Spin excitations in the two-dimensional strongly coupled dimer system malachite
Résumé
The mineral malachite, Cu-2(OD)(2)CO3, 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'/J(1) approximate to 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.