Spin-singlet ground state in two-dimensional S=1/2 frustrated square lattice:: (CuCl)LaNb2O7

We report on the magnetic properties of double-layered perovskite (CuCl)LaNb2O7 with a square lattice of S = 1/2 Cu2+ ions, prepared by topotactic ion-exchange reactions. Magnetic susceptibility exhibits a thermal-activated behavior, and the analysis of the data suggests competing ferro- and anti ferromagnetic exchanges for first- and second-nearest-neighbor bonds, respectively. Neutron scattering experiments provide explicit evidence for a spin-singlet ground state with an energy gap of 2.3 meV. The well-defined low-lying triplet mode is almost independent of Q despite densely packed magnetic ions in the CuCl plane. In addition, on the basis of the isolated dimer model, we found that the intradimer distance is as long as 8.8 angstrom, which corresponds to the fourth-nearest-neighbor bond. These results indicate a sizable geometrical frustration that should impose severe constraints on the propagation of the triplet excitation, as in the case of SrCu2(BO3)(2). This argument is further reinforced by the observation of a collective-triplets bound state at 5.0 meV.