Novel Quantum Spin Liquid States in the S=1/2 Three-Dimensional Compound Y_{3}Cu_{2}Sb_{3}O_{14}

The three-dimensional S=1/2 system Y_{3}Cu_{2}Sb_{3}O_{14} consists of two inequivalent Cu^{2+} sites, each forming an edge shared triangular lattice. Our magnetic susceptibility χ(T), specific heat C_{p}(T), ^{89}Y 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 ^{89}Y 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.