Long Lifetime Of The E-1(U) In-Plane Infrared-Active Modes Of H-Bn

We present an infrared reflectivity study of the E-1u in plane phonons of hexagonal BN as a function of temperature in the 40-680 K range. The infrared reflectance spectra of high-quality lamellar single crystals are accurately fitted using Lowndes' factorized form of the dielectric response, where the longitudinal-optic (LO) frequency is an independent adjustable parameter. From this analysis we obtain reliable values for the phonon damping of the IR-active E-1u phonons which couple to light and give rise to the phonon-polariton excitations in this hyperbolic material. Anharmonic coupling potentials are estimated from the temperature dependence of the damping parameters. The E-1u(LO) mode exhibits a substantially longer lifetime than its transverse-optic counterpart, as it is basically unaffected by isotopic-disorder scattering owing to the low density of phonon states around the E-1u(LO) frequency.