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We investigated the $5d$ element substitution effect on the thermal conductivity in $\mathrm{F}{\mathrm{e}}_{2}\mathrm{VAl}$-based superlattice thin films epitaxially grown on a MgO (100) substrate. We found unique crossover behavior in the period dependences of thermal conductivity for Ta-free $\mathrm{F}{\mathrm{e}}_{2}\mathrm{VAl}$ and $\mathrm{F}{\mathrm{e}}_{2}(\mathrm{V},\mathrm{Ta})\mathrm{Al}$-based superlattices. The $\mathrm{F}{\mathrm{e}}_{2}(\mathrm{V},\mathrm{Ta})\mathrm{Al}$-based superlattices with periods more than 40 nm appeared to have much lower thermal conductivity than Ta-free $\mathrm{F}{\mathrm{e}}_{2}\mathrm{VAl}$-based superlattices due to the substitution of V by Ta. Unexpectedly, at a shorter periodic length, less than 20 nm, $\mathrm{F}{\mathrm{e}}_{2}(\mathrm{V},\mathrm{Ta})\mathrm{Al}$-based superlattices appeared to have higher thermal conductivity than Ta-free $\mathrm{F}{\mathrm{e}}_{2}\mathrm{VAl}$-based superlattices despite such a heavy element substitution by Ta. This surprising experimental fact was well accounted for with theoretical calculations, which predicted the dominant contribution of phonons having a shorter mean free path in the Ta substituted samples. more...

The thermal transport properties of $M/\mathrm{F}{\mathrm{e}}_{2}(\mathrm{V},\mathrm{Ta})\mathrm{Al}/M$ ($M=\mathrm{Mo}$, W) multilayer thin films, which were epitaxially grown on the [100] MgO single-crystal substrates by means of a radio-frequency magnetron sputtering system, were systematically investigated using a picosecond pulsed laser heating time-domain thermoreflectance method. Not only the thermal diffusivity but also the boundary thermal resistance was strongly affected by both the Ta substitution for V in $\mathrm{F}{\mathrm{e}}_{2}\mathrm{VAl}$ and the elements in the metallic layer of top and bottom sides. From the first-principles density-functional theory calculations, we found that such changes in heat transport properties were directly related to the intrinsic phonon dispersion. In the $\mathrm{F}{\mathrm{e}}_{2}(\mathrm{V},\mathrm{Ta})\mathrm{Al}$ layer thickness region of less than 14 nm, the boundary thermal resistances of all thin-film series abruptly decreased with decreasing the thickness, which was due to the coherently ordered interfacial structure. more...