Advanced ac calorimetric technique for thermal diffusivity measurement at cryogenic temperatures

We have developed a new technique to research thermal transport properties of solid materials at cryogenic temperatures. It is based on an optical ac calorimetry for the measurement of thermal diffusivity. This paper presents new ac calorimeters with sophisticated laser heating systems that realize versatile intensity-modulation and beam-scan functions. They can provide three different measuring methods, a frequency-variation method, a distance-variation method, and a new type of constant wave number (CWN) method for various requirements. Typical experimental results for graphite materials, oxide superconductors and others are also presented. The CWN method was successfully tested to detect a small but clear anomaly accompanying with superconducting transition of Hg-1223. A large uni-axial anisotropy was observed for highly oriented pyrolitic graphite (HOPG) and pyrolytic graphite sheet (GS). The similar but rather small uni-axial anisotropy was observed for oxide superconductors, Y-123 single crystal and Bi-2212 single crystal. Y-123 was measured to further show a weak in-plane anisotropy.