Interface phonon contributions to thermal conductivity of superlattices

The thermal conductivities of (SrTiO₃)_n/(CaTiO₃)_n [STO_n/CTO_n] and other such superlattices (SLs) have been measured for several layer thicknesses, revealing a trend that dips to a minimum and then rises as a function of decreasing layer thickness. Distinct interface phonons have been detected by spatially resolved vibrational electron energy loss spectroscopy for the large thicknesses, while for n=1 and 2 the SL behaves as a “crystal of interfaces” in agreement with the experimental results. Here, we employ DFT-based machine-learning molecular-dynamics simulations to investigate the thermal conductivities in STO_n/CTO_n SLs, examining how interface phonons influence the thermal and vibrational properties for n = 1, 2, 4, 6, 8, and n>8. The simulations reproduce the thermal conductivity trends with n values and reveal the absence of true interfacial phonons in SL1 and SL2. In SL4, interfacial phonons are localized at the interfaces while other phonons are localized within the layers, significantly reducing heat transport. In thicker superlattices, interface phonons are localized and scatter bulk phonons. Through participation ratio (PR) analysis and spatial PR projections, we uncover how interface phonons evolve across different SLs. Additionally, spectral energy density analysis reveals anharmonicity effects around the interfaces, while frequency-dependent thermal-conductivity analysis identifies the contribution of localized interface phonons to the overall thermal conductivity.