This image shows the excitation with neutrons of a spin liquid on a honeycomb lattice. Image: Genevieve Martin, Oak Ridge National Laboratory.
This image shows the excitation with neutrons of a spin liquid on a honeycomb lattice. Image: Genevieve Martin, Oak Ridge National Laboratory.

An international team of researchers has found evidence of a mysterious new state of matter, first predicted 40 years ago, in a real material. This state, known as a quantum spin liquid, causes electrons – thought to be indivisible building blocks of nature – to break into pieces.

The researchers, including physicists from the University of Cambridge in the UK, measured the first signatures of these fractional particles, known as Majorana fermions, in a two-dimensional (2D) material with a structure similar to graphene. Their experimental results successfully matched one of the main theoretical models for a quantum spin liquid, known as a Kitaev model. The results are reported in Nature Materials.

Quantum spin liquids are mysterious states of matter thought to be hiding in certain magnetic materials, but they had not been conclusively sighted in nature. The observation of one of their most intriguing properties – electron splitting, or fractionalization – in real materials is a breakthrough. The resulting Majorana fermions could be used as building blocks for quantum computers, which would not only be far faster than conventional computers but able to perform calculations that they would find impossible.

"This is a new quantum state of matter, which has been predicted but hasn't been seen before," said Johannes Knolle of Cambridge's Cavendish Laboratory, one of the paper's co-authors.

In a typical magnetic material, the electrons each behave like tiny bar magnets. When a magnetic material is cooled to a low enough temperature, these individual 'bar magnets' will order themselves, such that all the north magnetic poles point in the same direction, for example. But in a material containing a spin liquid state, even when cooled to absolute zero, the bar magnets would not align. Instead, they would form an entangled soup caused by quantum fluctuations.

"Until recently, we didn't even know what the experimental fingerprints of a quantum spin liquid would look like," said paper co-author 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?’"

Knolle and Kovrizhin's co-authors, led by the Oak Ridge National Laboratory, used neutron scattering techniques to look for experimental evidence of fractionalization in crystals of the 2D material ruthenium chloride (RuCl3). The researchers tested the magnetic properties of the RuCl3 crystals by illuminating them with neutrons, and observing the pattern of ripples that the neutrons produced on a screen.

A regular magnet would create distinct sharp spots, but it was a mystery what sort of pattern the Majorana fermions in a quantum spin liquid would make. The theoretical prediction of distinct signatures made by Knolle and his collaborators in 2014 matched well with what experimentalists observed on the screen, providing for the first time direct evidence of a quantum spin liquid and the fractionalization of electrons in a 2D 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," added 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."

This story is adapted from material from the University of Cambridge, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.