Abstract
A quantum spin liquid state has long been predicted to arise in spin-1/2 Heisenberg square-lattice antiferromagnets at the boundary region between Néel (nearest-neighbor interaction dominates) and columnar (next-nearest-neighbor interaction dominates) antiferromagnetic order. However, there are no known compounds in this region. Here we use d(10)-d(0) cation mixing to tune the magnetic interactions on the square lattice while simultaneously introducing disorder. We find spin-liquid-like behavior in the double perovskite Sr(2)Cu(Te(0.5)W(0.5))O(6), where the isostructural end phases Sr(2)CuTeO(6) and Sr(2)CuWO(6) are Néel and columnar type antiferromagnets, respectively. We show that magnetism in Sr(2)Cu(Te(0.5)W(0.5))O(6) is entirely dynamic down to 19 mK. Additionally, we observe at low temperatures for Sr(2)Cu(Te(0.5)W(0.5))O(6)-similar to several spin liquid candidates-a plateau in muon spin relaxation rate and a strong T-linear dependence in specific heat. Our observations for Sr(2)Cu(Te(0.5)W(0.5))O(6) highlight the role of disorder in addition to magnetic frustration in spin liquid physics.