Extreme suppression of antiferromagnetic order and critical scaling in a two-dimensional random quantum magnet
Hong, Wenshan; Liu, Lu; Liu, Chang; Ma, Xiaoyan; Koda, Akihiro; Li, Xin; Song, Jianming; Yang, Wenyun; Yang, Jinbo; Cheng, Peng; Zhang, Hongxia; Bao, Wei; Ma, Xiaobai; Chen, Dongfeng; Sun, Kai; Guo, Wenan; Luo, Huiqian; Sandvik, Anders W.; Li, Shiliang
Sr_2CuTeO_6 is a square-lattice Néel 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 Sr_2CuTe_1-xW_xO_6 using neutron diffraction and μSR techniques, showing that the Néel order vanishes already at x=0.025±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 Sr_2CuTe_1-x W_x O_6, the μSR relaxation rate exhibits quantum critical scaling with a large dynamic exponent, z≈3, consistent with a random-singlet state.
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