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Patience and persistence lead to evidence of a quantum spin liquid

by Robin Yee
Dec 7 / 15

A CIFAR researcher has shared the first experimental evidence that a state of matter called a “spin liquid” exists at a temperature near absolute zero. The findings may advance a new field of study in modern physics, and contribute to the understanding of other states like superconductivity.

kagome.lattice
In a triangular arrangement of molecules called a “kagome lattice,” electrons are unable to align their spins, resulting in a “spin liquid” state.

CIFAR Senior Fellow Takashi Imai (McMaster University) and his student Mingxuan Fu published their findings in Science Nov. 6. They were able to demonstrate that a spin liquid state existed in a material called herbertsmithite at a temperature near absolute zero.

Spin is a property of electrons that determines its magnetic behaviour. In certain materials at low temperatures, spins tend to align. Theorists have debated for decades, however, whether particular triangular arrangements of electrons could ever prevent the spins from aligning themselves in a stable order. This proposed resulting fluctuation of spins, alternately attracting and repelling each other in a “love triangle” where no arrangement can satisfy all of the electrons, is named a spin liquid because of its shifting nature.

Evidence for the spin liquid state had previously been searched for in many candidate materials, including in ZnCu3(OH)6Cl2, or herbertsmithite, a copper mineral material named after its discoverer. The challenge was determining if the state existed near absolute zero, the lowest theoretically possible temperature, or if it ‘froze’ into a fixed pattern of spin orientations.

To investigate, the researchers used a sample of the mineral that had been synthesized with a specific form of oxygen that emits a signal detectable by nuclear magnetic resonance (NMR). The researchers used NMR to tease apart the signals of the crystal itself from the signals of the crystal defects.

“The beauty of NMR experiments is we can separately detect and probe the clean part of the sample, away from the defects,” Imai says. “That can be done only by NMR.”

It took two years for Imai and Fu to understand how to position the crystal in the system, and at one point they were almost ready to give up. At the end of their third attempt, “My student and I were measuring this together side by side, sitting in the lab,” Imai recalls. As the temperature dropped and they saw the results, he says, “We literally started screaming!”

“This is the smoking-gun evidence for a spin liquid ground state everybody was looking for over the last several decades,” Imai says.

“I’m expecting a lot of debate starting from our work,” Imai says, when asked what will come next. “This is not entirely the end of the road.” However, he is hopeful that a better understanding of spin liquids may eventually help inform research in high-temperature superconductivity.

“Our program has been supported by CIFAR for two decades,” Imai says. “The way materials research works is you keep looking and you keep measuring and then from time to time you hit something big. I truly appreciate CIFAR’s continuous support for this kind of research. People might not understand immediately, but once in a while something exciting happens.”