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Crossover from incoherent to coherent phonon scattering in epitaxial oxide superlattices


Elementary particles such as electrons1,2 or photons3,4 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.

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Figure 1: Measured thermal conductivity values for superlattices as a function of interface density at room temperature.
Figure 2: Measured thermal conductivity values for (STO)m/(CTO)n superlattices as a function of interface density at different temperatures.
Figure 3: Structural and microstructural characterization of superlattice samples from both series.


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The work on SrTiO3/CaTiO3 superlattices was supported by the US Department of Energy, Office of Basic Energy Sciences under Contract No. DE-AC02-05CH11231. The work on SrTiO3/BaTiO3 superlattices by A.S., C-H.L., D.G.S. and M.A.Z. was supported by the Defense Advanced Research Projects Agency (DARPA) and the US Army Aviation and Missile Research, Development, and Engineering Center (AMRDEC) through Grant No. W31P4Q-09-1-0005. TEM sample preparation for some of the SrTiO3/BaTiO3 superlattices was performed by the UCLA Nanoelectronics Research Facility. We acknowledge the use of instruments at the Electron Imaging Center for NanoMachines (EICN) supported by NIH (1S10RR23057 to Z.H.Z.) at the California NanoSystems Institute (CNSI), UCLA. Electron microscopy and spectroscopy of SrTiO3/BaTiO3 at Cornell by Y.Z. and D.A.M. was supported by the Army Research Office (ARO) grant W911NF-09-1-0415 and the electron microscopy facility of the Cornell Center for Materials Research (CCMR) by the National Science Foundation Materials Research Science and Engineering Centers (MRSEC) programme (DMR 1120296). P.E.H. is 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. J.R. acknowledges the fellowship from Link Foundation. The authors wish to express deep gratitude to D. G. Cahill for measuring the thermal conductivity of a significant number of the samples by TDTR, and for many thoughtful discussions. The authors also wish to acknowledge the contributions of CrysTec GmbH for providing the high-quality single-crystal substrates that were used in this study.

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J.R. and P.E.H. designed the experiments on STO/CTO superlattices and M.A.Z. designed the experiments on STO/BTO superlattices. A.K.Y., P.B.R., J.R. and A.S. grew and characterized the samples for STO/CTO superlattice work, and A.S. and C-H.L. grew the samples for STO/BTO superlattice work. R.C., J.C.D., B.M.F. and P.E.H. measured and analysed the thermal conductivity of samples. S.J.S., M.A.Z., Y.Z. and D.A.M. performed TEM studies. A.W.L. assisted in TDTR experiments at UVa. J.E.M. performed the Simkin–Mahan model calculations and J.R. carried out the coherence length calculations and scaling analysis to modify the Simkin–Mahan model. J.R., P.E.H., M.A.Z., A.K.Y. and R.C. co-wrote the manuscript. R.R., A.M., D.G.S. and M.A.Z. supervised the research. All authors contributed to the discussions and manuscript preparation.

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Correspondence to Ramamoorthy Ramesh or Mark A. Zurbuchen.

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The authors declare no competing financial interests.

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Ravichandran, J., Yadav, A., Cheaito, R. et al. Crossover from incoherent to coherent phonon scattering in epitaxial oxide superlattices. Nature Mater 13, 168–172 (2014).

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