EFFECT OF PHONON DISPERSION ON HEAT-PULSE SHAPE

The high-frequency portions of the acoustic branches of the vibrational spectra of solids have become increasingly available for time-resolution experiments through the recent development of heat-pulse techniques. These high-frequency phonons exhibit pronounced acoustic dispersion that manifests itself in a time evolution of heat-pulse shape. From a simple Born-von Kármán model we have calculated the effect of acoustic dispersion on heat-pulse propagation in α quartz. Results of the calculation indicate that dispersion manifests itself most prominantly on the heat-pulse width. Theoretical predictions are in qualitative agreement with observations on quartz, sapphire, and alkali-halide crystals. © 1968 The American Physical Society.