Specific-heat and magnetocaloric-effect study of the S=1/2 frustrated-chain antiferromagnet Cu$_{3}$(CO$_{3})_{2}$(OH)$_{2}$*

Azurite, Cu$_{3}$(CO$_{3})_{2}$(OH)$_{2}$, is a natural mineral in which the S=1/2 spins of Cu$^{2+}$ form frustrated chains. The magnetization of this material exhibits a plateau at 1/3 of the saturation value, at magnetic fields between 16 T and 26 T applied in the chain direction, and the specific heat at zero field shows a broad peak at 4 K followed by a sharp peak at 1.8 K [1]. We have measured specific heat up to 18 T and found that the broad peak gets smaller with increasing field and disappears above 10 T. The sharp peak begins to separate into a peak and a shoulder at 5 T, and the temperature difference between these anomalies becomes larger up to 13 T. These observations are well supported by magnetocaloric-effect data taken at the same time. In the plateau region, an exponential temperature dependence is observed in the specific heat at low temperatures, indicating an energy gap for low-lying excited states. Detailed specific-heat and magnetocaloric-effect data and the magnetic phase diagram up to 18 T will be presented. [1] H. Kikuchi \textit{et al,} Phys. Rev. Lett. \textbf{94}, 227201 (2005)