A large, cube-shaped metal frame reaches almost to the ceiling. The frame supports, at its center, four canisters, one inside the next, that insulate a central chamber that drops down to 7.5 millikelvin, or roughly -460°F.
In that coldest chamber resides a sensor called a Superconducting QUantum Interference Device, otherwise known as SQUID. “So that’s also our lab mascot,” says Katja C. Nowack, Physics. Her lab is building two scanning probe microscopes (SPM), each with a SQUID at its heart.
Why Such Extremely Low Temperatures?
The low temperature is needed in order for niobium, the metal from which the SQUIDs are made, to superconduct—to conduct electrons without electrical loss or resistance. In the superconducting state, SQUIDs are exquisitely sensitive magnetic field sensors. “The other reason for the low temperatures is that the physics that excites me happens at very low temperatures,” says Nowack.
Nowack studies phenomena that emerge from the complex and sometimes counterintuitive collective behavior of electrons in a material. For this, working at low temperature is often crucial. “At high temperature, the atoms and electrons in a material simply bounce around too wildly due to thermal energy,” she explains. “We need to quiet them down such that they feel more subtle interactions that bring out amazing phenomena.” Nowack’s research trajectory coincides closely with the rapidly advancing creation of new materials—particularly quantum materials—that could eventually revolutionize electronic devices as well as quantum information technology.
“The materials are getting more and more complicated,” Nowack says. “In order to understand what they do and how they work, you have to study them locally and in detail. You can’t get away with one or two characterization tools. We’re providing one piece of a multi-faceted picture. In the future, we hope to provide many puzzle pieces.”
Putting SQUIDs to Work
In scanning probe microscopy, a probe scans—or rasters—a sample material line by line to produce an image. The SQUID itself is a superconducting loop interrupted by two Josephson junctions, non-superconducting links, that enable the SQUID to detect and measure sensitive magnetic flux threading the loop.
“What I like about scanning probe microscopy is that it allows you to look at a broad range of materials and problems,” Nowack says. “With the SQUID, we can look at anything that has interesting magnetic signatures or patterns of current flow. We can also perform magnetic response measurements, driving small magnetic fields into the material to see how it reacts.”
The SQUID doesn’t have atomic spatial resolution as some other imaging techniques do, but the trade-off is that “the SQUID is a non-invasive probe that allows us to image functional devices,” Nowack says. “We can see what a material does when incorporated in a device, and we can troubleshoot why the device is not doing what it’s supposed to. That will help to improve materials and make interesting devices with them, which is always the next step.”
Nowack’s focus is now on getting both microscopes—a smaller station that reaches four Kelvin and the larger setup—up and running. The smaller SPM has already started producing images, and as the microscopes are refined, possibilities abound. “In the longer term, we want to build a whole toolbox of sensors that are sensitive to different physical properties and that can work in different parameter regimes,” Nowack says.
“The materials are getting more and more complicated,” Nowack says. “In order to understand what they do and how they work, you have to study them locally and in detail.
Measuring materials in high magnetic fields or sensing their local electrostatic potential are some of the capabilities Nowack hopes to include in her toolbox. Speeding up the SQUID’s response is also on the horizon. “If we modify the design such that we can measure much faster, you can start observing dynamic properties, and that’s very exciting, too,” she says.
Studies in Topological Phases of Matter, Advancing Quantum Computing and Spintronics
While the SQUID and SPM can be used to study a wide range of problems, Nowack does have a focus in mind: topological phases of matter. These materials exhibit unusual phenomena, often on their boundaries, that arise due to topological properties of the material—properties that are robust against changes to the material such as deforming it or adding impurities or imperfections. Examples include topological insulators that don’t conduct electricity inside the bulk material but are guaranteed to have unusual conducting states on their edges or faces.
Topological phases of matter have been actively studied for the last decade or so, says Nowack, and recent interest has spiked, as many believe they have the potential to advance quantum computing and spintronics. With the help of an Early Career Award from the United States Department of Energy, Nowack hopes to prove and probe the mysteries of their behavior through imaging.
In part, Nowack has already started this work. During her postdoctoral research at Stanford University, she used an SPM to study two-dimensional topological insulators. “This insulator doesn’t conduct anywhere on the inside, but it does conduct a current along its edges,” she says. “Theorists had predicted that the material would behave this way, and there had been a range of electrical characterizations, but we were the first to provide images of what was going on.”
Everyday applications of topological insulators and phases of matter remain a long ways off. But “there are visions of realizing topological quantum computation,” Nowack says, “because the phases can have pretty exotic quasi-particles, which you potentially could manipulate. It won’t be practical any time soon, but it’s possible.”
For now, the broad goal is to figure out what novel quantum materials can do and why. “It’s not always clear why these materials have the properties that they display,” Nowack says. “There’s an engineering aspect as well—if there’s a material that has a certain property but only at very low temperatures or under other unpractical conditions, you want to understand why. Then you can try to come up with the next material, which will have the same property but in more favorable conditions.”
Unpredictability and Surprise
During her PhD work at Delft University of Technology, Nowack built small quantum systems—“structures in which we could catch single electrons and play games with them,” she says. She took a turn in her career towards microscopy because she craved surprises.
“I spent the last three years at Delft doing an experiment where I knew exactly what I wanted to do and how the data was supposed to look,” she says. “It took me three years to get that data set, to prove it was possible, but we could have written the paper on day one.”
When Nowack searched for postdoctoral positions, she wanted to pursue something that might lead to more unpredictability. “I imagined that if I had a very specialized and nice microscope, I could still do engineered experiments, but there’s more room for surprises,” she explains. “That really motivated me to pursue scanning probe microscopy.”
Nowack is grateful to have landed at Cornell, where collaboration is real and thriving. “Collaboration adds a big fun factor and makes ambitious projects possible, and it’s not just that people at Cornell say they’re collaborating,” she says. “You can feel and see that it’s true.”