Creating and Deforming Quantum Materials to Discover New Physics

Caltech's newest condensed matter physicist Linda Ye, who grew up in Sichuan, China, specializes in inventing and characterizing new quantum materials that have applications in quantum computer science, materials science, and the development of robust electronics. "If you twist, elongate, and push on materials, you can create entirely new exotic states of matter," she says. "The electrons in the material end up sitting in a different world."

Ye, who is an assistant professor of physics, received her bachelor's degree from Tsinghua University in China in 2012, a master's degree from the University of Tokyo in 2014, and a PhD from MIT in 2020. She then served as a Marvin Chodorow Postdoctoral Fellow at Stanford University before joining the Caltech faculty in 2023.

For her PhD, Ye created what are referred to as topological materials. The word topology refers to a set of unusually stable electronic properties that arise from how atoms are arranged and connected inside the material. "These properties are robust and do not change when the materials are disturbed," she says.

Topological materials are so coined because the electron motion can be described with the same mathematics used to characterize the shapes of donuts and coffee mugs. For instance, a donut has a topological configuration that includes one hole. The number of holes, or the topology of the donut, is robust even if you deform its shape into a coffee mug. To change the donut's topological configuration, one would have to do something very drastic like tear another hole in it or roll it into a ball with no holes.

Different topological states within a material can translate to interesting properties. In fact, the 2016 Nobel Prize in Physics went to three researchers "for theoretical discoveries of topological phase transitions and topological phases of matter." Because of the important role played by the quantum mechanical nature of the electrons, these topological materials are also an essential part of a broader concept called quantum materials.

During Ye's postdoctoral work at Stanford, she switched gears and worked on strongly correlated materials, a different class of quantum materials with a longer history in research. In these materials, which include high-temperature superconductors, electrons interact with each other very strongly. She developed techniques to discover new states of matter in these materials, which involve deforming, or straining, the materials.

"One of the most important contributions from studies of strongly correlated electron systems in the past three or so decades, at least from my personal perspective, is the development of many new experimental techniques," Ye explains. "These techniques can, in turn, be used to tune and investigate much broader classes of materials."

Now, Ye is combining her past research areas to create materials that are both topological and strongly correlated, something that is not common in her field. She is using several methods, including synthesizing materials from scratch and deforming them, to look for unexpected quantum properties.

We sat down with Ye to learn more about her research and background.

Did you want to be a physicist when you were young?

I was pretty good at math and physics when I was young, but I first wanted to become an architect. I like architecture because it combines science and art, but, in the end, I found my path in physics, and it brought me here to Caltech. Looking back, I realize that being a condensed matter physicist is not entirely different than being an architect, as the materials we study are essentially an "architecture" of atoms. Many concepts are common between architecture and materials, too, such as the importance of symmetry and the overall organization of smaller constituents.

What do you like about Caltech?

The students here are really inquisitive, and they're also very focused on what they're doing. In general, I like that we have a very motivated community of both theoretical and experimental condensed matter physicists. We are a small but collaborative community.

How did you become interested in topological materials?

The concept of topological materials was beginning to come into shape when I became an undergrad, and my university invited many early pioneers to visit. They even offered an introductory course on the physics of topological materials, and this brought a lot of the excitement in this burgeoning field. At that time, I took the course and thought the combination between topology and physics was really cool, but I did not realize that I would specialize in these materials. For my PhD, I began working on what are called kagome metals. The name comes from a type of Japanese woven basket called kagome. We were working on a material with a lattice pattern that resembles the basket weave. The pattern has a hexagonal symmetry and is very similar to the honeycomb lattice of graphene. Others had theorized about the lattice pattern before, but our work was some of the first to really explore the electronic structure of this pattern in real materials experimentally.

For a material to be topological, the electrons have different wave functions, or behave differently, in the interior and on the boundary of the material. The difference leads to special conducting properties of the electrons on the surface. It's like a burrito wrapped with aluminum foil: Not only is the boundary different than the inside, but in this case, the inside of the material does not conduct electricity while the surface does. The kagome materials are fundamentally a new way to create topological electronic states of matter. We are thrilled that this field has expanded significantly, with numerous discoveries happening across the world.

How are the topological materials different from the strongly correlated ones?

The strongly correlated materials feature stronger interactions among the electrons. The most famous examples are the high-temperature superconductors, in which strong interactions between the electrons are the driving force of the superconductivity. The early generations of topological materials are often "single-particle," meaning that they are defined in a framework without interactions between the electrons. Traditionally, strongly correlated materials and topological ones are two separate directions in the bigger field of quantum materials. One goal of my lab is to combine the two directions to create unexpected states of matter.

How are you creating materials that have both properties?

That's a great question and one I hope to answer with my research group in the coming years. There is no universally agreed upon way of doing this. One of the ideas we are exploring now is to design materials with specific lattice geometries that inhibit the motion of electrons. In these so-called "flat band" systems, the electrons have very low kinetic energy and thus feel their mutually repulsive interactions very strongly, which can lead to exotic states of matter. Since we do our own crystal growth in the lab, we can flexibly design the lattice network electrons sit in and the chemical element they are derived from. This creates a little world where the electrons behave and interact differently.

We are also mechanically deforming these materials we have grown in our lab based on techniques I learned during my postdoc at Stanford. We apply a voltage at high temperatures to twist, elongate, and push or pull the material. This helps us tune the materials and create new phases of matter. In some sense, you are creating new materials by just deforming the lattice. We hope to use this to our advantage in uncovering new physics in many quantum materials.