Citation: Norman, Michael R. "Colloquium: Herbertsmithite and the search for the quantum spin liquid." Reviews of Modern Physics 88.4 (2016): 041002.
Web: https://arxiv.org/abs/1604.03048
Tags: Expository, Physical, Hardware
This paper gives a lovely review of Herbertsmithite, which was proposed as a candidate for a Z2 spin liquid on the Kagome lattice. The story is interesting because it gives a great case study on how people look for topological materials. The first big paper for physicists was this one:
> Helton, J. S., et al. "Spin dynamics of the spin-1/2 kagome lattice antiferromagnet ZnCu 3 (OH) 6 Cl 2." Physical review letters 98.10 (2007): 107204.
It is argued that all of their results are consistent with the material being a spin liquid. For the following fourteen years, there were a series of experimental and numerical papers which went back and forth for arguments for both gapless and gapped behavior. For instance, papers like this one argued for spin liquid behavior:
> Fu, Mingxuan, et al. "Evidence for a gapped spin-liquid ground state in a kagome Heisenberg antiferromagnet." Science 350.6261 (2015): 655-658.
Finally, in 2019, it was settled that this material is gapless, and is not a spin-liquid in the relevant regimes, using clean NMR measurements:
> Khuntia, P., et al. "Gapless ground state in the archetypal quantum kagome antiferromagnet ZnCu3 (OH) 6Cl2." Nature Physics 16.4 (2020): 469-474.
A relevant aspect of the story is the role of theory and numerics. Broadly, the theory was missing. If we understood antiferromagnets on Kagome lattices better it could have sped this process up a lot, but we don't. Similarly, the numerics were of very limited use. The biggest numerics were done on 48 sites, with most being done on 36 sites or less. This is not enough to get a convincing picture from numerics. These are exactly the sorts of computations that quantum computers would be good for. I imagine that having a quantum computer would have shortened the story of Herbertsmithite, so that it wouldn't have taken fourteen years to figure out that it is gapless.
There's a lovely one-paragraph discussion of this topic which is friendly to theorists in Xiao-Gang's review article:
> Wen, Xiao-Gang. "Colloquium: Zoo of quantum-topological phases of matter." Reviews of Modern Physics 89.4 (2017): 041004.