Researchers Discover Mysterious New Quantum State of Matter

April 5, 2016 | Johannes Van Zijl

Excitation of a spin liquid on a honeycomb lattice with neutrons.
Photo credit: Genevieve Martin, Oak Ridge National Laboratory

The discovery is an important step for physicists to understand the workings of quantum matter.

In an international collaboration, physicists from the University of Cambridge, have found evidence of a mysterious new state of matter, which was predicted to exist 40 years ago.

The new state, known as quantum spin liquid, causes electrons, which are generally thought of being indivisible elementary particles containing no components or substructure, to break into smaller pieces. It was believed that quantum spin liquids are mysterious states of matter that hid certain magnetic materials, but they had never been observed in nature before.

“This is a new quantum state of matter, which has been predicted but hasn’t been seen before,” said one of the study’s co-authors, Dr. Johannes Knolle of Cambridge’s Cavendish Laboratory, in a press release.

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In normal magnetic material, each individual electron in the material behaves like a tiny bar magnet. When the material is cooled to a low enough temperature, the “magnets” will arrange themselves across a large range, so that all the north magnetic poles point in the same direction, for example. But in material containing a spin liquid state, even if the material is cooled to absolute zero, these “bar magnets” would not arrange orderly, but rather form an entangled mesh caused by quantum fluctuations.

“Until recently, we didn’t even know what the experimental fingerprints of a quantum spin liquid would look like,” said co-author Dr Dmitry Kovrizhin, also from the Theory of Condensed Matter group of the Cavendish Laboratory. “One thing we’ve done in previous work is to ask, if I were performing experiments on a possible quantum spin liquid, what would I observe?”

The research team used a neutron scattering technique to look at experimental evidence of fractionalisation in alpha-ruthenium chloride (α-RuCl3), a compound with a honeycomb structure. They tested the magnetic properties of α-RuCl3 in powdered form by illuminating it with neutrons. The team then looked at the patterns produced on a projection screen as the neutrons scattered from the powdered sample.

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A normal magnet would scatter and produce clear sharp lines on the projection when used in a neutron-scattering technique, but it wasn’t known to physicists what sort of pattern the Majorana fermions in a quantum spin liquid would produce on the projection. It wasn’t until 2014 that Knolle and his colleagues predicted that quantum spin liquids would produce broad humps instead of sharp lines during a neutron scattering experiment. The latest findings match well with their earlier theoretical prediction, as they observed broad humps during the experiment, which indicated that they had detected the new state of quantum spin liquid in two-dimensional material.

“This is a new addition to a short list of known quantum states of matter,” said Knolle.

“It’s an important step for our understanding of quantum matter,” said Kovrizhin. “It’s fun to have another new quantum state that we’ve never seen before — it presents us with new possibilities to try new things.”

The new discovery of quantum spin liquid state makes use of a fascinating aspect of physics, electron splitting. It certainly requires further experimentation in the future, which would help physicists uncover the mysterious world of quantum matter.

The findings were reported in the journal of Nature Materials.

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