The new compound K3Cu3AlO2(SO4)4, representing a spin 1/2 distorted diamond chain system, was synthesized and its magnetic properties were studied by magnetic susceptibility, specific heat, and high-field magnetization measurements. Magnetic Cu2+ ions were found to form a highly one-dimensional distorted diamond chain with strong intrachain interactions. Short-range magnetic correlations were observed to develop in a two-stage process without magnetic long-range ordering by magnetic susceptibility and specific heat measurements. The exchange constant ratio J2/J1, where J2 is the magnetic interaction of the diamond backbone and J1 is the magnetic interaction of the monomers along the chain, was estimated to be 2.5, demonstrating that the magnetic ground state is composed of an alternating dimer-monomer spin-liquid phase. The absence of an M = 0 plateau in the high-field magnetization curve is also consistent with an alternating dimer-monomer model. These results strongly suggest that K3Cu3AlO2(SO4)4 is the first material exhibiting an alternating dimer-monomer spin-liquid ground state.
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