At its most fundamental level, classical physics considers liquids as being made up of atoms, ions or molecules that swirl and flow. In the quantum world, atomic spins can also be liquid-like. While the spins in these ‘quantum spin liquids’ are not mobile, the directions that they point in fluctuate, like a collection of overly excited Weebles. However, if these Weebles are to model quantum mechanical spins, their movements should be highly correlated and driven by quantum fluctuations rather than a classical stimulus, such as being poked by a cat.
Understanding quantum spin liquids is considered by many to be one of the grand challenges of physics and has been the focus of intense research for over 30 years. These exotic states of matter do not follow the classical rules of our everyday world. Instead, the laws of quantum mechanics define and control them, and this makes possible new and extraordinary types of behaviour.
Fluctuations play a key role in quantum spin liquids states, as they not only drive the spin dynamics, but are able to create excitations that are so well-defined that they act as particles themselves. They are correspondingly termed quasi-particles and their properties are quite different to those of the conventional building-blocks of matter. Correspondingly, entirely new directions for physics are being opened up. Possible applications include the emerging field of quantum information processing, where information is encoded in spins and quantum laws will govern how it is processed. It is easy to see why understanding the formation of quasi-particles and their new physics is captivating modern condensed matter physics.
One of the bottle-necks for this research is the scarcity of experimental materials which can be used to explore and test theoretical predictions. The finding of a new class, or flavour, of quantum spin liquids in the mineral kapellasite by an international team made up of researchers from France, Switzerland and the UK is attracting excitement as it could herald the discovery of new types of quantum properties.
In their article, Phys. Rev. Lett. 109, 037208 (2012), they combine data from a wide range of experimental techniques with theoretical modelling to characterize its spin correlations and their dynamics. Their results were very unexpected – they found that the spins form correlations based on an Archimedean solid: the highly symmetric cuboctahedron that is formed when the corners are taken from a cube to create a shape where every side is the same length. Dr Andrew Wills from the UCL and the LCN explained that to-date, research in this field had largely been based around simple models where the spins were only able to point up or down. Their latest work shows that very sophisticated ‘flavors’ of quantum spin liquids are possible, and raises the question of what quasi-particles and extraordinary physics they can host. ‘It is a very exciting time’, he said, ‘kapellasite has really opened our eyes to the breadth of the possibilities that are possible in the quantum world.’
As well as exploring the possibilities of quasi-particles in kapellasite, the team is working to discover other new types of quantum spin liquid.
Journal link: ‘Kapellasite: a kagome quantum spin liquid with competing interactions’, by B. Fåk, E. Kermarrec, L. Messio, B. Bernu, C. Lhuillier, F. Bert, P. Mendels, B. Koteswararao, F. Bouquet, J. Ollivier, A. D. Hillier, A. Amato, R. H. Colman, and A. S. Wills, Physical Review Letters 109, 037208 (2012)
Images from http://www.flickr.com/ and Physical Review Letters 109, 037208 (2012).