Novel Quantum Spin Liquid States in the S=1/2 Three-Dimensional Compound Y₃Cu₂Sb₃O₁₄

The three-dimensional S ¼ 1 2 system Y3Cu2Sb3O14 consists of two inequivalent Cu2þ sites, each forming an edge shared triangular lattice. Our magnetic susceptibility χðTÞ, specific heat CpðTÞ, 89Y nuclear magnetic resonance (NMR), muon spin relaxation (μSR), and electron spin resonance (ESR) measurements on this system confirm the absence of any long-range magnetic ordering and the persistence of spin dynamics down to 0.077 K. In 89Y NMR, we find an anomaly at about 120 K, which we suggest arises from a fraction of the spins condensing into a singlet (a valence bond solid, or VBS) state. A plateau in the muon relaxation rate is observed between 60 K and 10 K (signifying the VBS state from a fraction of the spins) followed by an increase and another plateau below about 1 K (presumably signifying the quantum spin liquid state from all the spins). Our density functional theory calculations find a dominant antiferromagnetic interaction along the body diagonal with inequivalent Cu(1) and Cu(2) ions alternately occupying the corners of the cube. All other near-neighbor interactions between the Cu ions are also found to be antiferromagnetic and are thought to drive the frustration.