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Quantum fluctuations can handle heat

by CIFAR Aug 28 / 14

Physicists have found that quantum materials can fluctuate between states of matter at surprisingly high temperatures.

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The V-shaped area shows the region within which quantum fluctuations continue as the researchers apply a perpendicular magnetic field and increase the temperature in a transverse field Ising chain system. 
Image courtesy of Takashi Imai

The finding came from an experiment that tested a decades-old theory with major implications for our understanding of superconductivity and other exotic states. Collaborators on the paper published in Physical Review X included CIFAR fellows Graeme Luke and Takashi Imai (both McMaster University), experimental physicists, associate fellow Subir Sachdev (Harvard University), a theoretical physicist, and other colleagues.

In classical physics, matter changes states when heated or cooled down, such as when water turns to ice or evaporates. Physicists call this a phase change. Quantum phase transitions, however, take place at −273.15 °C, or absolute zero temperature.

The researchers in this experiment chose to create quantum phase transitions using magnetic fields and then try to measure the point at which their model quantum material changed states of matter. Scientists call that the critical point — like the zero degrees Celsius at which water melts in the classical model.

“Instead of adding heat, we tweak a different kind of knob,” says Imai, a CIFAR senior fellow in the Quantum Materials program.

Molecules fluctuate at the critical point in both classical and quantum phase transitions. Looking closely at a balloon full of water vapour at the moments during which it condenses to water would reveal that some particles are water, while others are still vapour.

However, there is a major hitch when it comes to measuring the fluctuations of quantum phase changes.

Heisenberg’s uncertainty principle states that you cannot measure both the position and the momentum of a particle with perfect precision. That means there will always be some uncertainty to measurements of what state a quantum material is in. Physicists theoretically resolved the degrees of the quantum critical point’s fluctuations a few decades ago. But then in 1999, Sachdev posed a critical question: what happens when you add heat?

“This zero temperature quantum phase transition across quantum critical point is already a complicated problem but now we add temperature as well,” Imai says.

The researchers used a single crystal of Cobalt-niobate created by Luke as a model for the quantum material called a transverse field Ising chain system, which Imai describes as being somewhat like a tiny chain bracelet.

“Imagine there is a chain, but instead of metallic objects there are millions of tiny, tiny atomic bar magnets lined up along one direction — that’s an Ising chain.” Cobalt-niobate has millions of these atomic bar magnets within a crystal structure.

The researchers began at absolute zero and applied a magnetic field in a perpendicular direction, which induced the Ising chains to melt into a new state called quantum paramagnets, and they measured the quantum fluctuation. Then they added heat.

According to Sachdev’s theory, the quantum fluctuations should continue as the temperature increases. And the theory proved exactly correct. The fluctuations survived up to -266.15 degrees Celsius.

The important finding could also point to the mechanism of high-temperature superconductivity, a state at which electrons travel with zero resistance through particular quantum materials, at temperatures considered high from a quantum physics perspective — but are still lower than about -100 degrees Celsius. One theory suggests that if quantum fluctuations can survive to a high enough temperature, they might help electrons bind together to become superconducting.

“These fluctuations become the glue of superconductors,” Imai says.

He says the interdisciplinary collaboration and long-term support of CIFAR’s Quantum Materials program made it possible to finally test Sachdev’s 15-year-old theory.

“We are creating a perfect environment to attack these kinds of very difficult longstanding problems, because all kinds of expertise exist in our program and we regularly meet,” Imai says.

Other fellows in the Quantum Materials are also studying quantum critical points, including André-Marie Tremblay (Université de Sherbrooke), who has shown using a different model that fluctuations survive at high temperatures.

Room temperature superconductivity, if it is ever achieved, could revolutionize electrical systems, transit and other technology.

This research was supported by the Natural Sciences and Engineering Research Council of Canada.