I do it. On one side I really want to understand how nature works, especially when it seems to be complicated. On the other side I really like the tools I use on a daily basis, which is a combination of theoretical many-body physics, applied mathematics, and programming. Applying and extending these tools to a specific problem allows us to really get closer to the microscopic properties of a certain material and eventually also closer to a fundamental understanding of nature. Moreover, I am a people person. I like to communicate, discuss, and exchange ideas, which (normally) takes a big role in my daily life. It’s a great feeling when all of these aspects work together resulting in some “beautiful” data.”
What is the societal relevance of your research?
“At the Theory of Condensed Matter department, we mostly do fundamental research, partially curiosity-driven and partially driven by input from our experimental colleagues. Thus, in best cases, our impact is a fundamental understanding of structural or electronic properties of condensed matter which inspires or helps our experimental colleagues. Sometimes we can speculate about future applications of these findings, but normally these lie in the far future. However, in the case of layered or two-dimensional materials, there are already first application ideas in the form of sensors, electronic, and optical devices and we are actively working right now on fundamentally new device concepts based on so-called layered heterostructures.”
How do you experience IMM?
"IMM is a great institute and I’m very happy to be here. I am surrounded by professionals on every single level: from our secretary to the administration and IT departments, to the various outstanding researchers. All of them are not just very helpful but also very friendly which creates a most productive and enjoyable surrounding. Thus, it’s great to work at IMM and I hope to stay here for a long time.”
Text: Miriam Heijmerink