Quantum States of Matter in Crystals

David Hsieh, an assistant professor of physics at Caltech, is searching for new forms of matter that exhibit weird quantum properties in bulk. Find out the why, where, and how at 8 p.m. on Wednesday, October 15, in Caltech's Beckman Auditorium. Admission is free.

Q: What do you do?

A: I'm an experimental condensed-matter physicist. I'm searching for quantum phases of matter in crystals big enough to hold in your hand. A quantum phase occurs when the electrons in a crystal share a quantum state that creates an interdependence among them. This can lead to tangible phenomena that seem to defy the laws of everyday physics.  

The three familiar phases of matter—solids, liquids, gases—are governed by electrostatic forces. Likewise, free electrons interact with one another through electrostatic repulsions. If you just threw a bunch of electrons into a box, they'd eventually situate themselves as far away from one another as possible. These forces are not under our control, but when we embed electrons in a crystal, they swim in a lattice of ions that can facilitate many other types of interactions. By properly choosing those ions, we can actually exert a significant degree of control over the interactions and start creating new forms of quantum matter. My group is particularly interested in two types of interactions: electron-electron repulsion, and spin-orbit coupling.

Electron-electron repulsions are relatively weak in the metals we typically encounter in daily life. But under the right circumstances, the repulsions can get really, really big, and the material becomes a high-temperature superconductor. "High temperature" in this context means keeping the material at –135°C instead of –245°C, or in other words, keeping them really cold as opposed to really, really cold. Can a room-temperature superconductor be made? Nobody knows.

The other interaction that interests me is called spin-orbit coupling. Basically, an electron can be either "spin up" or "spin down," and most materials have an equal population of each all swimming around in random directions through the crystal. An atomic nucleus has a positive charge, so it emits an electric field. If the atom is really big and heavy, like lead or bismuth, the field is actually strong enough to torque the spins of passing electrons so that they all leave pointing in the same direction. The importance of spin-orbit coupling was given a huge boost about 10 years ago, when people began to think about so-called "topological order" in crystals. The hallmark of topological objects is that the bulk of that object doesn't carry electricity, but the boundaries carry it almost perfectly. This property cannot be induced in a non-topological system.

Q: What are these quantum phases of matter good for?

A: If you have something that carries electricity almost perfectly, the most straightforward application is microelectronic circuitry. Integrated circuits are made of semiconductors; the electricity that a semiconductor does not conduct gets dissipated as heat, which is why computer rooms are so heavily air-conditioned. A near-perfect conductor would generate very little heat. It could be a very "green" technology, so if you're running huge server farms, like Amazon.com or Google, the energy savings would be tremendous.

Moreover, the current would be spin-polarized—all the electrons' spins would point in the same direction—making topological materials ideal for wiring up spintronic circuits. Spintronics is an emerging computer technology that reads and writes information by using electric fields to manipulate spins, or magnetic fields to manipulate charge.

And if you start to assemble structures from both topological and conventional materials, you may get objects that might be used to build quantum computers.

I'd like to push further. Nobody knows what happens when you create both spin-orbit interactions and electron-electron interactions in the same crystal. A lot of condensed-matter physicists are going in that direction—it's an experimentally unknown territory.

We're also looking for what are called topological superconductors. Topological superconductors are predicted to have the potential to perform quantum computations in a fault-tolerant way, meaning that they would resist perturbations from the outside world that would otherwise crash the computer. There's a huge quantum-computing effort going on at Caltech, and engineering fault tolerance into the system is a key element.

Q: How did you get into this line of work?

A: Well, I wanted to do fundamental physics, but I also hoped to see societal benefits from my research within my lifetime. So I'm idealistic, but there's some pragmatism there, too. When I went to Princeton as a graduate student, I wanted to do experimental tests of string theory. But after a couple of years I grew increasingly attracted to condensed-matter physics, so I changed fields and wound up doing my PhD thesis on topological materials.

Named for the late Caltech professor Earnest C. Watson, who founded the series in 1922, the Watson Lectures present Caltech and JPL researchers describing their work to the public. Many past Watson Lectures are available online at Caltech's iTunes U site.