Spin excitations in the two-dimensional strongly coupled dimer system   malachite
  The mineral malachite, Cu2(OD)2CO3, has a quantum spin-liquid ground state and no long-range magnetic order down to at least T=0.4 K. Inelastic neutron scattering measurements show that the excitation spectrum consists of dispersive gapped singlet-triplet excitations, characteristic of spin-1/2 dimer-forming Heisenberg antiferromagnets. We identify a new two-dimensional dimerized coupling scheme with strong interdimer coupling J'/J1~0.3 that places malachite between strongly coupled alternating chains, square lattice antiferromagnets, and infinite-legged ladders. The geometry of the interaction scheme resembles the staggered dimer lattice, which may allow unconventional quantum criticality. 
E. Canevet, B. Fak, R. K. Kremer, J. H. Chun, M. Enderle, E. E.   Gordon, J. L. Bettis, M.-H. Whangbo, J. W. Taylor, D. T. A...
cond-mat.str-el