Thermal conductivity and spin state of the spin diamond-chain system azurite Cu₃(CO₃)₂(OH)₂

To investigate the spin state of azurite, Cu3(CO3)2(OH)2, we have measured the thermal conductivity along the c-Axis, κ_c, perpendicular to the spin diamond chains. It has been found that the temperature dependence of κ_c shows a broad peak at ∼100 K, which is explained as being due to the strong phonon scattering by the strong spin fluctuation owing to the spin frustration at low temperatures below ∼100 K. Furthermore, it has been found that the temperature dependence of κc shows another peak at low temperatures below 20K and that κ_c is suppressed by the application of magnetic field along the c-Axis at low temperatures below ∼35 K. In high magnetic fields above ∼8 T at low temperatures below ∼6K, it has been found that κ_c increases with increasing field. The present results have indicated that both spin-singlet dimers with a spin gap of ∼35K and antiferromagnetically correlated spin chains with the antiferromagnetic exchange interaction of ∼5.4K are formed at low temperatures, which is consistent with the recent conclusion by Jeschke et al. [Phys. Rev. Lett. 106, 217201 (2011)] that the ground state of spins in azurite in zero field is a spin-fluid one. In addition, a new quantum critical line in magnetic fields at temperatures above 3K has been proposed to exist.