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

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 $\mathrm{CaCu_3Ir_4O_{12}}$, crystallizing in the cubic $Im\bar{3}$ structure, provides a 3D lattice where $\mathrm{Cu^{2+}}$ $3d$ 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 $50~\mathrm{mK}$), along with the local microscopic probe muon spin relaxation ($\mu$SR) (down to $40~\mathrm{mK}$), to investigate the true magnetic ground state of $\mathrm{CaCu_3Ir_4O_{12}}$. Despite strong antiferromagnetic interactions ($\theta_{\mathrm{W}} \sim -200~\mathrm{K}$, 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) $\mu$SR characterizations confirm strong quantum spin fluctuations and the truly dynamic nature of the local moments down to $40~\mathrm{mK}$. These results establish $\mathrm{CaCu_3Ir_4O_{12}}$ as a promising 3D quantum-disordered magnet and a well-characterized platform for exploring fluctuation-dominated states in correlated $3d/5d$ oxides.