paper

Extreme Suppression of Antiferromagnetic Order and Critical Scaling in a Two-Dimensional Random Quantum Magnet

arXiv:2006.15749 · doi:10.1103/PhysRevLett.126.037201

Abstract

Sr $_{2}$ CuTeO $_{6}$ is a square-lattice NÃ©el 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 $_{2}$ CuTe $_{1 - x}$ W $_{x}$ O $_{6}$ using neutron diffraction and $I^¼$ SR techniques, showing that the NÃ©el order vanishes already at $x = 0.025 \pm 0.005$ . 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 $_{2}$ CuTe $_{1 - x}$ W $_{x}$ O $_{6}$ , the $I^¼$ SR relaxation rate exhibits quantum critical scaling with a large dynamic exponent, $z \approx 3$ , consistent with a random-singlet state.

6 pages + 6 pages supplemental material