Crystal orientation dependence of extreme near-field heat transfer between polar materials governed by surface phonon modes

Due to the rapid development of micro- and nano-manufacturing and electronic devices, heat transfer at the transition regime between radiation and conduction becomes increasingly important. Recent work has demonstrated the importance of nonlocal optical response and phonon tunneling. However, it remains unclear how the crystal orientation impacts these phenomena. In this work, we study this effect on heat transport across vacuum gaps between magnesium oxide by nonequilibrium molecular dynamics simulation. At 5 Å gaps, the overall thermal conductance exhibits 30% enhancement for the [100] orientation vs the [110] and [210] orientations, while becoming orientation-insensitive beyond 6 Å. When the gap size is extremely small, the crystal orientation significantly impacts the resonance frequencies of spectral thermal conductance, which are quite close to those of unique surface phonon modes distinct from bulk counterparts. As the gap size gradually increases, the spectral thermal conductance gradually converges to the predicted results of fluctuation-electrodynamics theory in the long-wavelength approximation. Our findings reveal how surface phonon modes govern extreme near-field heat transfer across nanogaps, providing insights for thermal management in electronic devices.