スピン1/2三角格子反強磁性体Li₂AMo₃O₈の非弾性中性子散乱

抄録

Li₂AMo₃O₈ (A = In or Sc) is a molecular based spin-1/2 triangular lattice system where spin-1/2 Mo₃O₁₃ clusters in place of Mo ions form the uniform triangular lattice. Their ground states are different according to the A site. Li₂InMo₃O₈ undergoes conventional 120° long-range magnetic order below T_N = 12 K whereas isomorphic Li₂ScMo₃O₈ exhibits no long-range magnetic order down to 0.5 K. Here, we report exotic magnetisms in Li₂InMo₃O₈ and Li₂ScMo₃O₈ investigated by inelastic neutron scattering (INS) using polycrystalline samples. Li₂InMo₃O₈ and Li₂ScMo₃O₈ show completely different INS spectra, representing their different ground states. Li₂InMo₃O₈ exhibits spin wave excitation which is quantitatively described by the nearest neighbor anisotropic Heisenberg model based on the 120° spin structure. In contrast, Li₂ScMo₃O₈ undergoes short-range magnetic order below 4 K with quantum-spin-liquid-like magnetic fluctuations down to the base temperature. Origin of the different ground states is discussed in terms of anisotropies of crystal structures and magnetic interactions.