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Vibrations in materials and nanostructures at sufficiently high temperaturesresult in anharmonic atomic displacements, which leads to new phenomena such asthermal expansion and multiphonon scattering processes, with a profound impacton temperature-dependent material properties including thermal conductivity,phonon lifetimes, nonradiative electronic transitions, and phase transitions.Nanoscale momentum-resolved vibrational spectroscopy, which has recently becomepossible on monochromated scanning-transmission-electron microscopes, is aunique method to probe the underpinnings of these phenomena. Here we reportmomentum-resolved vibrational spectroscopy in hexagonal boron nitride attemperatures of 300, 800, and 1300 K across three Brillouin zones (BZs) thatreveals temperature-dependent phonon energy shifts and demonstrates thepresence of strong Umklapp processes. Density-functional-theory calculations oftemperature-dependent phonon self-energies reproduce the observed energy shiftsand identify the contributing mechanisms.

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