Emergent Dynamic Magnetic Ground State in a Mixed 3d/5d Heavy Fermion System CaCu3Ir4O12

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Quantum-disordered magnetic ground states are challenging to identify in three-dimensional (3D) oxides, where strong exchange pathways typically favour long-range magnetic order or spin freezing. The quadruple perovskite CaCu₃Ir₄O₁₂, crystallizing in the cubic Im3 structure, provides a 3D lattice where Cu²⁺ 3d moments are coupled to an extended Ir 5d network, offering a rare platform for probing quantum-disordered magnetism in a mixed 3d/5d electron system. Here, we combine bulk probes, including DC and AC magnetic susceptibility, and heat capacity measurements (down to 50 mK), along with the local microscopic probe muon spin relaxation (μSR) (down to 40 mK), to investigate the true magnetic ground state of CaCu₃Ir₄O₁₂. Despite strong antiferromagnetic interactions (θW -200 K, with an applied-field dependence), no signature of long-range magnetic ordering or spin freezing is detected down to the lowest measured temperatures. Furthermore, our in-depth zero-field (ZF) and longitudinal-field (LF) μSR characterizations confirm strong quantum spin fluctuations and the truly dynamic nature of the local moments down to 40 mK. These results establish CaCu₃Ir₄O₁₂ as a promising 3D quantum-disordered magnet and a well-characterized platform for exploring fluctuation-dominated states in correlated 3d/5d oxides.