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$_6$

TL;DR

This study investigates the low-temperature magnetic properties of a disordered square-lattice antiferromagnet, revealing a valence-bond-glass state with a singlet gap, characterized by a temperature-linear specific heat component unaffected by magnetic fields.

Contribution

It demonstrates the emergence of a valence-bond-glass state with singlet gaps in a disordered $J_1$-$J_2$ Heisenberg model, expanding understanding of quantum disordered ground states.

Findings

Low-temperature specific heat shows a $T$-linear component for $0.1 \\leq x \\leq 0.5$

The $T$-linear component diminishes below 1.2 K and is field-insensitive

No long-range magnetic order observed down to 1.8 K

Abstract

The double-perovskite compounds Sr$_2$CuTeO$_6$ and Sr$_2$CuWO$_6$ 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$_2$CuTe$_{1-x}$W$_x$O$_6$ with randomness in the magnitudes of $J_1$ and $J_2$. It was found that the low-temperature specific heat for $0.1\leq x \leq 0.5$ 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 indicates that the ground state of Sr$_2$CuTe$_{1-x}$W$_x$O$_6$ is a valence-bond-glass state with singlet gaps.