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Thermal Conductivity and Spin State of the Spin Diamond-Chain System Azurite Cu3(CO3)2(OH)2

In order 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 $κ$c shows another peak at low temperatures below 20 K 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 ~ 6 K, 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 ~ 35 K and antiferromagnetically correlated spin-chains with the antiferromagnetic exchange interaction of ~ 5.4 K 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 3 K has been proposed to exist.