Filip Křížek's materials research was awarded the Lumina Quaeruntur fellowship by the Czech Academy of Sciences. The scientist is working on the development of new materials and components for a new generation of quantum technologies. He is striving to incorporate a magnetic element into already established semiconductor-superconductor materials. Filip's goal is to develop a material that will enable the most efficient and predictable use of the special properties that each of these three basic pillars of solid-state physics brings to the functionality of the resulting component.
Quantum computers are still a rarity. Their development and application depend, among other things, on materials research—that is, on the development of extremely precise, clean, and stable components with the appropriate properties. Is that what you do? What does your work look like?
My work focuses on investigating the unique quantum properties of electrons in complex heterostructures, which are created by combining high-quality crystalline layers with precisely selected properties. At temperatures close to absolute zero, a number of unique and often insufficiently described phenomena occur in electronic components made from these materials, which may play a significant role in the development of functional quantum technologies (e.g., quantum computers).
The cornerstone of our research is molecular beam epitaxy (MBE), a technique that allows individual layers of all three components (semiconductor, superconductor, magnet) to be deposited on a substrate with atomic precision and essentially without any impurities. Once the material is ready, it is used to manufacture electronic components of a specific shape and with theoretically well-predictable functionality. This is done using electron lithography – we prepare the target design on a computer, and the lithographer "prints" the pattern onto the prepared material with a resolution of tens of nanometers.
Components we design require high accuracy and resolution of lithography (approximately 15–20 nanometers), and thus until a new electron lithographer was purchased, it was not possible to even consider focusing our research in this direction. From my point of view, the success of this project requires high-quality equipment for material growth, advanced characterization, electron lithography, and low-temperature measurements, which combines the facilities of the Spintronics and Nanoelectronics Department with an investment in a new cryostat covered from the Lumina Quaeruntur prize money. The experimental work is therefore highly diverse in terms of the use of advanced techniques and, at the same time, takes us into uncharted waters in terms of understanding the interactions in these materials, thus placing considerable demands on their theoretical description and understanding.
What does the addition of a magnet to established semiconductor-superconductor materials bring?
Superconductivity is a property of certain materials in which the collective behaviour and correlation of electrons at very low temperatures results in a number of unique physical phenomena. The most basic characteristic is probably the resistance-free flow of electric current, but the related physics is much richer. To focus on the simplest aspect, the incorporation of semiconductors into superconducting components acts as a kind of "valve" enabling us to very finely tune the flow of "supercurrent" and other superconducting properties in our components using a locally easily generated electric field.
This allows us to study superconductivity in modes that would be achieved in pure superconductors rather by luck than by design. Again, to put it very simply, the addition of magnetic material allows, in addition to the important local tuning of fine magnetic fields (another more sophisticated "valve"), the use of certain spintronic concepts related to the manipulation of electron spin, here directly in the superconducting state. Theoretical predictions in such components open up a lot of promising possibilities. However, the main obstacle is that the presence of magnetic material in most cases "kills" superconductivity. Therefore, our project focuses primarily on incorporating relatively unconventional antiferromagnets, altermagnets, and ferromagnetic semiconductors.
Lumina Quaeruntur is a prestigious award by the Czech Academy of Sciences. It is an award for distinguished and promising scientists who significantly advance the frontiers of knowledge with their research topics. How do you feel about it?
I see it as the culmination of everything I've done so far. I started at the Institute of Physics of the Czech Academy of Sciences, where we produced semiconductor nanowires for solar cells. I continued with my PhD in Copenhagen, where things moved in a slightly different direction. I was in a group connected to Microsoft, which also dealt with nanowires, but the motivation was to develop materials for topological quantum computing. The idea was to connect semiconductors and superconductors in a quasi-one-dimensional system (e.g., in the form of nanowires). It was a fairly complex project – in addition to growing nanocrystals in the MBE laboratory, I also worked on the complete preparation of quantum electronic components using lithography and their measurement at low temperatures.
Then I returned to Prague and planned to stay there. But a friend from my doctoral studies, who worked at IBM's Swiss branch, needed someone on a scholarship to help in Werner Wegscheider's laboratory at ETH Zurich with the production of hybrid (superconductor + two-dimensional semiconductor) materials. A slight problem was to agree on how long I would stay there. They did not understand why someone would want to cut short their great three-year offer, but in the end, I managed to shorten it to only a year and a half. I really like it in Prague and at FZU. The experience I gained in Zurich allowed me to move from researching hybrid materials with naturally quasi-one-dimensional semiconductor geometry to technically more demanding, but technologically more scalable and controllable two-dimensional electron gases. At that time, these were basically only being prepared in one or two laboratories in the world, and due to their connection to Microsoft, they were not readily available to the wider scientific community.
After a while, colleagues from Copenhagen contacted me. They were interested in the material I had been preparing in Zurich. I tried to produce it according to my original Zurich recipe, even though some very experienced colleagues warned me that it probably wouldn't work. Our MBE in Prague does not just focus on semiconductors, so it's a bit "dirty" because in our ultra-clean equipment we use a much wider range of materials than our Swiss colleagues. Dirty in our context still means one of the best levels of vacuum that can be achieved in larger experimental facilities (from 10–12 to 10–11 millibars), but two-dimensional electron gases struggle with any level of impurities. The whole story would take a long time to tell, but the important thing is that we can now prepare the material in our ultra-clean, but at the same time slightly dirty, conditions.
So, several things came together: I regained access to Copenhagen, where my colleagues are intensively engaged in quantum transport and want to work with materials that I know how to produce. Secondly, contrary to expectations, I can grow them in our MBE facility without limiting our other projects focused on different materials in any way. No one thought this was possible. On top of that, we got two new devices from OP JAK infrastructure grants—an electron lithographer and an ion beam electron microscope—which let me make the nano-sized parts I need from materials and do more advanced forms of material characterization.
What will the prize money help you to achieve?
Lumina has the huge advantage of providing money for investments. What we really lack now is an adequate cryostat. The most commonly used superconductor is aluminium because it has various suitable properties, but unfortunately it also has a critical temperature of 1.2 kelvin, which is also the limit of our vector magnet without overcoming many inconveniences. However, we would need to achieve even lower temperatures because most of the phenomena studied are very sensitive to thermal noise. If we use funds from Lumina and add resources from the department, we can purchase a cryostat that reaches temperatures of up to 50 millikelvins. At the same time, it is possible to purchase a dry cryostat, without the need for liquid helium. At today's prices and given the time-consuming nature of the measurements, liquid helium would consume all of Lumina's funds. If we manage to purchase the cryostat, together with MBE growth technology, lithography, and characterization methods, we will have everything we need for the production and calibration of components. We can then share these with partner laboratories with more specifically focused cryostats and delve deeply into the physics of our materials.
So Lumina mainly means an investment in equipment for you?
For me, Lumina means students above all else. I see a lot of potential for doctoral projects in our field – students can learn a lot from us and achieve interesting results. What's more, there's a lot we want to do, and I have to admit that I really need help with this project. I enjoy working in the lab, but four extra pairs of experienced hands make a big difference. For now, I have in mind a small group, starting with two doctoral students – one who would focus more on the material part and the other who would focus on characterizing the components. At the same time, I think the project also provides opportunities for younger students. Routine tasks include, for example, measuring and analysing the quantum Hall effect or spin-orbit interaction in two-dimensional electron gases. Compared to the rest of the project, this is relatively trivial, but I think younger students would benefit from such a topic. And it would certainly make our work much easier and save us time.
How did you get into materials physics? You chose a business academy as your high school, so it probably wasn't because you had a great physics teacher at elementary school.
I got into science by accident, through a somewhat convoluted path. My mom wanted me to study. But after graduating from business school, you don't have much of the knowledge necessary for entrance exams. So I happened to take the Scio tests and was accepted to the Faculty of Economics at the Prague University of Economics and Business. I didn't enjoy it very much, but I was getting by, and also worked at the same time. After three semesters, I still didn't like it very much, but dropping out of school would probably disappoint my family.
Fortunately, someone told me that the Faculty of Nuclear and Physical Engineering at the Czech Technical University accepts students without entrance exams. So I gave up on the Prague University of Economics and Business and enrolled there – peace and quiet at home and a cheaper tram pass for me. I went to the introductory seminars before the start of the semester, and it seemed to me that studying physics might make more sense for me, so I actually started my studies. But with my knowledge of mathematics and natural sciences from business school, it wasn't easy. In the first semester, I only passed one exam, mathematical analysis, and achieved the minimum number of credits. But then I somehow caught up, quit my job, and really focused on my studies. In our fourth year, we had a class with Tonda Fejfar from the Institute of Physics, and they awarded fake Nobel Prizes for our year-end projects. I won the vote, and that gave me the courage to ask Tonda if I could do my thesis with him. It sounds a little absurd, but during our studies we didn't feel that it was possible to just ask and then do a thesis at FZU. Today, we sit here and often hope that a student will just ask us one day... And that we won't have to search high and low for them.
I probably chose the materials partly because I never thought I was particularly brilliant. For me, my work is a little more alchemy than physics. The growth process in MBE is essentially impossible to predict theoretically, and success is strongly based on experience and some form of acquired intuition for the subject. I hope this isn't too arrogant, but I feel like I'm pretty good at it. And I enjoy it. Of course, the best thing is when we can work on a material for which someone has an interesting vision, and we can help them materialize it through our efforts. It's important to me that people trust us that when we give them some material, we try to prepare it as best we can and characterize it as much as possible ourselves so that before releasing it we really understand it.
Author: Věra Ondřichová
The interview was also published on the Vědavýzkum.cz portal.
Photo: René Volfík, Jana Plavec
