Han purple pigment loses a dimensionPI: Greg Boebinger, National High Magnetic Field LaboratoryFlorida State University, University of Florida, Los Alamos National Laboratory NSF Award Number: DMR-0084173 Han purple (BaCuSi206) was used more than 2,000 years ago to color the Chinese terracotta warriors. It also provides a model system for quantum spin physics: containing a stack of two-dimensional (2D) square lattices of paired electron spins. In high magnetic fields, the electron spins tend to align, undergoing a quantum phase transition at ~23 teslas that is the spin analog of Bose-Einstein condensation (BEC). Magnetization measurements up to 45T and down to 0.03 K at the National High Magnetic Field Laboratory show that the spins in Han purple lose a dimension at the QCP, going from 3D to 2D due to interlayer geometric frustration. Two-dimensionality is evidenced by linear power law scaling, e.g. by the linear temperature dependence of the magnetization near the transition. Dimensional reduction at a quantum phase transition might account for some of the mysterious properties of other strongly correlated materials near quantum critical points (e.g. the high temperature superconductors), but until this experiment on Han purple, a reduction in dimension at the onset of collective behavior had not been experimentally observed in any bulk material. Xi’an warriors superimposed on Han purple crystals grown at Stanford University (left). The high magnetic field phase diagram of the spin system, (below) showing the quantum critical point at 23T near which 2D behavior is observed (white region) in this model spin system. S.E. Sebastian, N. Harrison, C.D. Batista, L. Balicas, M. Jaime, P.A. Sharma, N. Kawashima, I.R. Fisher, Dimensional Reduction at a Quantum Critical Point,Nature 441 (2006) 617.