Like human social behavior, the behavior of electrons in relation to each other is difficult to predict. In strongly correlated systems, each electron impacts how those around it act, their orientation and movement, and this leads to diverse behavior in the whole. The unpredictability and diversity, says Eun-Ah Kim, Physics, is what makes research in the field of condensed matter physics so incredibly rich.
“What’s so exciting is that no matter how much you understand an individual electron as a fundamental particle, you will never be able to predict all the diverse social phenomena electrons can exhibit when they are brought to interact with each other in different ways and when the group is placed in a different environment,” Kim says.
Kim works to understand how various kinds of electron behavior come about and how she can control or predict for highly desirable behaviors such as superconductivity. Superconductivity—the flow of electrons without resistance—could revolutionize energy storage and transport. “We want to try to peep into the secretive social life of electrons to understand their behavior and to control it,” Kim says. “Just like a peaceful nation is desirable, there are desirable behaviors for electrons, which we want to know how to bring about.”
Getting to Know the Intricacies of Superconductivity
Anyone who has ever charged a laptop knows that the adapter heats up. This heat is caused by the resistance of electrons as they bump through the wires, and it’s energy that we can’t get back. On a large scale—in wires running from power plants to homes, for instance—the energy loss is huge.
“If we could replace that wire with superconducting wire, which does not have any resistivity, does not have the electrons bumping and losing energy to heat, then we save a lot,” Kim says. Superconductors are also perfect diamagnets. They repel magnets. They could therefore be used for levitating trains and to advance various imaging tools.
One problem is that known superconducting materials only superconduct at very low temperatures. Finding a superconductor that operates at a higher temperature, for applied use, is a holy grail of condensed matter physics.
“Just like a peaceful nation is desirable, there are desirable behaviors for electrons, which we want to know how to bring about.”
Kim is working with one of the most promising candidates: iron selenide. In 2014 researchers found that a single atomic layer of iron selenide was shown to exhibit superconducting behavior at the highest temperature ever recorded—about 80 Kelvin, or around -316 ˚F. Kim and her team have made recent progress to understand and explain the material’s behavior.
“We are understanding how the superconductivity is coming about and the rules of other social phenomena that are going on—how the spins are forming a very exotic state,” Kim says. “Our new perspective on the spin state of the system and how that’s affecting superconductivity is having an impact, and it opens up doors to new investigations. That’s one thing I’m really excited about.”
Kim works on a number of other promising materials that exhibit interesting behavioral phenomena, including topological superconductors for applications in quantum computing.
Electrons—Small, Fast, and Difficult to Track
At the microscopic level, trying to see and track electron behavior is no easy task. “They’re so fast, so small, they behave quantum mechanically, so you cannot keep track of them and know what they’re doing,” Kim says. “In that sense, they are secretive.”
To uncover their secrets, Kim uses a variety of methods, one of which is simulations—models of electron activity that can help predict behaviors and what they mean. A recent breakthrough in Kim’s lab is the incorporation of a type of high-powered computing, called neural network machine learning, to help determine the phase of a material.
“We want to know: Is it a superconductor, a metal, or an insulator?” Kim says. “We never thought of it this way, but it’s really a multiple choice question, and a trained neural network can answer it very fast, like answering whether an animal is a cheetah or a cat.”
Kim and her team first input data they have about a system from experiments and other simulations, training the network to give the correct answers in circumstances where they already know the answers. The algorithms that make up the neural network then can use new inputs and how they are weighted to make predictions of the unknown—in this case, the phase. With this method, Kim says a computation that may have taken seven years can be completed in seven minutes. “What wasn’t possible before now can be done,” she adds.
The capability is especially useful in wading through messy, complex data. “We’ve been improving the precision of our measurements through the development of new technologies, but those improvements have brought out the reality of so much fluctuation in the data—how do I interpret this, how do I handle it?” Kim says. “We believe using the trained neural networks will open doors to solving new problems and allow us to assess this new noisy data.”
With Just One Principle, So Much
What drew Kim to physics, at a young age, was that it made her feel powerful. “What was appealing to me about physics was that a single principle can be applied at so many different scales,” Kim says. “If I just understand one principle, I can understand so much at once. That was empowering.”
Kim remembers learning as a young child that the workings of a small motor she could hold in her hand could be explained with the same principle, reversed, that explains the operation of huge power plants. Later, in high school, she realized that the rainbow she saw when the light hit a classmate’s straight, black hair came about by the same principle—the principle of interference—as the rainbows on soap bubbles and in oily puddles.
“There was this connection between mathematics, which gives you a rigorous and exact statement about the world, and the world itself—reality, nature, how the world works,” Kim says. “You just need to know a few facts. It’s very economical.”
She was also fascinated by phases—the fact that you can freeze water, for example, and it will expand and become something different. “That’s so fascinating,” Kim says. “I really liked the richness of the material world, and then in college I learned that, in condensed matter physics, you get to study these social phenomena of electrons, like peeping into a secret. That’s an appealing feeling to me.”
Outside the Box
Kim says she has recently come to appreciate how her powers of intuition and empathy—characteristics often described as feminine—help her as a scientist. “Science is becoming more and more global and communicative and collaborative, and it’s very important that I can figure out what another person is thinking beyond what’s on the surface,” she says.
This ability breaks down traditional borders and paves the way for radical collaboration. “There is a spirit at Cornell which encourages collaboration as well as thinking the wrong thoughts, thinking outside the box,” Kim says. “That has had an impact on me and has encouraged me to take bold steps.”