“We create a model framework in which we can verify the physics”
Exploring quantum mechanical interactions with two-dimensional materials
Adrian Paulus has been fascinated by physics for as long as he can remember. Initially, it was the solar system and space travel that sparked his interest, but he eventually became fascinated by precise measurements and tiny structures. At the Walter Schottky Institute, he now works with crystals measuring just a few micrometers and materials that are so thin that they are described as two-dimensional.
By Veronika Früh
“When people ask me what I do, I always say I work a lot with adhesive tape.” Adrian Paulus sits in a sunny seating area at the Garching research campus and laughs as he sums up his work like this. The 26-year-old is an MQV fellow at the Walter Schottky Institute (WSI) and is working on his Ph.D. on two-dimensional (2D) materials. Standard Tesafilm is actually an important everyday tool for him in the fabrication of his two-dimensional samples. He uses the adhesive tape to peel off individual layers from centimeter-sized crystals, a process known as exfoliation. What makes the initial crystals so special is that the atoms within them are strongly bonded in two dimensions but only very weakly bonded in the third dimension. The physicist exploits this property to produce individual monolayers. “As a lab assistant, I would actually like to have a group of enthusiastic elementary school children who would simply spend the whole day exfoliating for me in their craft class. That would be great,” he jokes.
But there's a bit more to it than that, the doctoral student admits. It takes a lot of finesse, the right angle, and the right speed to get a good flake, a small two-dimensional piece. Adrian checks under the microscope to see if the exfoliation was successful. “These crystals are usually several tens of micrometers in size. That means you really have to search with the microscope, scanning about a centimeter to find them,” he explains. “That can be very tedious at times, but sometimes it's also very meditative.” And a bit like gambling. This makes it all the more exciting when Adrian discovers a flake under the microscope: “You feel like a winner when you find one, especially if it's particularly large or beautiful!”
Adrian then layers the individual flakes on top of each other to form a sample using special stamps. “In the community, it's often compared to Lego bricks,” he explains, “but I don't think that's a good comparison because you can't twist Lego bricks against each other.” And it is precisely this twisting that is important for the experiments that are to be carried out later with the sample: “By twisting the 2D materials relative to each other, you create a higher-level grid, and with this grid you implement the physics you want to investigate.” Adrian therefore prefers to compare the stacking of the individual material layers to a sandwich. Different ingredients play different roles in the system – “for example, you have the bread, which gives the system mechanical stability” – and depending on the other ingredients, the taste is different. "Building such stacks from 2D materials is also an art in a way. To the untrained eye, every sample looks the same, but to us, some samples have a certain beauty," he explains.
The excitement of pushing boundaries
Adrian is still relatively early in his doctoral studies. “It surprised me a little, but it's definitely a big adjustment,” he says, describing his first few months as a doctoral candidate. The independence and freedom to carry out his measurements on his own, on the one hand, and the greater responsibility for his own research work, on the other – “you realize that you're suddenly on a different level.” The adjustment was particularly surprising for Adrian because he had already written his master's thesis in the same group. After completing his bachelor's degree in physics and applied computer science, he moved from Göttingen to Munich to pursue a master's degree in Quantum Science & Technology (QST).
Adrian Paulus, 26
Position
Institute
Walter Schottky Institute – Chair of Semiconductor Nanostructures and Quantum Systems (SNQS)
Degree
Physics, Applied Computer Science, Quantum Science & Technology
Adrian works with two-dimensional materials as an experimental platform for analog quantum simulation. This allows him to test quantum mechanical interactions that cannot be simulated using computational methods due to their complexity.
It had long been clear that Adrian would pursue a career in “something related to physics.” He says that he certainly picked up some of this interest at home, coming from a “scientific household.” He was fascinated by the solar system even in kindergarten, and space travel captivated him throughout his childhood.
The decisive factor in his choice of physics was a visit to the “GEO600” gravitational wave detector near Hanover. “They talked about how they had to achieve extreme precision in order to measure gravitational waves,” Adrian explains. “That's when I realized that it's not enough to just understand mechanics to build something that's really cutting edge.”
What's more, physics gives you a really good overview of how the world works. In his second semester at Geog-August-Universität Göttingen, he began studying applied computer science in addition to physics – “driven by the same desire to understand complex things.”
He was particularly fascinated by the field of neuroinformatics, which attempts to understand how the brain works. This fascination with pushing boundaries, both in science and his own, is what drives him to this day and carried him through the demanding double degree program. “It was pretty intense,” Adrian recalls with a laugh, adding that he didn't have much time for anything else.
When it came to deciding what to do after his bachelor's degree – “Should I continue with this neuro stuff, or should I do quantum stuff?” – Adrian finally opted for quantum computing. “Because I found computers extremely exciting with my background in computer science, I thought quantum computing was cool,” he says. “And then Munich was simply the place to be in a way.” The fact that you can study for the QST master's degree at both Munich universities, Ludwig-Maximilians-Universität München and Technical University of Munich, convinced him. The physicist also has a personal connection to Munich: he was born here and lived here until he was four years old. During his master's program, he became aware of the MQV doctoral scholarship, almost automatically, as he explains, because there were already several doctoral fellows in the group where he was writing his master's thesis. Of course, he thought about switching to another group or institute for his doctorate. In the end, however, he was very happy in Dr. Finley's group at WSI and decided to stay.
2D materials as an experimental platform
After several weeks spent exfoliating in the clean room and stacking in the gray room, a slightly less particle-free room, Adrian's sample is ready. “And ideally, you then measure it,” says the doctoral candidate, casually summarizing the majority of his work, which is researching the physics. The sample is cooled down to a few Kelvin in the cryostat, the optics are aligned for the best possible signal, and the parameter range to be investigated is defined. Depending on what you want to find out, a measurement then can take up to several weeks.
The 2D materials serve as an experimental platform for analog quantum simulation. “Analog quantum simulation means putting a quantum mechanical model into an experimental platform. Then you can test the complexity of quantum mechanics in a real experiment or simulate it, which is not possible with computer-based models,” Adrian explains. It is comparable to a wind tunnel used to test special properties where the complexity of the world becomes too great to verify numerically. “We create a model framework in which the physics can be verified,” the doctoral student sums it up.
Learning more about complex quantum mechanical interactions
The atomically thin semiconductor materials Adrian works with belong to the family of transition metal dichalcogenides, abbreviated TMDs, “which is much easier to write,” says the physicist with a laugh. These materials are great because they can be examined directly with light. “You look at the spectrum of these materials and then you see physics. It sounds a bit dumb, but that’s how it is,” he continues enthusiastically.
Adrian explains that, essentially, two basic types of measurements can be carried out when looking at the optical properties of these materials. On the one hand, the emission from a laser-excited system can be measured, i.e., the light that the excited system emits again. “This spectrum depends, for example, on how many electrons there are in the system,” says the physicist. The lattice structure of his samples can be filled with electrons. “At certain electron fillings, order forms. The system crystallizes, so to speak, and the electrons have to remain more or less in place,” he explains. “What you then see are sudden changes in the intensity of the emission, which becomes brighter, or sudden jumps in the wavelength.” Adrian can then draw conclusions about the system based on these signatures.
Secondly, Adrian looks at the reflection or absorption of the material. To do this, the sample is stroked with white light – “that's what it's called!” – and the light is absorbed more strongly at points where the layers resonate with the wavelength of the light. The absorption strength is also characteristic of the material and, like the emission strength, allows conclusions to be drawn about the system. “These are the two basic techniques you start with. Then, if you want, you can build more complex experiments around them,” Adrian summarizes. “But the interesting thing is that you can already learn an awful lot about your system with such simple techniques.”
Learning more about complex quantum mechanical interactions is what drives Adrian's research. For him personally, however, there is an even stronger motivation: “It's just fun!” Protein research, which always comes up quickly when talking about quantum simulation and is therefore also mentioned briefly by Adrian, does not really play a role in his work. The field of application that is much more relevant to him is that 2D materials are very clean and defect-free, making them a suitable platform for measuring certain natural constants with extreme precision. This allows the definitions of these constants to be improved. More accurate measurements are not only important for science, but also for example for the semiconductor industry, which manufactures chips in the nanometer range. “If you can measure more accurately, it improves the world as a whole,” says the physicist. “But much of it is simply pure basic research. And that's important too, even if no application comes out of it,” he says. For him, it's about satisfying intellectual curiosity: “It's just exciting, for its own sake!”
No worries during gymnastics
Physics also plays a role in Adrian's favorite hobby, gymnastics. “It may be a cliché, but there is indeed a bit of physics involved,” he says. Gymnastics is actually very much about mechanics. He started the sport at the age of five when he was living in Texas with his family for a few years. It has been a constant in his life ever since. Nowadays, Adrian doesn't have much time for training alongside his work in the laboratory, but he still manages to take part in a few competitions each year. For him, gymnastics is the perfect way to switch off. “It's a sport that demands your full concentration. It usually makes you forget your worries in the lab or your everyday concerns,” he explains. “And it's just fun to spin around in the air like that.”
When it's not possible to be twirling through the air, Adrian likes to take a walk along the Isar river to clear his head. The fact that the river is only a few minutes' walk from his workplace is a definite plus for the physicist here at the Garching research campus. “Of course, it doesn't help much to just go outside during production,” he admits with a laugh. The sample has to be built, after all, even if that sometimes means starting over or staying longer. “But when you're thinking about physics or the world, a walk along the Isar definitely helps.” And even if exfoliation sometimes seems tedious or frustrating when the sample breaks at the last moment, “most of the time it's fun because it's almost like meditating.” And when a particularly beautiful sample emerges, the frustration is quickly forgotten – “a satisfying feeling.”
Published on 27 June 2025; interview on 22 April 2025.