Quantum disordered state in the J₁-J₂ square-lattice antiferromagnet Sr₂Cu(Te_0.95W_0.05)O₆

The B-site ordered double perovskites Sr₂Cu(Te_1−xW_x)O₆ provide an excellent arena for investigating exotic phases expected for the J₁-J₂ square-lattice Heisenberg antiferromagnet. Here, combining magnetic susceptibility and specific-heat measurements with electron spin resonance (ESR) and muon spin rotation/relaxation (μSR) techniques, we explore a spin-liquid-like state in the vicinity of the Néel critical end point (x = 0.05–0.1). The specific heat and the ESR and muon relaxation rates give evidence for an energy hierarchy of low-energy excitations, reminiscent of randomness-induced singlet states. In addition, the weak transverse μSR data show a fraction of frozen magnetic moments in the random-singlet background. The origin of a random-singlet-like state near the phase boundary is discussed in terms of concomitant exchange randomness and local strain generated by the W⁶⁺-for-Te⁶⁺ substitution.