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Ultrafast time-domain thermoreflectance (TDTR) is utilized to extract the through-plane thermal conductivity (ΛLSCO) of epitaxial La0.5Sr0.5CoO3− δ (LSCO) of varying thickness (<20 nm) on LaAlO3 and SrTiO3 substrates. These LSCO films possess ordered oxygen vacancies as the primary means of lattice mismatch accommodation with the substrate, which induces compressive/tensile strain and thus controls the orientation of the oxygen vacancy ordering (OVO). TDTR results demonstrate that the room-temperature ΛLSCO of LSCO on both substrates (1.7 W m−1 K−1) are nearly a factor of four lower than that of bulk single-crystal LSCO (6.2 W m−1 K−1). Remarkably, this approaches the lower limit of amorphous oxides (e.g., 1.3 W m−1 K−1 for glass), with no dependence on the OVO orientation. Through theoretical simulations, origins of the glass-like thermal conductivity of LSCO are revealed as a combined effect resulting from oxygen vacancies (the dominant factor), Sr substitution, size effects, and the weak electron/phonon coupling within the LSCO film. The absence of OVO dependence in the measured ΛLSCO is rationalized by two main effects: (1) the nearly isotropic phononic thermal conductivity resulting from the imperfect OVO planes when δ is small; (2) the missing electronic contribution to ΛLSCO along the through-plane direction for these ultrathin LSCO films on insulating substrates.
Bibliographical noteFunding Information:
This work was supported by the National Science Foundation (NSF) through the University of Minnesota MRSEC under Award Number DMR-1420013 (X.W.W., X.J.W., J.W., and C.L.). J.W. and C.L. acknowledge support (for scattering characterization) from the Department of Energy through the University of Minnesota Center for Quantum Materials, under DE-FG02-06ER46275 and DE-SC-0016371. T.L.F. and X.L.R. acknowledge the support from National Science Foundation (Award No. 1150948). J.Z. would like to thank the support from the National Natural Science Foundation of China (Grant No. 51336009 and No. 51373184). Work at Argonne National Laboratory (crystal growth and characterization) was sponsored by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Science and Engineering Division. Research at UCM sponsored by Spanish MINECO/ FEDER MAT2015-66888-C3-3-R and by the ERC Proof of Concept Grant MAGTOOLS. Electron microscopy work at ORNL was sponsored by the Materials Sciences and Engineering Division of the U.S. Department of Energy and ORNL’s Center for Nanophase Materials User Program.
© 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
- nanoscale epitaxial LSCO
- oxygen vacancies
- thermal conductivity
- time-domain thermoreflectance
How much support was provided by MRSEC?
Reporting period for MRSEC
- Period 4
11/1/14 → …
Project: Research project