Spin correlations in the S = 1/2 square-lattice antiferromagnet Sr₂CuTe_1-xW_xO₆ with a spin-liquid-like ground state

The S = 1/2 square-lattice Heisenberg model has a predicted, but never observed, quantum spin liquid ground state for J₂/J₁ = 0.4-0.6 [1]. Isostructural double perovskites Sr₂CuTeO₆ and Sr₂CuWO₆ are physical realisations of this model with Néel (J₁ ≫ J₂) and columnar (J₂ ≫ J₁) antiferromagnetic order, respectively. The solid solution Sr₂CuTe_1-xW_xO₆ has a spin-liquid-like ground state in a wide region of x = 0.05-0.6 [2]. Here we show using polarized neutron scattering, that the spin correlations in two spin-liquid-like samples x = 0.2 and 0.5 are distinctly different and related to the parent phases [3]. Moreover, our neutron diffraction experiment on the W-rich ordered samples reveals two magnetic phases with antiferromagnetic and ferromagnetic order along c. Disorder in the interlayer J_c is therefore the likely cause of the suppression of magnetic order at x ≈ 0.6. These results highlight the complex magnetism of Sr₂CuTe_1-xW_xO₆ and hint at a new quantum critical point at x ≈ 0.3, where the spin correlations cross over from Néel to columnar-like.

[1] P. W. Anderson, Science 235, 1196 (1987). [2] O. Mustonen et al., Nat. Commun. 9, 1085 (2018). [3] Mustonen et al., arXiv:2309.14422 (2023).