U of T researchers achieve lowest levels of temperature and pressure to understand quantum materials

A team of researchers at the University of Toronto’s Ultracold Atoms Lab may have discovered a lower limit to the rate at which atomic gases near absolute zero demagnetize.

It has long been known that certain substances do extraordinary things in extreme environments. Superconductivity and unusual forms of magnetism, among other phenomena, have been observed in various substances, known as quantum materials, in very hot or very cold temperatures. Yet, while experiments with these quantum materials lead to astonishing results, the mechanisms behind some of these observations leave theoretical physicists baffled, making quantum materials one of the most mysterious frontiers in physics today.

Despite having a wide range of significant practical applications, including magnetic information storage materials, hyper-efficient energy transport systems, and models for peculiar astronomical phenomena, very little is known about why exactly these bizarre quantum materials behave the way that they do, from a theoretical perspective.

Researcher and associate professor at U of T Joseph Thywissen, and his teammates seek answers to these questions by studying a simulator system of ultracold gases, whose properties resemble that of the enigmatic quantum materials but whose components interact at slower and therefore more observable rates.

“The reason there’s still mysteries is because, if you want to see how quickly something relaxes in a material, you’ve gotta be quick,” explains Thywissen. “Electrons relax in femtoseconds, on the scale of 10 to the minus 15 seconds. Whereas in our lab our atoms relax in milliseconds, 10 to the minus three seconds. The difference between those two is something you can and cannot see in a lab,” he says.

Spin, in this case, is an attribute inherent to quantum particles and is the key to many real world effects, such as magnetism. There are still several outstanding fundamental questions about this.

Thywissen’s team captured individual atoms in a container, and manipulated their spin direction with a magnetic field into an uneven state. They then measured how long it took for the spin directions of the particles to diffuse along a spiral pattern until the system relaxed to equilibrium.

At an extremely low temperature — almost at absolute zero — and an extremely sparse distribution of particles — practically a vacuum — the researchers have determined a lower limit to the speed of the diffusivity process that can be described with surprising mathematical simplicity.

Though this speed limit was determined from experimental data, the discovery is novel in the field of atom mechanics. The next step is to elucidate the implications of this new measurement in terms of understanding quantum materials. Thywissen is hopeful that the observation can be transformed into a universal principle for both atomic and electron systems.

“What we do is we investigate this other strange but strangely similar system, which we believe has behind it the same principles at work, the same equations,” he says, adding, “There are other people who have seen these kinds of limits without really understanding where they come from. Ours is a mode of relaxation that others hadn’t measured with atoms before, but it is another piece of a puzzle that seems like it’s assembling a picture.”

There is still a long way to go with research in quantum materials. “I do believe that someone will turn this into an equation, into a theory, and into an understanding. But for now, that’s yet to be understood,” Thywissen says.

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