barium copper silicate enters a new state-called the quantum critical point, the three-dimensional material "loses" a dimension to form a 'Flatland' (of sorts). Just as in the novella Flatland that posited a planar world, the spins strongly interact only in two dimensions. Effects from the third dimension are negligible.
Dimensional reduction at a quantum critical point. 2006. S. E. Sebastian et al. Nature 441: 617-620.