陰山 洋 氏 （京都大学大学院理学研究科）
"Spin-Liquid State in Two-Dimensional Frustrated Square Lattice System (CuCl)LaNb2O7"
Soft chemical approach carried out under relatively mild conditions allows access to metastable phases with novel physical and chemical features, providing a diverse range of applications that include catalysis, ionic conductions and low-dimensional magnetism [1]. My talk will deal with (CuCl)LaNb2O7, a double-layered perovskite of the Dion-Jacobson type that can be obtained by the ion-exchange reaction between RbLaNb2O7 and CuCl2. Here, the Cu2+ ions carrying S = 1/2 form the square lattices which are separeted with respect to each other by the non-mangetic perovskite slabs. The magnetic susceptibility and inelastic neutron scattering experiments of a powder sample of (CuCl)LaNb2O7 have revealed that the ground state is in a spin-liquid phase with an energy gap of 2.3 meV to the lowlying excited state [2]. Interestingly, the dispersion of the triplet excitation is nearly flat, in spite of the two-dimensional magnetic network. We consider that the geometrical frustration most likely arising from antiferromagnetic next-nearest bonds makes the triplet excitation extremely localized, as in the case of the Shastry-Suhterland compond SrCu(BO)2 [3]. To unveil the properties of the spin-liquid state, we recently measured the magnetization at low temperatures. It is found that the spin gap closes at a surprisingly low filed of 10.3 T than that expected from the zero-field spin gap. Moreover, the gapless phase is stable over a wide range of field, suggesting strong correlations of triplet excitations. If time allows, I wish to show some of the results of other experiments (specific heat, muon spin resonance, X-ray diffraction etc.).
[1] H. Kageyama et al.; J. Phys.: Condens. Matter 16 (2004) S590.
[2] H. Kageyama et al.; J. Phys. Soc. Jpn. 74 (2005) 1702.
[3] H. Kageyama et al., Phys. Rev. Lett. 84 (2000) 5876.