Valence-Bond-Glass State with Singlet Gap in the Spin-1/2 Square-Lattice Random $J_1$-$J_2$ Heisenberg Antiferromagnet Sr$_2$CuTe$_{1-x}$W$_x$O$_6$

Abstract

The double-perovskite compounds Sr2CuTeO6 and Sr2CuWO6 are magnetically described as quasi-two-dimensional spin-1/2 square-lattice J1−J2 Heisenberg antiferromagnets with predominant J1 and J2 exchange interactions, respectively. We report the low-temperature magnetic properties of Sr2CuTe1−xWxO6 with randomness in the magnitudes of J1 and J2. It was found that the lowtemperature 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 state of Sr2CuTe1−xWxO6 is a valence-bond-glass state with singlet gaps.