I’ve been feeling I’ve not been posting enough. I’ll try to get back into it…
I’ve been studying quantum spin liquids for many years now. Or maybe I should say looking for them? Quantum spin liquids have both theoretical and experimental definitions. In theory, they are exotic phases of quantum matter whose ground states are intrinsically and inextricably in quantum superposition. From this quantum entanglement of the ground state they obtain the ability to support exotic excitations that behave like particles different from the elementary ones in our world: emergent non-local “quasiparticles”. In experiment, they are materials containing spins that avoid order at the lowest temperature, for which the conventional magnetic excitations – magnons – are absent, and which show some anomalous properties, e.g. unusual thermodynamics. Somehow it has been hard to bring these two meanings together. Either this is a failing of theory, a lack of the right measurements, or maybe we just haven’t found the right material yet.
The last option would make things easy – maybe we will just find a new system where the agreement is obvious. So I get excited when a new material with some promise appears. About a year ago, I heard from Gang Chen, my former student, now a professor at Fudan university in China, of YbMgGaO4. This is a material with a triangular lattice of Yb ions, each of which hosts an effective S=1/2 quantum spin. Typical of 4f rare earth ion spins, it has very short-range but highly anisotropic interactions with its neighbors. Guided by Gang, the experimentalists identified it as a spin liquid: at least the spins remain correlated but disordered down to millikelvin temperatures. How and why, remains to be understood.
One intriguing thing is some similarity to the thermodynamic properties observed in the more famous organic spin liquid materials, which have been studied for many years. And both share the triangular lattice structure. The reason the new YbMgGaO4 material is of particular interest, though, is a practical one. The Yb spins have large magnetic moments, and they are dense, so it is a good material for inelastic neutron scattering studies. This is the most powerful (i.e. data-rich) technique to study magnetism we know today, so this is a big deal. Moreover, single crystals big enough for such measurements are now available. The above images are from papers this year by two different groups, arXiv:1607.02615 (left), and arXiv:1607.03231 (right), which carried out the first such measurements, and show a smooth continuum of excitations, i.e. an apparent absence of conventional magnons. Nice pictures.
For now I won’t speculate (at least in writing!) about what’s going on, but it is certainly interesting and I’m sure it will be an active research subject for the near future. Let me also mention that in a material like this, the chemistry is largely independent of the rare earth element, so Yb can be substituted for almost any other 4f atom. This means a wide variety of magnetic systems can be found with identical geometry. So YbMgGaO4 is just the first of many new materials we can probably expect to see studied soon. Something to look forward to!