SrCu2(BO3)2
Strontium copper borate is a unique material. The S=1/2 antiferroagnetic dimers in this one are located on a very peculiar frustrated lattice - so called Sutherland-Shastry lattice, with the dimers being pefectly orthogonal to each other. This geometry strongly suppresses the effect of interdimer interactions on the gapped ground state. What happens, however, once these couplings are tuned to the critical value is a big open problem. In SrCu2(BO3)2 it is possible to do such things experimentally by means of applying pressure to the sample. The difficulty is, the pressure needed is rather high!
Taking advantage of Raman-active "pantograph" mode that stretches the dimers, we have traced the spin-spin correlations through the pressure-induced quantum phase transition by means of optical spectroscopy. A remarkable sing change in the spin correlations was discovered, that awaits the theoretical explanation.
S. Bettler, L. Stoppel, Z. Yan, S. Gvasaliya and A. Zheludev, Sign switching of dimer correlations in SrCu2(BO3)2 under hydrostatic pressure, external page Phys. Rev. Research 2, 012010(R) (2020); external page arXiv:1907.09316