Thermal diffusion, interfacial thermal barrier, and ultrasonic propagation in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathit{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 ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$ (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 surface, 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 ∼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.