Understanding the mysterious ‘and’ of emergence

In physics, as in life, the whole is often more than the sum of its parts – and that’s what fascinates researcher Max Metlitski about condensed matter.

Just as a symphony becomes more than the notes of each instrument, so it is with electrons.

“The collective behaviour of the electrons can be very different from the properties that each individual electron exhibits by itself,” Metlitski says. This principle of “collective is different” is called emergence, and it is everywhere. Consider even a drop of water: it is only when you put trillions of its tiny v-shaped molecules together that you get phenomena like liquidness and surface tension, and objects like a drop of dew.

Exactly how this happens, even in the case of dew drops, is still poorly understood. As early-20th-century British physicist Arthur Eddington put it, “We used to think that if we know one, we knew two, because one and one are two. We are finding that we must learn a great deal more about ‘and.’”

The mysterious “and” – the emergent behaviour of a system – is what Metlitski is seeking to shed some light on. “I was fascinated by this idea of emergence – that in systems described by simple physical laws, you can have very complicated emergent behaviour.”

This goes well beyond dew drops. Condensed matter incorporates systems that do things like resisting magnetic fields or conducting electricity with no resistance. “It’s a challenging field with many open questions,” Metlitski says.

Metlitski’s parents came to Canada from Russia and settled in Vancouver when he was a teenager. He was and is fascinated by literature, especially Russian poetry, but in high school he was selected to attend a physics olympiad event that included a week of intense physics lectures. He loved it, and the experience pointed him toward his future career.

Even so, the path to the condensed matter subfield was not exactly straight. In university, Metlitski started studying high energy physics and turned his attention to what happens in the extreme conditions of neutron stars (extremely dense, rapidly spinning remains of stars that have gone supernova and shed their outer layers). These stellar cores have collapsed to the point where neutrons and protons are jampacked together. The particles pair up as they cool, leading them to exhibit emergent properties like superfluidity and superconductivity.

“I was studying superconductors inside of neutron stars,” Metlitski remembers, “and then somebody said, ‘Why not study superconductors which are here on Earth?’” So he did.

Condensed matter physics is almost a reverse of high energy physics, Metlitski says. High energy physics, as a field, is on a quest to take nature apart to discover its basic building blocks. Condensed matter takes the known basic building blocks and tries to understand what happens when you put many of them together. Even forces, which particle physics treat as fundamental rules, can be emergent phenomena when viewed through a condensed matter lens.

Metlitski did his doctoral work at Harvard under the supervision of Subir Sachdev, a pioneering condensed matter physicist who has since been named Perimeter’s Cenovus Energy James Clerk Maxwell Chair (Visiting). After his PhD, Metlitski took a postdoctoral fellowship at the Kavli Institute for Theoretical Physics at the University of California, Santa Barbara. Along the way, his work on superconductors earned him the 2014 William L. McMillan Award, which recognizes outstanding contributions by a young condensed matter physicist.

In October 2015, Metlitski became Perimeter’s newest faculty member, strengthening the Institute’s rapidly growing condensed matter team. Perimeter is, he says, a “very prestigious place to be.” He adds that it allows him the freedom to focus on his research, as well as the ability to work with “great colleagues.”

The research program Metlitski is building at PI goes beyond superconductors to incorporate all the highly coherent states of matter where there are strong interactions and correlations between particles. That includes, for example, the Bose-Einstein condensates in which atoms share a single quantum state near absolute zero, and exhibit strange properties such as being able to slow down light.

It also involves trying to understand the “strange metal” regime in high-temperature superconductors like cuprates, which are copperoxide ceramics that have been altered to add or remove electrons in a process known as doping.

Above a certain temperature, typically a few degrees Kelvin, most superconductors (like mercury, lead, and aluminum) lose their superconducting abilities and become conventional metals with well-understood properties.

A few “high-temperature” superconductors can superconduct at temperatures of up to 138K (which is still very cold). Stranger and more intriguing still are the cuprates: even after they cease to be superconducting, they turn into very unusual metals whose properties are difficult to explain theoretically.

Metlitski grapples with that strangeness. “We don’t even understand the normal state of these materials very well,” he says. “If we could understand the normal state, then maybe in the long run we could design materials that would be room-temperature superconductors.”

That would have all sorts of applications, from super-efficient electrical grids to better magnetic technologies. “Your MRIs could be a lot cheaper,” he adds.

Equally intriguing to Metlitski are quantum spin liquids, in which quantum effects can cause the internal “spin state” of particles in a crystal to become liquid-like and to exist in a state of flux. This phenomenon could be exploited to create “topological quantum computing,” a more stable form of quantum computing.

But though the potential payoff of condensed matter physics is obvious, that’s not what Metlitski finds motivating. He is all about the mysterious “and.”