The excitation of a spin liquid on a honeycomb lattice with neutrons. Credit: Image courtesy of University of Cambridge
This state, quantum spin liquid, causes electrons – thought to be indivisible building blocks of nature – to break into pieces. The researchers, including physicists from the Uni of Cambridge, measured the first signatures of these fractional particles, known as Majorana fermions, in a 2D material with a structure similar to graphene. Their experimental results successfully matched with one of the main theoretical models for a quantum spin liquid, known as a Kitaev model. Quantum spin liquids are mysterious states of matter which are thought to be hiding in certain magnetic materials, but had not been conclusively sighted in nature.
The observation of one of their most intriguing properties – electron splitting, or fractionalisation – in real materials is a breakthrough. The resulting Majorana fermions may be used as building blocks of quantum computers, which would be far faster than conventional computers and would be able to perform calculations that could not be done otherwise.
In a typical magnetic material, electrons each behave like tiny bar magnets. And when a material is cooled to a low enough temperature, the ‘magnets’ will order themselves over long ranges, so that all magnetic poles point in the same direction. But in a material containing a spin liquid state, even if that material is cooled to 0K, the bar magnets would not align but form an entangled soup caused by quantum fluctuations.
Knolle and Kovrizhin’s co-authors, led by Dr Arnab Banerjee and Dr Stephen Nagler from Oak Ridge National Lab used neutron scattering techniques to look for experimental evidence of fractionalisation in alpha-ruthenium chloride (a-RuCl3). They tested magnetic properties of a-RuCl3 powder by illuminating it with neutrons, and observing the pattern of ripples that the neutrons produced on a screen when they scattered from the sample.
A regular magnet would create distinct sharp lines, 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 by Knolle et al in 2014 match well with the broad humps instead of sharp lines which experimentalists observed on the screen, providing for the first time direct evidence of a quantum spin liquid and the fractionalisation of electrons in a 2D material.
“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.” http://www.cam.ac.uk/research/news/new-state-of-matter-detected-in-a-two-dimensional-material
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