Estimation of in-plane thermal diffusivity in YBCO bi-crystalline thin film by use of the low temperature laser scanning microscopy
MATSEKH Arkadiy, 木須 隆暢, 井上 昌睦 （九大）； 吉積 正晃, 須藤 泰範, 和泉 輝郎, 塩原 融 （SRL）
arkadiy*super.ees.kyushu-u.ac.jp
Abstract:We have experimentally investigated thermal diffusion in a 0.5 mm thick SrTiO3 based YBCO bicrystalline thin films by means of one-dimensional low temperature scanning microscopy （LTLSM）. Bolometric response of the sample to local heating produced by modulated laser beam radiation is obtained as a function of the beam position. Grain boundary （G.B.） being a weak link is temperature sensitive， thus it can be used as a local thermal sensor. In experiments DC bias current was higher than the GB critical current， but much lower than the intra-grain one. By changing modulation frequency we showed， that at 10 Hz modulation frequency， detected G.B. trace was found to be about 300 micrometers. Upon the frequency increase， the trace diminished and for 200 kHz was about 10 micrometers. Theoretical analysis shows that thermal length is inversely as the square root of the frequency， and phase delay is in direct p! roportion. We have shown phase delay between modulating and response signals does not depend on the thickness for 150 nm and 400 nm films examined. Experimentally obtained phase delay is in a good agreement with theoretical dependence. Effective thermal diffusivity coefficient calculated from the above mentioned dependence is close to the one of the SrTiO3 and indicates that heat diffusion is much influenced by the substrate. Hence this result demonstrates the potential of the LTLSM for obtaining effective in-plane thermal diffusivity of multilayered superconducting tapes and films.We also analyzed an important subsequent problem of this study which is the spatial resolution of the LTLSM. According to above results it can be as poor as several tens micrometers if determined by simple thermal healing length （how deep can heat penetrate into the sample）. To analyze the spatial resolution we made an LTLSM experiment with another extreme case of the uniform response from the whole sample within the superconductive transition using the sample as a non-linear resistor， generating uniform response to laser irradiation. The results obtained experimentally indicate， that spatial resolution can be as good， as 1-10 micrometers which is much better， than a simple thermal healing length. This analysis is also important for a fundamental understanding of the LTLSM. This work is partly supported by the New Energy and Industrial Technology Development Organization （NEDO） as Collaborative Research and Development of Fundamental Technologies for Superconductivity Applications， Project for Development of Materials & Power Application of Coated Conductors， M-PACC and also by JSPS: KAKENHI （20360143）.