An international team of scientists, led by Dr. Arnab Banerjee from Oak Ridge National Laboratory, has found evidence of a mysterious state of matter – called a quantum spin liquid – in a 2D material.
The excitation of a spin liquid on a honeycomb lattice with neutrons. Image credit: Genevieve Martin, Oak Ridge National Laboratory.
This new state, first predicted more than four decades ago, causes electrons to break into pieces.
Dr. Banerjee and co-authors 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, reported in the journal Nature Materials, matched with one of the main theoretical models for a quantum spin liquid, known as a Kitaev model.
According to the team, “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 – fractionalization – in real materials is a breakthrough.”
“This is a new quantum state of matter, which has been predicted but hasn’t been seen before,” said co-author Dr. Johannes Knolle, from the University of Cambridge Cavendish Laboratory, UK.
In a typical magnetic material, the 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 the north magnetic poles point in the same direction, for example.
But in a material containing a spin liquid state, even if that material is cooled to absolute zero, the bar magnets would not align but 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 co-author Dr. Dmitry Kovrizhin, also from the University of Cambridge 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 physicists used neutron scattering techniques to look for experimental evidence of fractionalization in alpha-ruthenium chloride (a-RuCl3).
They tested the 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 match well with the broad humps instead of sharp lines which the scientists 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,” Dr. Knolle said.
“It’s an important step for our understanding of quantum matter. 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,” Dr. Kovrizhin added.
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A.Banerjee et al. Proximate Kitaev quantum spin liquid behaviour in a honeycomb magnet. Nature Materials, published online April 4, 2016; doi: 10.1038/nmat4604
