The diffusion of a "warm" band of phonons, generated by continuously saturating a spin resonance at one end of a Sr${\mathrm{F}}_2$:${\mathrm{Tm}}^{2+}$ crystal, was measured throughout the rest of the crystal by optically monitoring the spatial distribution of the spin temperature. We interpret the experimental results in terms of a phonon transmission coefficient into the helium (∼4%) and a mean free path determined by diffuse scattering (∼25%) off the crystal surfaces.

• Received 19 August 1968

DOI:https://doi.org/10.1103/PhysRevLett.21.987