High-temperature phonons in h-BN: momentum-resolved vibrational spectroscopy and theory

Vibrations in materials and nanostructures at sufficiently high temperatures result in anharmonic atomic displacements, which leads to new phenomena such as thermal expansion and multiphonon scattering processes, with a profound impact on temperature-dependent material properties includ-ing thermal conductivity, phonon lifetimes, nonradiative electronic transitions, and phase transi-tions. Nanoscale momentum-resolved vibrational spectroscopy, which has recently become possi-ble on monochromated scanning-transmission-electron microscopes, is a unique method to probe the underpinnings of these phenomena. Here we report momentum-resolved vibrational spectros-copy in hexagonal boron nitride at temperatures of 300, 800, and 1300 K across three Brillouin zones (BZs) that reveals temperature-dependent phonon energy shifts and demonstrates the pres-ence of strong Umklapp processes. Density-functional-theory calculations of temperature-dependent phonon self-energies reproduce the observed energy shifts and identify the contributing mechanisms.