Coherent phonon transport in Epitaxial Oxide Heterostructures

Coherent transport of phonons was unambiguously observed [1] in superlattices of complex oxides, (SrTiO$_{3})_{\mathrm{m}}$/(CaTiO$_{3})_{\mathrm{n}}$ and (SrTiO$_{3})_{\mathrm{m}}$/(BaTiO$_{3})_{\mathrm{n}}$ manifested by a minimum in thermal conductivity as a function of superlattice interface density. To gain further insights into coherent regime of phonon transport, we systematically changed acoustic impedance mismatch between superlattice constituents and studied its effect on the transition from incoherent to coherent regime of heat transport. Further, in an attempt to manipulate transport of a broad range of phonons in the coherent regime, controllable disorder is introduced to attain both short-range and long-range phonon scattering. Quasi-periodic, controllable disorder is introduced by randomly stacking alternating layers of SrTiO$_{3}$ and CaTiO$_{3}$, for a given average interface density and volume fraction. In conclusion, our studies elucidate the effect of periodicity and impedance mismatch on both coherent and incoherent phonon scattering in epitaxial oxide heterostructures.\\[4pt] [1] J. Ravichandran, A. Yadav, R. Cheaito et al., accepted in \textit{Nature Materials }(2013).