We report the crystal structure and unconventional magnetic ordering of Cs_2Cu_3CeF_{12}, which is composed of buckled kagome lattice of Cu^{2+} ions. The exchange network in the buckled kagome lattice is fairly anisotropic, so that the present spin
system can be divided into two subsystems: alternating Heisenberg chains with strong antiferromagnetic exchange interactions and dangling spins. Although the direct exchange interactions between neighboring spins were found to be all antiferromagnetic, ferromagnetic magnetic ordering of the dangling spins was observed. Magnetization exhibits a plateau at one-third of the saturation magnetization. These observations can be understood in terms of the indirect interaction between dangling spins mediated by the chain spin.
Hexagonal antiferromagnets Cs$_2$Cu$_3$MF$_{12}$ (M = Zr, Hf and Sn) have uniform Kagome lattices of Cu$^{2+}$ with S = 1/2, whereas Rb$_2$Cu$_3$SnF$_{12}$ has a 2a by 2a enlarged cell as compared with the uniform Kagome lattice. The crystal data of
Cs$_2$Cu$_3$SnF$_{12}$ synthesized first in the present work are reported. We performed magnetic susceptibility measurements on this family of Kagome antiferromagnet using single crystals. In the Cs$_2$Cu$_3$MF$_{12}$ systems, structural phase transitions were observed at $T_t = 225$ K, 172 K and 185 K for M = Zr, Hf and Sn, respectively. The magnetic susceptibilities observed for $T > T_t$ are almost perfectly described using theoretical results obtained by exact diagonalization for the 24-site Kagome cluster with $J/k_B = 244$ K, 266 K and 240 K, respectively. Magnetic ordering accompanied by the weak ferromagnetic moment occurs at $T_N = 23.5$ K, 24.5 K and 20.0 K, respectively. The origins of the weak ferromagnetic moment should be ascribed to the lattice distortion that breaks the hexagonal symmetry of the exchange network for $T < T_t$ and the Dzyaloshinsky-Moriya interaction. Rb$_2$Cu$_3$SnF$_{12}$ is magnetically described as a modified Kagome antiferromagnet with four types of neighboring exchange interaction. Neither structural nor magnetic phase transition was observed in Rb$_2$Cu$_3$SnF$_{12}$. Its magnetic ground state was found to be a spin singlet with a triplet gap. Using exact diagonalization for a 12-site Kagome cluster, we analyzed the magnetic susceptibility and evaluated individual exchange interactions. The causes leading to the different ground states in Cs$_2$Cu$_3$SnF$_{12}$ and Rb$_2$Cu$_3$SnF$_{12}$ are discussed.
We synthesized single crystals of the new hexagonal compounds A$_2$Cu$_3$SnF$_{12}$ with A=Cs and Rb, and investigated their magnetic properties. These compounds are composed of Kagom{e} layers of corner-sharing CuF$_6$-octahedra. Cs$_2$Cu$_3$SnF$_{1
2}$ has the proper Kagom{e} layer at room temperature, and undergoes structural phase transition at $T_mathrm{t}simeq 185$ K. The temperature dependence of the magnetic susceptibility in Cs$_2$Cu$_3$SnF$_{12}$ agrees well with the result of the numerical calculation for $S=1/2$ two-dimensional Heisenberg Kagom{e} antiferromagnet down to $T_mathrm{t}$ with the nearest exchange interaction $J/k_mathrm{B}simeq 240$ K. Although the magnetic susceptibility deviates from the calculated result below $T<T_mathrm{t}$, the rounded maxima were observed at approximately $Tsimeq (1/6)J/k_mathrm{B}$ as predicted by the theory. Cs$_2$Cu$_3$SnF$_{12}$ undergoes three-dimensional magnetic ordering at $T_mathrm{N} = 20$ K.
We investigated the crystal structure of Rb$_2$Cu$_3$SnF$_{12}$ and its magnetic properties using single crystals. This compound is composed of Kagome layers of corner-sharing CuF$_{6}$ octahedra with a 2a x 2a enlarged cell as compared with the prop
er Kagome layer. Rb$_2$Cu$_3$SnF$_{12}$ is magnetically described as an $S$=1/2 modified Kagome antiferromagnet with four kinds of neighboring exchange interaction. From magnetic susceptibility and high-field magnetization measurements, it was found that the ground state is a disordered singlet with the spin gap, as predicted from a recent theory. Exact diagonalization for a 12-site Kagome cluster was performed to analyze the magnetic susceptibility, and individual exchange interactions were evaluated.
Soft and hard X-ray photoelectron spectroscopy (PES) has been performed for one of the heavy fermion system CeRu$_2$Si$_2$ and a $4f$-localized ferromagnet CeRu$_2$Ge$_2$ in the paramagnetic phase. The three-dimensional band structures and Fermi surf
ace (FS) shapes of CeRu$_2$Si$_2$ have been determined by soft X-ray $h u$-dependent angle resolved photoelectron spectroscopy (ARPES). The differences in the Fermi surface topology and the non-$4f$ electronic structures between CeRu$_2$Si$_2$ and CeRu$_2$Ge$_2$ are qualitatively explained by the band-structure calculation for both $4f$ itinerant and localized models, respectively. The Ce valences in CeRu$_2X_2$ ($X$ = Si, Ge) at 20 K are quantitatively estimated by the single impurity Anderson model calculation, where the Ce 3d hard X-ray core-level PES and Ce 3d X-ray absorption spectra have shown stronger hybridization and signature for the partial $4f$ contribution to the conduction electrons in CeRu$_2$Si$_2$.
