Arnaud Théry: from axion detectors in Paris to quantum materials in Halle

Postdoc Arnaud Théry on hunting axion dark matter in Paris, broken symmetry in superconducting junctions in Halle, and what running does for a head full of physics.

In early June, twenty-eight people from the Max Planck Institute of Microstructure Physics ran the Halle Firmenlauf, split across seven relay teams. The course was 2.5 kilometres, in a field of about 4,700 runners. Arnaud Théry was one of the quickest of the institute group. He had run it the year before and wanted to be faster this time, so he trained through the spring. "I was expecting to be quite fast," he says.

Most of that training happened with colleagues. "A lot of us are foreigners, so when we arrive in Halle we don't have friends here," he says. "It's hard to meet friends except in the lab." Through the winter he ran most often with Banabir Pal, a senior scientist in his group; when his own motivation dropped, a running partner brought it back.

What he works on now

He works on quantum materials and superconductivity, mostly on Josephson junctions made from complex materials. A Josephson junction is a sandwich. Take two superconductors, slip a thin layer of some other material between them, send a current through, and measure what comes out. The filling determines almost everything. The same component sits at the heart of many quantum computers, where it forms the qubit. Arnaud cares less about the qubit than about the filling. "We're much more focused on the material," he says. "Its properties, which can be topological." In plain terms: the electrons inside these materials are only allowed to move in particular states, and those states can do unusual things.

One of the unusual things is a diode effect. Push current through an ordinary wire and it behaves the same in both directions. Reverse it and nothing changes. In the materials Arnaud studies, that symmetry breaks. The current flows more easily one way than the other, and the critical current, the most the material carries while staying superconducting, comes out different in each direction. "It's very easy to measure," he says, "but it's actually quite an advanced phenomenon, because it's not something that happens in normal systems. Symmetry breaking in the material. To be very specific, time-reversal and inversion symmetry breaking." The material has stopped treating the two directions of time and space, and with them the two directions of current, as the same.

One measurement has his attention right now. He sends radio-frequency signals into a sample that shows the diode effect and watches for Shapiro steps, flat plateaus that appear in the voltage. As he turns up a magnetic field, the steps shift, and no clean theory says exactly how they should. "There's no precise model that tells us what to expect," he says. "We're measuring and trying to find something interesting without having a model beforehand." That gap, where the data has run out ahead of the theory, is the place he likes to work. He is collecting the numbers and making sense of them at the same time.

What it might be good for

Arnaud is honest about where this work sits. He is comfortable in the fundamental camp and doesn't pretend a product is close. "To me, my interest is more fundamental. We know these systems won't have an application in the next few years," he says. "But even very fundamental work, just understanding new systems that weren't known, opens up possibilities for further applied research."

The possibility he can actually point to is memory. A junction that conducts differently in each direction is, in a sense, already holding a bit of information. Make it from superconductors and you get computation with almost none of the usual energy waste.

There is a larger reason he finds materials worth a career. They stand in for the rest of physics. "Material physics is kind of a little model for general physics," he says. Inside a crystal you can build quasiparticles that behave like particles from high-energy physics. You can take a classical system and make it act with the complexity of a quantum one. Someone working on something as distant as climate physics can find a version of their problem sitting in a slab of material on his table. "There are experiments in our field that relate to most fields in physics. That's very interesting." That is his simple motivation. "I don't really think about the long-term aim. It's more like discovery. I'm interested in pretty much all of physics. Just finding interesting measurements, measurements that surprise you, and then understanding why."

Before Halle: chasing the axion

Arnaud did his PhD in Paris, and it aimed at one of the biggest open questions in physics. Most of the matter in the universe is invisible. We see its gravity and have no idea what it is made of. One leading candidate is a hypothetical particle called the axion, and his thesis built an instrument to look for it.

The instrument was not the usual kind. "Usually detectors are really simple," he says. "They're just cavities trying to detect a photon." His instrument combined a non-linear circuit, Josephson junctions again, with a magnetic crystal. The mix could be tuned across frequencies and run with less noise, which means more of the search covered and more precision where it counts. "Our detector was based on more interesting, more quantum physics," he says. Catching the axion himself was never the real goal. "My aim wasn't really to detect dark matter. There are so many groups trying. It was to prove that our detector might give a more interesting perspective, that maybe people should think about more advanced detectors like ours. And I think we're kind of succeeding."

He is clear about what that kind of search does to the person doing it. "Dark-matter detection demands a lot of patience," he says. "If you just think about dark matter, it's either you fail or you make a major discovery, and there's no in-between." The way through is to stop measuring yourself against the detection of a new particle. "You have to realise it's not really about that. It's also about improving the detector, doing really interesting, complex physics." The output of his PhD reflects that. "We got a few interesting papers that don't all focus on dark matter detection, but are about the physics going on inside the detector."

How he ended up at MPI

The move to Halle was a bit of a coincidence. Arnaud was finishing his PhD and hunting for a postdoc, ideally outside France. His supervisor in Paris is a good colleague of Stuart Parkin and had been invited to speak at the institute's PhD retreat. That year he couldn't make it, so he handed the slot to Arnaud.

Arnaud had no experience giving long talks, and then he realised he was the only PhD student in a room of senior researchers. "Everyone invited there is famous scientists, not PhD students. I was quite stressed." The talk landed well. The group was welcoming. Afterwards Banabir, who leads his subgroup, mentioned they were looking for a postdoc who knew radio-frequency work and advanced measurements, which described his PhD almost exactly. "He asked if I'd be interested, and I said yes. So I was really lucky."

The difference from Paris is mostly one of scale, and Arnaud values both ends of it. In Paris he was in a small group and saw his supervisor daily. "He would come into the lab, say hello, ask how you're doing. If I wanted to talk to him, it was very easy." The group in Halle is much larger and spans many different topics, and it runs differently: a postdoc is trusted to drive his own work. "For a postdoc it's really good. I feel more independent, and I can supervise students a bit. It's good to have both, the smaller group and the bigger one."

The institute, the people, the glove box

What Arnaud praises most about MPI is not any single machine. It is the people within walking distance of his desk. "Anyone working here is very privileged for the possibilities they have to do good science," he says. Paris had very good equipment, but only for the group's own narrower set of questions. Halle is broad. "Because so many people work on different fields, if you need advice on a subject you're interested in but not expert in, you can just go to someone, and they'll advise you. The institute itself holds a lot of knowledge, and it's easy to talk to people."

He has his favourite instruments. He runs and looks after a BlueFors dilution refrigerator, the kind of cryostat that brings a sample to a fraction of a degree above absolute zero. Most of his measurements are standard direct-current work with some radio frequency layered on. Then there is the glove box, where some of his materials are born. It is used for anything that oxidises the moment it meets air or shouldn't be touched directly. Inside it he and his colleagues grow the crystals that go into the junctions and peel them into flakes. "We exfoliate the crystals. That's most of the material we use."

Arnaud feels very privileged to have easy access to a great cleanroom. A much larger one is coming as part of the institute's new building, though it isn't running yet. Arnaud is curious about it without being impatient. "I'd be really curious to work in it. But I'm already happy with the one we have."

Teaching, and remembering you're good

The part of the postdoc Arnaud didn't expect to enjoy this much is working with students and sharing the knowledge he got during his PhD. He has started helping to supervise PhD researchers, not running their projects but working alongside them. "It feels good to feel a bit knowledgeable. It's some kind of accomplishment when you've understood something well and you can share it." A PhD, in his telling, can sometimes be a lonely kind of ownership. "When it goes wrong, it's really on you. When it goes well, you're happy, but it's always your project only." A postdoc gets to spread the effort around. "I feel like I can help on other projects more, and I really enjoy that."

He offers one piece of advice that sounds aimed at the students and is probably also aimed at himself. Measurements can disappoint. Data comes out looking wrong, or dull, or both. When it does, he says, it helps to remember the company. "We're only surrounded by extremely clever people, so we tend to forget that we're actually good at what we do. One measurement that doesn't go well is not the end of our lives."

That is partly what the running is for. "After a long day in the lab I get home and I'm still thinking about physics, and that's not very healthy, because just thinking randomly about physics doesn't help," he says. "When you go running, you're fully present. It's the end of the day. It's like cleaning." So most evenings he laces up, often with Banabir, and lets the day empty out.