An Explorer of Quantum Borderlands
Transition zones are always interesting—look at marine tidepools, for example. Maggie McKee in Quanta interviews Suchitra Sebastian, a physicist who looks at quantum transition zones:
Suchitra Sebastian is a fringe physicist. Not a crackpot — she lectures at the University of Cambridge and has published a string of papers inScience and Nature. But she likes to venture into the borderlands between forms of matter that other physicists have already explored. There, in the liminal space where the particles in a material begin to change from one configuration to another, new quantum effects appear. “A lot of it is really exciting phenomena that emerges before it’s theoretically predicted,” Sebastian said with delight.
Last year, she and her colleagues discovered what appeared to be electrons looping their way through an insulator, a type of material that by definition prevents such movement. The observation, in a substance called samarium hexaboride, is still not understood. But Sebastian says one possibility is that what was looping was not electrons but an entirely new kind of subatomic building block.
Interactions between electrons create wavelike disturbances — known as quasiparticles — that serve as the basic components of almost every complex material. The known quasiparticles tend to act like heavier versions of electrons, but not so in this case. “In samarium hexaboride, the possibility is that the electron itself has broken apart,” said Sebastian. “So instead of thinking of the electron as the building block, we would need to think of fractional parts of the electron as building blocks.” These fractional quasiparticles would create an entirely new way to understand the universe of materials.
Sebastian herself moves between very different worlds. Before delving into science, she worked as a management consultant, and now she performs in experimental theater pieces when she’s not in the lab. “I kind of intensely do different things,” she says. “If I spend too much time doing the analytical physics side, I’m, like, gasping for oxygen.” Research, she said, “is not about drawing within the lines. It’s about discovery and creativity.”
Quanta Magazine spoke with Sebastian about her research and her unconventional path to science. An edited and condensed version of the interview follows.
QUANTA MAGAZINE: You search for quantum effects that are entirely new to science. How do you go about looking for them?
SUCHITRA SEBASTIAN: One option is to look at many, many, many, materials. You can say, “I’m trying to find the one that does something very different.” You may never find it. What I realized is you can use external conditions — pressures or temperatures or magnetic fields — to manipulate a material and move it into a region where it does something really interesting and where new quantum properties emerge.
Where is that magical region?
Water can be water, or it can be ice, or it can be steam. These are the same material but in different phases. In the quantum world you can also havedifferent phases. You can have the same material and the same electrons, but the interactions can result either in the substance organizing into one kind of material — so under certain conditions, you can have a magnet — or you exert pressure on the material — you press it — and then it quantum configures in a slightly different way and the magnet transforms into a superconductor. The region I’m excited about is the region between these phases, which is a quantum critical region. Between one phase and another phase you get this intermediate region, where it’s unknown what might happen, and you can have completely new forms of matter emerging.
What gives rise to the quantum effects?
Simple materials with weak electron interactions can be modeled just in terms of the electrons’ propensity to hop around, which can be averaged over the entire material. But in more strongly interacting materials, the repulsive force due to interactions between each of the trillions and trillions of electrons is stronger than their propensity to hop around. In this case, the resulting collective effects are almost impossible to predict and can be dramatically different from individual electron behavior.
Last year, you found an unexpected quantum effect in a material called samarium hexaboride. What was so surprising? . . .