Static and dynamic spin properties in the quantum triangular lattice antiferromagnet Ag2CoO2

In Ag2CoO2, Co forms triangular lattice layers, which are separated by the metallic (Ag-2) block. The magnetic susceptibility and heat capacity measurements show that this material exhibits an antiferromagnetic transition at T-N = 17.5 K and the Weiss temperature (T-Theta) and the effective moment are -274 K and 1.62 mu(B), respectively, indicating that the Co ion carries spin (S) 1/2 and has a strongly frustrated state with T-Theta/T-N = 15.7. A density functional theory calculation confirmed that the valence state of the Co ions is 2+ and the low-spin state with S = 1/2 is realized at reduced on-site Coulomb interaction on Co. We performed elastic and inelastic neutron scattering experiments in a powder sample of Ag2CoO2. Although no noticeable magnetic Bragg peaks were observed below T-N, distinct magnetic excitations were observed in the inelastic neutron scattering experiments. The excitations are consistent with those expected for the S = 1/2 Heisenberg triangular lattice antiferromagnet. These results suggest that the ordered moment is reduced due to the quantum effect, which explains the absence of the magnetic Bragg peaks. Our results thus suggest that Ag2CoO2 is a good candidate to realize a quantum Heisenberg triangular lattice antiferromagnet.