Size effect of out-of-plane thermal conductivity of epitaxial YBa2Cu3O7−δ thin films at room temperature measured by photothermal radiometry

Abstract

The out-of-plane (c-axis) thermal conductivities of high-temperature superconducting thin films (YBa2Cu3O7-δ: YBCO) have been measured by photothermal radiometry (PTR) at room temperature. The YBCO samples are in c-axis-aligned epitaxially grown thin films with thicknesses of 250, 500, and 1000 nm. PTR is a noncontact measurement technique for the thermal conductivity and is based on the detection of infrared radiation emitted from a sample heated by a frequency-modulated laser beam. By changing the modulation frequency up to about 1 MHz, the thermal conductivity of thin film can be determined by a curve-fitting analysis of phase-lag data in the frequency domain. The uncertainty of measured thermal conductivity is estimated to be better than ±7%. The experimental results for thermal conductivity exhibit apparently positive film thickness dependence, and their absolute values are less than half of those for single crystal at the smallest thickness. The results indicate a size effect that cannot be explained by the very short phonon mean free path that the kinetic theory predicts. By employing a simple model taking into account phonon boundary scattering, the possible mean free path to interpret the present results is substantially larger than the prediction. The conclusion supports the validity of quite broad spectral distribution of phonons responsible for the thermal conductivity of YBCO.