Tianli Feng, Maria Varela, Neven Biškup, Xuewang Wu, Hong Zheng, Xiulin Ruan, John F. Mitchell, Jie Zhu, Xiaojia Wang, Chris Leighton, Jeff Walter
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.
Nanoscale epitaxial La0.5Sr0.5CoO3–δ (LSCO) films possess controlled oxygen vacancy ordering (OVO) via lattice mismatch accommodation with substrates. In this work, the glass-like through-plane thermal conductivity of single-crystal LSCO films is experimentally demonstrated, which is independent of the OVO orientation. The essential reason, as revealed by theoretical calculations, is a combined effect of the oxygen vacancies, Sr substitution, size effect, and weak electron/phonon coupling.