Post date: Jul 8, 2014 3:06:48 PM
Our work on the thermal conductivity of cubic boron nitride under pressure is now available online at Physical Review Letters. Congrats to Saikat Mukhopadhyay for all his work on this project!
Saikat Mukhopadhyay and Derek Stewart, "Polar Effects on the Thermal Conductivity of Cubic Boron Nitride under Pressure", Physical Review Letters, 113, 025901 (2014)
http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.113.025901
While the presence of polar bonds leads to LO-TO splitting in the phonon dispersion of materials, the impact of polar bonds on thermal conductivity has been unclear. Since optical branches do not participate directly in thermal transport, the presence of a LO-TO gap is typically assumed to have little influence on thermal transport. However, optical branches do play a key role in phonon-phonon scattering. Therefore, the best candidate for observing an effect due to polar bonds on the thermal conductivity would be a system with (1) a small number of phonon branches (i.e. small unit cell), (2) optical branches in close proximity to acoustic branches to ensure acoustic-optical interactions, and (3) a small phase space for phonon-phonon scattering. Cubic boron nitride (c-BN) satisfies all of these requirements and provides the perfect candidate for our study. In this work, we have used pressure to manipulate the phonon dispersion and LO-TO splitting of cubic boron nitride. Using a first principle Boltzmann transport approach, we have calculated the thermal conductivity of c-BN for a wide range of pressures and temperatures. We find that the change of thermal conductivity with pressure in c-BN goes through an interesting non-linear regime at low to intermediate pressures. This non-linear regime is something that has not been observed in similar non-polar materials like diamond. Through a careful analysis of phonon-phonon scattering and the changes in phonon dispersion with pressure, we relate this non-linear regime to the polar nature of c-BN. This work could also have importance implications for thermoelectric polar materials where there is significant LO-TO splitting at the Gamma point.