Valence-bond-glass state with a singlet gap in the spin-1/2 square-lattice random J_1-J_2 Heisenberg antiferromagnet Sr_2CuTe_(1-x)W_xO_6

摘要

The double-perovskite compounds Sr_2CuTeO_6, and Sr_2CuWO_6 are magnetically described as quasi-two-dimensional spin-1/2 square-lattice J_1-J_2 Heisenberg antiferromagnets with predominant J_1 and J_2 exchange interactions, respectively. We report the low-temperature magnetic properties of Sr_2CuTe_(1-x)W_xO_6 with randomness in the magnitudes of J_1 and J_2. It was found that the low-temperature specific heat for 0.1 ≤ x ≤ 0.5 has a large component proportional to the temperature T above 1.2 K, although the low-temperature specific heat for the two parent systems is approximately proportional to T~3. With decreasing temperature below 1.2 K, the T-linear component decreases rapidly toward zero, which is insensitive to the magnetic field up to 9 T. This is suggestive of the singlet excitation decoupled from the magnetic field. The NMR spectrum for x = 0.2 exhibits no long-range order down to 1.8 K. These results indicate that the ground slate of Sr_2CuTe_(1-x)W_xO_6, is a valence-bond-glass state with singlet gaps.