Spin-singlet ground state in the two-dimensional frustrated triangular lattice: YbAl₃C₃

We present the highly frustrated phenomena observed in YbAl ₃C₃ with a spin gap. The inelastic neutron spectra of low-lying magnetic excitations at low temperatures are quite similar to those of SrCu₂(BO₃)₂ and (CuCl)LaNb₂O ₇ that exhibit a dimer ground state in a two-dimensional quantum spin system. The structure of the first excited triplet state is directly clarified by inelastic neutron scattering experiments with a higher energy resolution. Furthermore, it becomes clear that the characteristic magnetic excitations remain within a high-temperature region. The result of the specific heat measurement also shows an anomaly associated with the formation of dimerized Yb ions. We conclude that a longrange magnetic order is suppressed by a large geometrical spin frustration, resulting in the stabilization of the dimer ground state.