Mechanical and Civil Engineering Seminar

Phonons, the energy quanta of lattice vibration, are the dominant or important heat carriers in condensed matters. Coupling of phonons with electronic and spin excitations can lead to unwanted electro-thermal effects and thermomechanical failures in electronic devices, as well as thermoelectric and spin caloritronic phenomena, which can be utilized to convert heat directly into electricity. In this seminar, the intriguing phonon behaviors in materials with extreme simplicity or complexity will be examined. Topics to be discussed include nanoscale thermal transport measurements for observing ballistic phonon transport and the resulted size-dependent thermal conductivity in one-dimensional (1D) carbon nanotubes and two-dimensional (2D) materials, as well as in complex three-dimensional architectures of these simple 1D and 2D building blocks. The findings on these hexagonal crystal structures are compared to those observed for cubic phase boron arsenide, where the very different atomic masses of the two constituent elements result in a large energy gap in the phonon dispersion, low phonon-phonon scattering rate, and potentially record high thermal conductivity. The observed phonon behaviors in these hexagonal and cubic crystals are contrasted with those obtained from inelastic neutron scattering measurements on higher manganese silicides and spin ladder compounds, where the presence of two incommensurate sublattices give rise to both separated and coupled phonon modes in the highly complex structures, including a twisting motion that is also present in 1D nanotubes. Inelastic light scattering measurements provide further insights into the coupling among different phonon modes in simple crystals such as graphene, as well as the coupling between phonons and magnons, the energy quanta of spin waves, in complex ferromagnetic materials. These discoveries provide the basis for engineering the simple and complex materials for electronics, spintronics, thermal management, and thermoelectric energy conversion applications.