THERMAL-DIFFUSION, INTERFACIAL THERMAL BARRIER, AND ULTRASONIC PROPAGATION IN YBA2CU3O7-X THIN-FILMS - SURFACE-SELECTIVE TRANSIENT-GRATING EXPERIMENTS

The application of interface-selective transient grating experiments to the measurement of anisotropic thermal diffusion constants in thin films of YBa2Cu3O7-x (Y-Ba-Cu-O) is reported. The 50-350-nm-thick Y-Ba-Cu-O films, grown on MgO(100) substrates, are studied using four experimental grating geometries. These provide a measurement of the time-dependent heat flow in the regions adjacent to the Y-Ba-Cu-O/MgO interface, the free film sirface, and the bulk of the film. Measurements yield a detailed understanding of the flow of heat from the film into the substrate as well as the direction-dependent diffusion constants. A temperature-dependent thermal barrier that significantly restricts heat transfer from the film into the substrate is observed and quantified. The rate of flow through Y-Ba-Cu-O/MgO interface is approximately 100 times less than the rate of flow in the Y-Ba-Cu-O film. The anisotropic diffusion constants inside the film have been measured in the 17-300-K temperature range, and the values are found to be similar to those obtained using conventional techniques on bulk samples. The frequency, wave vector, and acoustic damping rate of high-frequency (> 1-GHz) acoustic waves are measured in the same films.