Low temperature thermal properties of amorphous materials

The knowledge derived from thermal measurements are important for non-crystalline materials because the structural disorder severely limits the precision of the information from diffraction and scattering experiments. To show the difference between the crystalline and amorphous substances, the heat capacities of glassy and crystalline potassium meta-phosphate are presented. A large excess heat capacity was found for the glassy sample at temperatures between 10 to 40 K with a maximum value at 25 K and is reproduced well by a distribution of harmonic oscillator frequencies with an excess density of vibrational states at 41 cm(-l) relative to the crystal. This is identified with low energy excitations and Boson peaks generally known to occur in glassy substances in scattering experiments. For glassy hydrocarbons neutron scattering and calorimetry give the frequency dependence and absolute magnitude of the excess vibrational density of states.