PHCC
Piperazinium hexachlorodicuprate, or PHCC for short, is a pretty amazing dimer system. At normal conditions the couplings between the individual S=1/2 antiferromagnetic dimers are not strong enough to break the singlet-like ground state and the material is gapped. However, moderate magnetic field or hydrostatic pressure may close the spin gap. While the former scenario is a rather typical example of "magnetic Bose-Einstein condensation", the latter has a twist: by increasing the pressure one actually drives PHCC through a cascade of rather different magnetically ordered states! This can be easily captured by muSR, neutrons or even light scattering.
Another scenario for PHCC is related to the possibility of replacing chlorine with bromine. This modifies the superexchange between the copper ions and thus creates disorder in the magnetic bonds. Studying the fate of boson condensation in the presence of randomness is another exciting research topic.
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V. N. Glazkov, T. S. Yankova, J. Sichelschmidt, D. Hüvonen, A. Zheludev, Electron spin resonance study of anisotropic interactions in a two-dimensional spin gap magnet PHCC, external pagePhys. Rev. B 85, 054415 (2012); external pagearXiv:1111.3223v1