Novel Quantum Spin Liquid states in the S = 1/2 three-dimensional compound Y₃Cu₂Sb₃O₁₄

The three-dimensional $S = 1/2$ system Y₃Cu₂Sb₃O₁₄ consists of two inequivalent Cu²⁺ ions, each forming edge shared triangular lattices. Our magnetic susceptibility $χ(T)=M/H$, specific heat Cₚ(T), ⁸⁹Y nuclear magnetic resonance (NMR), muon spin relaxation ($μ$SR), 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. From ⁸⁹Y NMR we find evidence of a transition at about 120 K which we suggest to arise from a fraction of the spins condensing into a singlet (a valence bond solid VBS or a quantum spin liquid QSL) state. A plateau in the muon relaxation rate is observed between 60 K and 10 K (signifying the VBS/QSL state from a fraction of the spins) followed by an increase and another plateau below about 1 K (presumably signifying the VBS/QSL 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 neighbour interactions between the Cu ions are also found to be antiferromagnetic and are thought to drive the frustration.