Nanoscale Phonon Spectroscopy Reveals Emergent Interface Vibrational Structure of Superlattices
Eric Hoglund a, De-Liang Bao b, Andrew O'Hara b, Jordan Hachtel c, Zachary Piontkowski d, Joseph Matson l, Ajay Yadav e, Ryan Haisimaier f, Roman Engel-Herbert f, Jon Ihlefeld a, Jayakanth Ravichandran g, Ramamoorthy Ramesh e, Joshua Caldwell l, Thomas Beechem d h, Sokrates Pantelides b i, Patrick Hopkins j k, James Howe a
a Materials Science and Engineering, University of Virginia
b Department of Physics and Astronomy, Vanderbilt University
c Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, EE. UU., Oak Ridge, United States
d Sandia National Laboratories
e Department of Materials Science and Engineering, University of California Berkley
f Department of Materials Science and Engineering, Pennsylvania State University
g Department of Chemical Engineering and Materials Science, University of Southern California
h Center for Integrated Nanotechnologies, Sandia National Laboratories,
i Department of Electrical Engineering and Computer Science, Vanderbilt University
j Department of of Mechanical and Aerospace Engineering, University of Virginia
k Department of Physics, University of Virginia, Charlottesville
l Department of Mechanical Engineering and Electrical Engineering, Vanderbilt University
Proceedings of Electron Beam Spectroscopy for Nanooptics 2021 (EBSN2021)
Online, Spain, 2021 June 14th - 15th
Organizers: Mathieu Kociak and Nahid Talebi
Poster, Eric Hoglund, 051
Publication date: 8th June 2021

Perovskite oxides offer an abundance of properties with technological importance that are tunable by creating superlattices (SLs). Most research so far focuses on electronic properties, but such SLs can also feature unique thermal properties, which are dictated by lattice vibrations.  Here we combine atomic-resolution integrated differential phase contrast in a scanning transmission electron microscopy, vibrational electron energy-loss spectroscopy, and density-functional-theory calculations to probe the phonon spectra and oxygen atom positions for a series of SrTiO3-CaTiO3 SLs with one to twenty-seven unit-cells per layer. We show large-period SLs feature bulk plus localized interface vibrational modes. As layer thickness decreases, the bulk modes disappear while the interface modes underpin an emergent phonon structure, which is also reflected in the macroscopic vibrational response measured by UV-Raman and Fourier transform infrared spectroscopy. This behavior correlates to period thickness reaching the length scale of TiO6 octahedral coupling at the interfaces, thus reflecting the dominant role of interfaces as size decreases and the need for spatially quantified structural and vibrational data to enable complete understanding of SL behavior.

ERH and PEH appreciate support from the Office of Naval Research through a MURI Program, Grant Number N00014-18-1-2429. Theory at Vanderbilt University was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Science and Engineering Directorate grant No. DE-FG02-09ER46554 and by the McMinn Endowment. Calculations were performed at the National Energy Research Scientific Computing Center (NERSC), a U.S. Department of Energy Office of Science User Facility located at Lawrence Berkeley National Laboratory, operated under Contract No. DE-AC02-05CH11231.

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