Unusual Magnetic State with Dual Magnetic Excitations in the Single Crystal of S = 1/2 Kagome Lattice Antiferromagnet CaCu3(OH)6Cl2 ∙ 0.6H2O

We have succeeded in preparing single crystals of CaCu3(OH)(6)Cl-2 center dot 0.6H(2)O, a candidate for the S = 1/2 Kagome lattice antiferromagnet. Magnetic properties of the compound are dominated by the nearest neighbor antiferromagnetic interaction J(1), and the next nearest neighbor ferromagnetic J(2) and an antiferromagnetic J(d) across a hexagon, which is different from related compounds Kapellasite and Haydeeite with ferromagnetic J(1). Magnetic susceptibility exhibits a sudden increase below 13K and a cusp anomaly at T* = 7.2K in the ab-plane, whereas only a moderate enhancement is observed below T* along the c-axis. A tiny peak detected in heat capacity at T* indicates the occurrence of a magnetic phase transition. The low temperature magnetic heat capacity was reproduced by assuming a two-dimensional spin-wave component and a temperature-linear term. The spin-wave contribution suggests a magnon excitation in a short-range ordered region, whereas the relatively large T-linear term 5.9 mJ/(Cu-mol center dot K-2) at H = 0T of this insulating compound suggests the existence of an unusual quasi-particle excitation below T*. They apparently reveal the unconventionality of the ground state of this S = 1/2 Kagome lattice antiferromagnet.