Crossover from incoherent to coherent phonon scattering in epitaxial oxide superlattices

Journal name:
Nature Materials
Volume:
13,
Pages:
168–172
Year published:
DOI:
doi:10.1038/nmat3826
Received
Accepted
Published online

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.

At a glance

Figures

  1. Measured thermal conductivity values for superlattices as a function of interface density at room temperature.
    Figure 1: Measured thermal conductivity values for superlattices as a function of interface density at room temperature.

    a, (STO)m/(CTO)n superlattice. b, (STO)m/(BTO)n superlattice. The black line represents the modified Simkin–Mahan (SM) model with disorder correction for (STO)m/(CTO)n superlattices and the blue dots represent the Simkin–Mahan model with disorder and volume fraction corrections for (STO)m/(BTO)n superlattices. The blue line is used as a guide to eye. The black dot–dash horizontal lines refer to the experimentally measured thermal conductivity for STO/CTO 50:50 alloy and the STO/BTO 25:75 alloy and are designated as the alloy limits. Error bars in the figures represent the error due to standard deviation and uncertainties in the measurement. Detailed error analysis is available in the Supplementary Information.

  2. Measured thermal conductivity values for (STO)m/(CTO)n superlattices as a function of interface density at different temperatures.
    Figure 2: Measured thermal conductivity values for (STO)m/(CTO)n superlattices as a function of interface density at different temperatures.

    The minimum in thermal conductivity becomes deeper at lower temperatures and the interface density at which the minimum occurs moves to smaller values at lower temperatures as expected. The solid lines are guides to the eye. The shift of the minimum is shown using dashed lines projected onto the x axis for different temperatures.

  3. Structural and microstructural characterization of superlattice samples from both series.
    Figure 3: Structural and microstructural characterization of superlattice samples from both series.

    a,b, High-resolution, short angular-range θ–2θ XRD scan of a (STO)6/(CTO)6 superlattice centred on the NGO 220 substrate peak (a) and (STO)74/(BTO)1 superlattice peaks centred on the STO 002 substrate peak (b). Both the superlattice peaks and the thickness fringes suggest the high degree of interface abruptness in the samples. c, A high-resolution reciprocal space map of the (STO)2/(CTO)2 superlattice centred on the NGO 332 substrate peak. The map clearly shows that the superlattice film is coherently strained to the substrate. The colour scale bar indicates intensity in arbitrary units (log scale). d, Surface topography of a 200 nm (STO)2/(CTO)2 thick superlattice film on a STO (001) substrate. The image clearly shows the presence of smooth step edges with unit-cell height. e,f, High-angle annular dark-field (HAADF) STEM images of (STO)2/(CTO)2 (e) and STEM-EELS image (dimensions 35 nm×3.6 nm) of a (STO)30/(BTO)1 superlattice (f) revealing the presence of atomically sharp interfaces with minimal intermixing in the samples studied. A schematic of the crystal structures is shown on the right in f. Chemical formulae of the component materials of the superlattice are colour-coded to match the false-colour of the atomic-resolution STEM-EELS image on the left (Sr, orange; Ba, purple; Ti, green).

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Author information

  1. These authors contributed equally to this work

    • Jayakanth Ravichandran,
    • Ajay K. Yadav &
    • Ramez Cheaito

Affiliations

  1. Applied Science and Technology Graduate Group, University of California, Berkeley, California 94720, USA

    • Jayakanth Ravichandran &
    • Ramamoorthy Ramesh
  2. Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA

    • Jayakanth Ravichandran,
    • Ajay K. Yadav,
    • S. J. Suresha,
    • Joel E. Moore &
    • Ramamoorthy Ramesh
  3. Department of Materials Science and Engineering, University of California, Berkeley, California 94720, USA

    • Ajay K. Yadav,
    • Pim B. Rossen &
    • Ramamoorthy Ramesh
  4. Department of Mechanical and Aerospace Engineering, University of Virginia, Charlottesville, Virginia 22904, USA

    • Ramez Cheaito,
    • John C. Duda,
    • Brian M. Foley &
    • Patrick E. Hopkins
  5. Department of Materials Science and Engineering, Cornell University, Ithaca, New York 14853, USA

    • Arsen Soukiassian,
    • Che-Hui Lee &
    • Darrell G. Schlom
  6. School of Applied and Engineering Physics, Cornell University, Ithaca, New York 14853, USA

    • Ye Zhu &
    • David A. Muller
  7. Department of Electrical and Computer Engineering, University of Virginia, Charlottesville, Virginia 22904, USA

    • Arthur W. Lichtenberger
  8. Department of Physics, University of California, Berkeley, California 94720, USA

    • Joel E. Moore &
    • Ramamoorthy Ramesh
  9. Kavli Institute at Cornell for Nanoscale Science, Cornell University, Ithaca, New York 14853, USA

    • David A. Muller &
    • Darrell G. Schlom
  10. ARPA-E, US Department of Energy, 1000 Independence Avenue, Washington DC 20585, USA

    • Arun Majumdar
  11. Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA

    • Ramamoorthy Ramesh
  12. Department of Materials Science and Engineering, University of California, Los Angeles, California 90095, USA

    • Mark A. Zurbuchen
  13. Western Institute of Nanoelectronics, Department of Electrical Engineering, University of California, Los Angeles, California 90095, USA

    • Mark A. Zurbuchen
  14. California NanoSystems Institute, University of California, Los Angeles, California 90095, USA

    • Mark A. Zurbuchen
  15. Present address: Department of Physics, Columbia University, New York, New York 10027, USA

    • Jayakanth Ravichandran

Contributions

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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The authors declare no competing financial interests.

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