EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Phonons are quasiparticles describing the lattice vibrations in a crystal, and they are responsible of transporting heat and sound. Phonon transport in bulk materials is usually described by a diffusive picture. At nanoscale lengths, however, boundaries and interfaces play an important role. At this scale, when the phonon mean free path is larger than the length phonon travel before meeting a boundary, transport becomes ‘ballistic’. Moreover, if the phonons' phase relation is preserved, wave effects become dominant, and coherent transport takes place. Under these conditions, the material becomes homogeneous, with a modified phonon dispersion relation where phonons do not scatter at the interfaces anymore. Materials that show coherence effects have potential applications in thermo-optics, thermoelectrics, as phonon sources, filters, and detectors, heat-waveguides, and more. [1,2]

In this work, computer simulations are used to design potential superlattices that feature significant phonon coherence. The dynamics of the atoms within the superlattices is simulated using classical molecular dynamics. The fluctuations of the atomic velocities are tracked throughout the simulation, and their correlations are used to calculate the coherence length. [3] The coherence lengths of GaN/InN superlattices, and structures made of monolayers of InN in a GaN matrix, are investigated. It is shown that acoustic modes feature high phonon coherence lengths (up to a few 100 nm), and that superlattices with a low period feature higher phonon coherence lengths than superlattice with a long period. Our results, as well as drawbacks and possible improvements of the used method, will be discussed.