The mineral malachite, $\mathrm{Cu}{}_2\left(\mathrm{OD}\right){}_2\mathrm{CO}{}_3$, 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 distinct two-dimensional dimerized coupling scheme with strong interdimer coupling $J^{\prime }/J_1\approx 0.3$ 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.

• Received 18 December 2014

DOI:https://doi.org/10.1103/PhysRevB.91.060402