Kikuchi et al. Reply: In the preceding Comment [1], Gu and Su (GS) reported the finite temperature transfer matrix renormalization group (TMRG) method results for the distorted diamond chain (DDC) model. They pointed out that the double-peak behavior of T found in experiment cannot be reproduced by our parameter set J 1 :J 2 :J 3 1:1:25:0:45 [2], but well fitted by J 1 :J 2 :J 3z 1:1:9: ÿ 0:3 with J 3x =J 3z J 3y =J 3z 1:7.In response to GS's Comment, we have performed the additional density matrix renormalization group (DMRG) and the exact diagonalization calculations for the magnetization curve MH at T 0 of the DDC model with GS's parameter set. As can be seen from Fig. 1, the DMRG MH curve with GS's parameter set does not well explain the experimental results.The positional relations between Cu 2 ions corresponding to J 1 and J 3 are very similar to each other as can be seen in the schematic view of the crystal structure of Cu 3 CO 3 2 OH 2 in Fig. (1b) of our previous Letter [2]. The distance of two Cu 2 ions corresponding to J 1 is 327.5 pm with bond angle 113.7 and that to J 3 is 329.0 pm with bond angle 113.5 . Thus it is unlikely that J 1 is antiferromagnetic without the XXZ anisotropy while J 3 is ferromagnetic with strong XXZ anisotropy. Further, as far as we know, such a strong XXZ anisotropy has not been observed at all in the S 1=2 spin systems of Cu 2 ions.The double-peak behaviors of T and CT are not necessarily attributed to the frustration effect. The mechanism for the double-peak behaviors will be as follows. In the case of J 2 J 1 , jJ 3 j as lowering the temperature, spins coupled by J 2 are going to form singlet dimers at first. The remaining spins are nearly free because they are separated PRL 97,

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The magnetic susceptibility, high field magnetization, and specific heat measurements of Cu3(CO3)2(OH)2, which is a model substance for the frustrating diamond spin chain model, have been performed using single crystals. Two broad peaks are observed at around 20 and 5 K in both magnetic susceptibility and specific heat results. The magnetization curve has a clear plateau at one third of the saturation magnetization. The experimental results are examined in terms of theoretical expectations based on exact diagonalization and density matrix renormalization group methods. An origin of magnetic anisotropy is also discussed.

Azurite (Cu3(CO3)2(OH)2) is a model substance of a diamond spin chain, one of frustrated antiferromagnetic quantum spin chains. The high field magnetization up to 60 T, magnetic susceptibility, magnetic entropy, muon spin relaxation and 1 H-NMR of azurite have been measured using single crystals. A distinct 1/3 magnetization plateau is confirmed to be present in the magnetization curve. Two resonance peaks of 1 H-NMR were observed. By analyzing temperature dependencies of their hyperfine shift, we found two different local susceptibilities which correspond to susceptibility of dimer and monomer spins, respectively. New field induced phase transition was found to occur at a critical field where the magnetic plateau onsets. A short range magnetic ordering developed in a peculiar two-stage process, which was revealed by temperature dependence of the magnetic specific heat and susceptibility.