There are many kinds of frustration. There’s the kind of frustration of electrons in some materials at extremely low temperatures that forces them to abandon their preference for classical states such as spin up or spin down and instead enter unusual states of quantum superposition — a favorite subject of MIT Pappalardo Fellow in physics Itamar Kimchi. Then there is the kind of scientific frustration when a research path down which one travels trying to explain these phenomena turns into a dead end.
“Especially in the kind of work I do, you don’t know what the research problem is until you’ve solved it, because you are exploring with a flashlight in the dark things that you only understand once you’ve finished exploring them, that you didn’t even know were there until you’ve understood them,” says Kimchi, who works closely as a postdoc with professor of physics Senthil Todadri.
“One of the things people don’t talk about that much, but I think is helpful to recognize for young people who may go into science, is this emotional roller coaster of scientific research, especially of this basic science, where you’re in the dark even when you try to define the research problem, you don’t know what’s going on, you run into a dead end, but then when you do discover something new that nobody discovered before, it is this high when things make sense, and it’s really something new. That’s beautiful. It’s an emotionally intensive exploration,” Kimchi, 30, says.
Kimchi ’08 double majored in mathematics and physics at MIT before obtaining his doctorate at the University of California at Berkeley. He returned to MIT as a Pappaladro Fellow in 2015.
Early interest in physics
Interest in math and physics came early to Kimchi, who was born in Jerusalem, Israel, but attended high school in the United States. “I knew I liked math and logic and understanding things. Popular science books that I was exposed to a little bit, like about relativity or the universe, were kind of fun. It felt like there was something neat about there being mysteries that you could uncover and then maybe part of that seems to combine nicely with thinking logically and mathematical reasoning,” he recalls.
Coming to MIT as an undergraduate, Kimchi was hooked from his first physics class. “I immediately could tell that what teachers in physics here wanted me to learn really fit what I wanted to learn as well, much better than any other class or department,” Kimchi says. “I felt like it was a good fit for the kind of problem sets I wanted to work on. The way that the class was taught, I think, really brought me into physics. That’s what cemented it.” He graduated Phi Beta Kappa.
It wasn’t until graduate school that Kimchi specialized in condensed matter physics. “It’s neat when you can use sophisticated math to try to understand real experimental results. That’s the draw of hard condensed matter physics theory, the correlated electron systems, for me. You have these really complicated quantum systems, and you see strange experimental results. And it’s always fun to try to understand them, and sometimes also it turns out that you need some powerful mathematics to make sense of the behavior, and that’s a kind of neat synergy or combination,” Kimchi explains.
In his graduate work, Kimchi discovered he liked the richness in the field of condensed matter physics and that he could accomplish projects on a short time scale. He notes that it overlaps with materials science, chemistry, theoretical mathematics, and computer science. “My main project was on a series of magnetic insulator compounds where the magnetic atom is iridium, which is very heavy, far down in the periodic table. The nucleus is big, with a big charge,” Kimchi explains. “So the electrons move around it very fast, and that means that special relativity has some effect on the electronic properties of the material, in particular spin-orbit coupling.” His work on possible quantum spin liquid phases in lithium iridium oxide was published in a Physical Review B paper in 2014. Iridium is a transition metal, making this compound a transition metal oxide. “It turns out that it’s only when the electrons move very fast that their orbital motion can interact with this internal relativistic spin. The effects on the material properties, you don’t need to know special relativity for that, you just see the effects when you measure the magnetic properties of the iridium oxide.” These interactions can also lead to strange quantum entangled states of these electrons, that cannot be described by electronic band theory.
Looking back on the work, Kimchi says when he began making theoretical predictions about this particular interplay of spin-orbit coupling and lattice crystal structure in iridium oxides, it wasn’t clear that the crystal structures that he envisioned could be made. But when experimentalist James Analytis moved to Berkeley as a professor, he was able to grow crystals that turned out to form these crystal structures very closely related to the ones Kimchi was studying theoretically. “Then we could understand their magnetic properties, which are very unusual. People are still working on understanding them. The framework for understanding these properties that I and other people are using still comes from the initial theoretical work I did when I didn’t even know if these crystal theoretical structures could be made,” Kimchi says.
“We developed a model for this family of materials that involved frustration and spin-orbit coupling and crystal structure, all three tied up together. The model has some unusual predictions, one about the possibility of a spin liquid type of entangled state,” Kimchi explains. He is continuing work on other predictions from the model proposed in the paper about unusual spiral states, in which spins point in a very unusual spiral configuration. More recently, Kimchi co-authored work on these spiral states with Radu Coldea at Oxford University. “In this recent paper, I discuss what makes them unusual. It’s a little bit technical but turns out to be a very striking qualitative difference from usual spirals, this one is the first of its kind actually, and it comes from these ingredients of frustration and spin-orbit coupling and crystal structure that are in this model,” Kimchi says.
Dance of the electrons
Kimchi likens these mysterious behaviors of electrons to a quantum mechanical dance. “It can mean a lot for how the material behaves because the electrons are doing something completely different,” he says. He tries to develop theories that point to new experimental directions. “The role I see for myself is as a theorist who works with experimentalists. That’s been my role in my PhD as well,” he says.
“All the kinds of work that I do are understanding things that nobody has thought of before. It’s really thinking about the possibilities for how electrons can collectively act with quantum mechanics. So what are possibilities that people haven’t even considered?” Kimchi says.
“Before you can talk about technological applications, you need to understand it quantitatively, but before you can understand things quantitatively, you need to understand them qualitatively, when there is something completely new that you can’t even understand qualitatively. For example, this qualitatively new type of spiral, where you’re just understanding, what is this thing seen in the material? What’s different about it from everything that came before, but how can we use what we know about what came before to understand it?” he says.
Frustration at triangles
In a very light atom, two properties of the electron, its momentum and its spin, are to a good approximation independent of each other. But in the heavy-element-based materials that Kimchi sometimes studies, relativistic effects couple the spin to the electron motion. At low temperatures, there may sometimes be just a few or one electron per atom (the outermost electron, i.e. a valence electron) involved in the low-temperature behavior, and its various states have low energy. Repulsive interactions between the electrons might lead a compound that should be metallic to instead become an insulator, with the electrons “stuck,” as in a traffic jam, on each atom.
In such a so-called Mott insulator, an essential question is what happens to the electron spin. In many cases, the electrons on successive atoms form an alternating line of spin up and spin down electrons, preferring to alternate the spin on every site. In the context of his work, Kimchi says, magnetic frustration refers to cases where the electron doesn’t know whether it wants to be spin up or spin down. For example, this happens in crystals with structures where there are triangles in the lattice of the magnetic atom. In materials with this triangular type of arrangement, the crystal structure doesn’t allow a checkerboard pattern of alternating up/down spins. “This kind of frustration from the triangles is a way to amplify the effects of quantum mechanics,” Kimchi says. Electrons may be spin up at the first point of the triangle and spin down at the second, but the frustration at the third point can force the electron into a quantum superposition of both spin up and spin down. (This type of quantum superposition from coupling of spin up and spin has been shown in nitrogen-vacancy center diamond.) Spin-orbit coupling can have frustration effects similar to the triangles, again amplifying the collective quantum effects in the material.
He is currently working on quantum magnetic insulators that have both magnetic frustration and some disorder, which means there are irregularities in the arrangement of atoms. “There is some really interesting interplay between this quantum frustration and disorder, such as from impurities or other material randomness, and I am exploring how they combine. They can actually enhance each other’s effects, it turns out,” Kimchi says.
At Berkeley, Kimchi was advised by Professor Ashvin Vishwanath, who also was a Pappalardo Fellow in Physics at MIT from 2001 to 2004. Like Kimchi, Vishwanath — who is now at Harvard University — also worked closely with Senthil Todadri. During his PhD work, among other projects, Kimchi analyzed the role of spin-orbit coupling in quantum spin liquids, in which all of the spin moments of the electron are thought to be in a quantum superposition across the material. “Because they are quantum mechanical, one of the fundamental issues is there is no general direct way to see this quantum superposition,” he says. “One of my goals is to try to understand what are other experiments that one could do on other materials to try to understand this elusive behavior better.”
“There is a set of experiments that collectively form sufficient evidence that some kinds of spin liquids do occur in these certain materials, but what they are exactly, there is still some key understanding that’s lacking,” he suggests. “It’s a very hard problem, but it’s very interesting because what you have is a chunk of material where the electrons are in a quantum superposition state of the magnetic moments of their spins involving a subtle dance across the entire chunk of material.”
Although he was born in Israel, Kimchi considers himself to be more American, but he cherishes the multicultural research environment at MIT. “One of the fun things about science is getting to know people from across the world, how international it is,” he says.
Kimchi is married to Mollie Kimchi-Schwartz, who also is a physicist and a staff scientist at MIT Lincoln Laboratory, where she works on quantum information and quantum computing. He enjoys traveling, hiking, and being outdoors.