Magnetic liquid all in a spin

Two-dimensional materials have revealed the fingerprint of a new and apparently mysterious quantum state of matter in which electrons seemingly break apart, a phenomenon that was predicted forty years ago. The new state, a quantum spin liquid, displays the characteristics predicted for Majorana fermions in a two-dimensional material with a structure similar to graphene.

Physicists from the University of Cambridge, England, and their colleagues say that the measurements of Majorana fermions in alpha-ruthenium chloride honeycomb magnet match the main theoretical model for a quantum spin liquid, the Kitaev model. [A. Banerjee et al. Nature Mater. (2016). DOI: 10.1038/nmat4604].

The observation of one of the most intriguing properties of such materials, electron splitting, or more properly fractionalization, represents a breakthrough in this field. It might be possible to use Majorana fermions as the building blocks of a quantum computer. "This is a new quantum state of matter, which has been predicted but hasn't been seen before," explains team member Johannes Knolle.

Of course, in a conventional magnetic material, the electrons behave like tiny bar magnets and on cooling the material to a sufficiently low temperature, the poles of these "magnets" align over a long range. In a material containing a spin liquid state, cooling it to as close to absolute zero as is physically possible does not lead to alignment of the electrons and instead they exist in an entangled "soup" because of quantum fluctuations.

"Until recently, we didn't even know what the experimental fingerprints of a quantum spin liquid would look like," explains co-author and condensed matter scientist Dmitry Kovrizhin. "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?"

Colleagues Arnab Banerjee and Stephen Nagler of the Oak Ridge National Laboratory, Tennessee, USA, used neutron scattering experiments to search for the evidence of fractionalization in a powder of the material. A conventional magnetic material would generate sharp lines in the scattering pattern, but the researchers did not know precisely what to expect from Majorana fermions in a quantum spin liquid. However, the pattern that emerged matched theoretical predictions of distinct signatures by Knolle and his collaborators carried out in 2014. Broad humps were apparent rather than sharp lines providing the first direct evidence of a quantum spin liquid and the fractionalization of electrons in a two dimensional material.

"This is a new addition to a short list of known quantum states of matter," explains Knolle. "It's an important step for our understanding of quantum matter," adds 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."

David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the popular science book "Deceived Wisdom".