Properties of the low dimensional Sr$_{2}$Cu(W$_{1-x}$Mo$_{x}$)O$_{6}$ spin system*

Low-dimensional spin systems have gained much attention in solid state physics. Such systems could have a ground state with no long-range magnetic order and an energy gap in the spin excitation spectrum, offering the possibility of a quantum spin-liquid phase. Quantum fluctuations causing the spin-liquid state are particularly strong in systems with reduced dimensionality and a low spin value; and magnetic frustration can further enhance the fluctuations. Among various low-dimensional spin systems, the S = 1/2 Heisenberg frustrated square lattice model is especially interesting due to its relevance to high-T$_{C}$ superconducting cuprates, whose undoped parent materials are S = 1/2 square-lattice antiferromagnets. Sr$_{2}$CuWO$_{6}$ and Sr$_{2}$CuMoO$_{6}$ have been found to be quasi-two-dimensional S = 1/2 magnetic systems with a square lattice of Cu-ions. These compounds show low-dimensional magnetic properties, with no clear indication of long-range order in magnetic susceptibility. I will discuss the materials properties and the observation of long range magnetism by neutron diffraction and other techniques.