Extreme Suppression of Antiferromagnetic Order and Critical Scaling in a Two-Dimensional Random Quantum Magnet
Wenshan Hong, Lu Liu, Chang Liu, X. Y. Ma, A. Koda, Xin Li, Song Jianming, Wenyun Yang, Jinbo Yang, Peng Cheng, Hongxia Zhang, Wei Bao, Xiaobai Ma, Dongfeng Chen, Kai Sun, Wenan Guo, Huiqian Luo, Anders W. Sandvik, Shiliang Li
Abstract
${\mathrm{Sr}}_{2}{\mathrm{CuTeO}}_{6}$ is a square-lattice N\'eel antiferromagnet with superexchange between first-neighbor $S=1/2$ Cu spins mediated by plaquette centered Te ions. Substituting Te by W, the affected impurity plaquettes have predominantly second-neighbor interactions, thus causing local magnetic frustration. Here we report a study of ${\mathrm{Sr}}_{2}{\mathrm{CuTe}}_{1\ensuremath{-}x}{\mathrm{W}}_{x}{\mathrm{O}}_{6}$ using neutron diffraction and $\ensuremath{\mu}\mathrm{SR}$ techniques, showing that the N\'eel order vanishes already at $x=0.025\ifmmode\pm\else\textpm\fi{}0.005$. We explain this extreme order suppression using a two-dimensional Heisenberg spin model, demonstrating that a W-type impurity induces a deformation of the order parameter that decays with distance as $1/{r}^{2}$ at temperature $T=0$. The associated logarithmic singularity leads to loss of order for any $x>0$. Order for small $x>0$ and $T>0$ is induced by weak interplane couplings. In the nonmagnetic phase of ${\mathrm{Sr}}_{2}{\mathrm{CuTe}}_{1\ensuremath{-}x}{\mathrm{W}}_{x}{\mathrm{O}}_{6}$, the $\ensuremath{\mu}\mathrm{SR}$ relaxation rate exhibits quantum critical scaling with a large dynamic exponent, $z\ensuremath{\approx}3$, consistent with a random-singlet state.