Breakdown of atomic spin-orbit coupling picture in an apparently isolated pseudo-one-dimensional iridate: Sr3 NaIrO 6

Abstract

In the presence of strong atomic spin-orbit coupling (SOC), tending to the j−j coupling limit, 5d4 iridates are speculated to possess a nonmagnetic Jeff=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+(5d4) 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 χ, 23Na nuclear magnetic resonance (NMR), muon spin relaxation/rotation (μSR), and heat capacity Cp 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 μSR 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 d4 iridates is unlikely to offer a ground state which will be consistent with a purely atomic j−j coupling description.