Breakdown of atomic spin-orbit coupling picture in an apparently isolated pseudo-one-dimensional iridate: Sr3NaIrO6

In the presence of strong atomic spin-orbit coupling (SOC), tending to the j - j coupling limit, 5d(4) iridates are speculated to possess a nonmagnetic J(eff) = 0 singlet ground state from atomic consideration, which invariably gets masked due to different solid-state effects (e.g., hopping). Here, we try to probe the trueness of the atomic SOC-based proposal in an apparently one-dimensional system, Sr3NaIrO6, with well-separated Ir5+ (5d(4)) ions. But all the detailed experimental as well as theoretical characterizations reveal that the ground state of Sr3NaIrO6 is not nonmagnetic. However, our combined dc susceptibility chi, Na-23 nuclear magnetic resonance (NMR), muon spin relaxation/rotation (mu SR), and heat capacity C-p measurements clearly refute any sign of spin freezing or ordered magnetism among the Ir5+ moments due to geometrical exchange frustration, while in-depth zero-field and longitudinal field mu SR investigations strongly point towards an inhomogeneous quantum spin liquid (QSL)-like ground state. In addition, the linear temperature dependence of both the NMR spin-lattice relaxation rate and the magnetic heat capacity at low temperatures suggest low-lying gapless spin excitations in the QSL phase of this material. Finally, we conclude that the effective SOC realized in d(4) iridates is unlikely to offer a ground state which will be consistent with a purely atomic j - j coupling description.