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Interview: Prof. Christoph Stampfer, RWTH Aachen University

Hi Christoph, we know you from the early days of Zurich Instruments when you were still at ETH Zurich. Back then, you worked on taming the wonder material graphene when you made the first graphene quantum dots. What changed since then? How has the field progressed?

Many things changed. When we started to make graphene quantum dots in 2007, graphene was still considered as a very new material. In particular, it was still the only two-dimensional material people worked with. It was only in 2010 that hexagonal boron nitride (hBN), a kind of insulating brother of graphene, entered the arena; now there are many available 2D materials including semiconductors, superconductors, ferromagnets and many more. And it took until 2013 for the dry-transfer process to get established, a real key step for making high-quality graphene-based devices. Moving from graphene on SiO2 to hBN/graphene/hBN heterostructures also improved dramatically the electronic quality of devices based on synthetic graphene, that is, graphene grown by chemical vapor deposition. Nowadays, we can routinely achieve room-temperature carrier mobilities on the order of 100’000 cm2/(Vs), outperforming all other known materials. This makes graphene a highly interesting material for applications in the area of high-frequency electronics and integrated opto-electronics. Within the EU Graphene Flagship project, major efforts have been taken to make this true; two years ago we also founded the Aachen Graphene & 2D Materials Center to bridge the gap from basic science to applications.

From a more fundamental point of view, I find it particularly interesting that – thanks to the weak van der Waals force – one can stack 2D materials nearly arbitrarily on top of each other. This allows researchers to make superlattices and to observe interesting physics connected to moiré patterns. More recently, superlattices with magic-angle – around 1.1 degrees – twisted bilayer graphene gave rise to superconductivity and interaction-induced insulating states. This really came as a surprise, and it is a prime example of how exciting modern solid-state physics can be. There is a significant research effort in this field right now. In Germany, for example, a Priority Programme on “2D Materials - Physics of van der Waals [hetero]structures” was established very recently.

What happened with graphene quantum dots – are you still working in this area?

After years of trying to obtain clean quantum dots in etched graphene nanostructures, one of the very few ways to confine electrons in this gapless material, we moved to bilayer graphene. In bilayer graphene, we can open a band gap by applying a perpendicular electric field, which breaks the inversion symmetry in this material and allows for electrostatic confinement of electrons. Actually, this had been known for many years but needed the introduction of hBN and of ultra-flat graphitic gates to get it to work in practice. Nowadays there are around three groups working on such systems, including Klaus Ensslin’s group at ETH Zurich and us, and it is fantastic to see how much cleaner this system is. We are now at the point where we can routinely achieve single-electron occupation in such quantum dots, and we are on the way to understand the system such that we can set out to benchmark its suitability for spin and valley qubits.

You have a full set of Zurich Instruments lock-in amplifiers: how do they help you in your work?

We are doing experiments on graphene-based nanoelectromechanical systems, including highly tunable graphene resonators; this is where we started using the UHFLI 600 MHz Lock-in Amplifier, as at the time no one else was offering such measurement frequency range. We now have two of them and are extending this type of experiments. We also have several MFLI Lock-in Amplifiers, which are used in the lab on several experiments on graphene and bilayer graphene quantum devices. Very recently, we also acquired an AWG from Zurich Instruments to help us understand the physics of potential spin and valley qubits in bilayer graphene.

This sounds fascinating – how did you get to this point in your career? Tell us about your journey as a scientist.

I have tried to escape science several times but in the end remained. I grew up in the German-speaking part of Italy (South Tyrol) and in the last 5 years of school I went to a technical high school where I enjoyed the training to become an industrial electronics technician. Fortunately, the graduation of this school allowed me to go to university; however, the Matura was not accepted by ETH Zurich in Switzerland and so I decided to go to Vienna to study electrical engineering at TU Wien. During my semiconductor devices course in the 2nd semester, my professor convinced me to study technical physics, which I did; I completed my degree with a master’s thesis in theoretical physics. To keep my plan to work in the field of microelectronics alive, I switched to mechanical and process engineering and went to Christofer Hierold’s group at ETH Zurich to do a PhD in micro- and nanosystems technology. I then moved from the department of mechanical and process engineering to the physics department and did my postdoc with Klaus Ensslin: that was the first time I worked at low temperatures. In 2009 I moved to Aachen, where I got a Junior-Professor position at RWTH Aachen University and Forschungszentrum Jülich. In 2013 I was fortunately promoted to full professor.

You also have an extended interest in educational aspects. Tell us a bit more about your physics app Phyphox.

Phyphox stands for physical phone experiments and it is an advanced sensor app that turns your smartphone into a small physics lab. This gives science teaching both at school and university level a completely new twist. This app really turns you into an explorer in no time, allowing you to do experiments and live out your curiosity. When preparing the 1st-semester introductory lectures on experimental physics in 2015, I asked myself, how can we motivate students to do experiments themselves? My experience is that watching experiments does not really reflect reality. This is how I started to experiment with the smartphone. The first experiment I did at home with my brother: we simply put the smartphone into a paper roll, let it roll around the floor and then analyzed the data. It was fantastic to see how quickly we got a nice and interesting set of data. We decided to make it more professional so that we could really use it in a kind of “flipped classroom” way for our experimental physics lectures.

The students like working with this app. The Phyphox App was released in September 2016. We now have more than 850.000 installations, and the app has been translated into 16 languages. It is free and we will keep it free. Actually, we just turned into an open-source project.

What do sport and art mean to you?

For me, sport and art are very much connected to science. I like sports in terms of competition, and I like to work and collaborate with sportive people. I think sport has some spirit that has a positive impact on motivation and healthy competition. In my childhood, I did ski racing for nearly seven years and the training connected with it has shaped me quite a bit. Art is also important as it is connected to creativity, which is important in science. I used to paint quite a bit, especially in Zurich during my PhD when I was quite active. I still love going to art exhibitions and choosing nice color scales for figures, which sometimes the students don't appreciate much.

Prof. Christoph Stampfer, RWTH Aachen

Christoph Stampfer heads the quantum device and 2D materials group at the 2nd institute of physics at RWTH Aachen University. His research focusses on graphene and related 2D materials as well as on quantum transport and applications to quantum technologies.

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