Coherent transport in Superlattices – Ramez joint first author on paper published in Nature Materials!

We have demonstrated the cross over from incoherent to coherent phonon transport in superlattices through thermal conductivity measurements of strontium titanate (SrTiO3)/calcium titanate (CaTiO3) superlattices with different periodicities.  Our work was recently published in Nature Materials (“Crossover from incoherent to coherent phonon scattering in epitaxial oxide superlattices,” Nature Materials doi:10.1038/nmat3826).  Congratulations to Ramez who is a joint first author on this work!!! This work summarizes a major collaboration with several groups around the US, including Profs. Ramamoorthy Ramesh, Arun Majumdar, Darrell Schlom, Mark Zurbuchen, David Muller, and Joel Moore.  We appreciate the generous support from the Army Research Office and the National Science Foundation.

Abstract

Elementary particles such as electrons^1, 2 or photons^3, 4 are frequent subjects of wave-nature-driven investigations, unlike collective excitations such as phonons. The demonstration of wave–particle crossover, in terms of macroscopic properties, is crucial to the understanding and application of the wave behaviour of matter. We present an unambiguous demonstration of the theoretically predicted crossover from diffuse (particle-like) to specular (wave-like) phonon scattering in epitaxial oxide superlattices, manifested by a minimum in lattice thermal conductivity as a function of interface density. We do so by synthesizing superlattices of electrically insulating perovskite oxides and systematically varying the interface density, with unit-cell precision, using two different epitaxial-growth techniques. These observations open up opportunities for studies on the wave nature of phonons, particularly phonon interference effects, using oxide superlattices as model systems, with extensive applications in thermoelectrics and thermal management.   We are grateful for financial support from Army Research office (ARO) grant W911NF-13-1-0378.  TDTR measurements on the SrTiO3 /CaTiO3 superlattices at the University of Virginia were supported by the National Science Foundation (NSF) grant CBET-1339436.