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Interview: Philip Moll

Hello Philip. Could you tell us about the research your group is currently involved with?

My group and I are interested in new materials with unusual electronic properties on the mesoscale. By building electronic circuits from newly discovered materials, we can probe the basic physics of materials early after their discovery and test their performance in a chip environment.

At the Max-Planck-Institute for Chemical Physics of Solids, my group enjoys the interdisciplinary environment of strong expertise in both chemistry and solid state physics we need to drive this research. The main materials of focus are unconventional and high-temperature superconductors, strongly correlated metals as well as topological semimetals. We use strong magnetic fields to study these materials, mainly for quantum oscillations for fermiology and to probe critical fields of superconducting devices.

In Dresden you’re using both, HF2LI and MFLI lock-ins, some of them with options. How do they fit in with your experimental setup?

A typical measurement involves resistance measurements in a helium cryostat and strong magnetic fields. Currently, the strongest magnet delivers 16 T, yet we are currently building up a new setup for a 20 T magnet to study Dirac- and Weyl-semimetals in the quantum limit. For these measurements, the MFLI is our workhorse lock-in amplifier that we use daily for high precision, low-noise measurements of small signals.

Typical devices feature multiple electrical terminals which are measured in parallel while applying a current bias to the sample. As we focus on highly conductive materials, we are mostly concerned with signals on the nV level. We use multiple MFLI units triggered together to measure the various terminals in parallel. In these projects, we are mainly concerned with the dc-response and use low frequencies (<200 Hz). The HF2LI is used for resonant experiments at higher frequencies.

So what features of our lock-ins do you find especially beneficial?

We chose the HF2LI specifically for its stable Phase-locked Loop, that I came into contact with during my time at UC Berkeley. We use it to track fast changes of a resonating circuit, which can be conveniently done using its FPGA-based PLL option. The ultimate goal is to track frequency changes of a resonating circuit in a pulsed field magnet. These powerful magnets can deliver up to 100 T over a few milliseconds. With the fast logic response of the FPGA, we plan to track the frequency change during the duration of the pulse. The HF2LI works well for us in higher frequency applications, yet we mostly need precision measurements of low signals at lower frequencies. So I was happy to hear about the development of the MFLI, expanding the same electrical design into small signal levels at low frequencies, and I ordered some of the first units right when they hit the market. I like the low-noise input stage and the flexibility of the software, and as a frequent user of other lock-in amplifiers with fewer bits on the ADC, I appreciate that I never saw digital noise on these units.

But honestly, what I like most about these lock-ins that they do not have any front panel displays. In the beginning, I was quite skeptical of going for an all software solution, but now I would not go back. The real danger with lock-in amplifiers is that they can be garbage in garbage out boxes. A lock-in without a scope attached is a sure recipe for disaster as you need to check the time domain information to make a judgment if the signal is actually valid for demodulation. But, you know, it gets late in the lab, you want to start your overnight measurement, do you really want to bring that scope over? So the lab reality is that many students often skip this step. This is completely eliminated with the MFLI, as you have to open your browser to set it up and you immediately see your input signal in the digital scope. This has clearly reduced the number of artifact measurements, as sample issues like contact non-linearities or ground leakage are immediately spotted.

How do you see your research area changing over the next few years - are there any new developments that we need to look out for?

I want to go further into the integration of novel materials into devices, using more of micro- and nanotechnology tools that were developed for silicon technology. There is much to be learned about unconventional metals with strong electronic correlations, and the development of large-scale dry dilution refrigerators is shifting the paradigm that materials requiring sub-Kelvin temperatures are only an academic curiosity which can never be commercially applied. The advent of quantum information technology is heavily driving this development, and, in the decades to come, we may see dilution-refrigeration becoming a standard tool in the computer clusters of Google and others.

While most of these applications today use traditional superconducting materials, this also allows thinking about new applications based on unconventional materials such as heavy fermion superconductors. Building actual devices made from such materials and testing their performance may lead to interesting technologies. One general development in applied quantum materials is the growing complexity of devices, and when it comes to electronic experiments, they grow in the number of contact terminals. There are some interesting solutions on the market for parallel measurements of multiple channels, and I would like to see a multi-channel version of the MFLI in the future.

You started your research career here in Zurich before moving to the USA, and then returning to Europe. What differences have you seen in the scientific environments of Switzerland, California and Germany, and what advice do you have for people looking to make similar steps?

With the global connections of the scientific community, the questions of interest are rather similar worldwide, but the approaches and the culture of research are still very diverse. I thoroughly enjoyed all the places my path took me to so far, and I always try to learn from their individual strengths. At ETH Zurich, I learned a lot about well-structured research, building collaborative projects with the leading experts that were literally just around the corner. The availability of research infrastructure is exceptional, for my particular research I enjoyed good access to well-maintained micro-structuring equipment at the electron microscopy center SCOPE-M. To me, the strategy to invest in world-class infrastructure and attracting leading researchers alike is one of the main advantages of Switzerland as a science hub.

Going to Berkeley was a very interesting change, and I certainly found the cliché of the dynamic research approach in the US to be very true. What a vibrant campus full of activity and interactions, between faculty, researchers and students alike. The timescales are much shorter, and the willingness to try even crazy ideas without the fear of possible failure is certainly something I took with me to my group.

My next move to the Max-Planck-Society was yet another 180° change. For once, it is my first position as an independent researcher. Of course, I had to learn about how to lead a group, and also learn to step aside and let others do things I very much enjoy, such as working on experimental setups. Luckily, I was able to assemble a great team of postdocs and students. We have great colleagues at the institute and it is a wonderful place to focus on science.

Finally, how do you spend your time outside of the lab?

Ha, this is a good question. Since well before my time at the university, I was always curious to try out new and crazy things. I may have a problem there, but I really enjoy playing around with ideas in the laboratory. So in my free time, I like to try some of the crazier ideas, and while most of them don’t work I thoroughly enjoy having tried something new. And sometimes, it does work of course. So most of my free time is spent in the lab, which probably explains why I know all the night guards at the institute. When I actually am out of the laboratory, I simply enjoy spending time with my wife and friends. I do like to cook, and sailing/boating and scuba diving in summer and skiing in wintertime.

Philip Moll

Philip Moll is leading the Physics of Microstructured Quantum Matter (MPRG) group at the Max-Planck-Institute for Chemical Physics of Solids in Dresden, Germany.

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