Low-temperature group velocity and elasticity in CaWO4

The CRESST experiment looks for evidence of dark matter particles colliding with nuclei in CaWO4, using cryogenic bolometers sensitive to energy deposition (∼10 keV) with high accuracy. Calibration of the energy deposited in the phonon system depends upon the details of the evolution of the non-equilibrium energy in the CaWO4 absorber. Phonon images sensitively reveal variations in angular phonon flux in a single-crystal sample and provide accurate data for the group velocity along non-symmetry directions. Our measurements, over a wide range of propagation angles, provide new and precise values for phonon group velocities and uncover significant discrepancies between the experimentally observed transport of non-equilibrium energy and that predicted using values of the low-temperature elastic constants widely cited in the literature.