Atomic Scale Control of Thermal Conductivity in LaMnO ₃ /SrMnO ₃ Superlattices

ABSTRACT

We report atomic scale structure and phonon thermal transport in (LaMnO ₃ ) _m /(SrMnO ₃ ) _n /SrTiO ₃ (100) superlattices (LMO/SMO SLs) with the thickness of individual layers m , n   =  1 − 10 u.c. and the thickness ratio m / n =  1,  2. Optical transient thermal reflectivity measurements reveal pronounced differences in the thermal conductivity between SLs with m / n =  1 and SLs with m / n =  2. State‐of‐the art electron microscopy techniques and ab initio density functional calculations enable us to assign the origin of this difference to the differences in the oxygen octahedral rotation (OOR) angle φ _OOR within the LMO layers in these SLs. Namely, LMO and SMO layers within the ( m / n =  1) SLs possess the same φ _OOR = 180°, which is characteristic of a cubic structure. In contrast, the m / n =  2 SLs reveal a significantly smaller value φ _OOR = 165° in LMO and a large φ _OOR = 180° in SMO. The analysis of experimental data shows that the thermal conductance of the LMO/SMO interfaces in m / n =  2 SLs with OOR misfit is low 0.3 GWm ⁻² K ⁻¹ , whereas a surprisingly large value of interfacial conductance 1.8 GWm ⁻² K ⁻¹ was determined for m / n =  1 SLs without OOR misfit. Moreover, a minimum of thermal conductivity of κ _min = 0.3–0.4 Wm ⁻¹ K ⁻¹ has been observed at the interface density 1/(Λ = m+n)∼0.11–0.25 u.c. ⁻¹ , denoting the crossover between the interface‐ to volume‐dependent thermal conductivity. Our results point out the importance of MnO ₆ octahedral rotations/tilts as an effective structural control parameter to tune thermal transport and magnetism in LMO/SMO SLs by changing the LMO/SMO thickness ratio.