The Boson Peak and Disorder in Hard Sphere Colloidal Systems

The Boson peak is believed to be the key to the fundamental understanding of the anomalous thermodynamic properties of glasses, notably the anomalous peak in the heat capacity at low temperatures; it is believed to be due to an excess of low frequency vibrational modes and a manifestation of the structural disorder in these systems. We study the thermodynamics and vibrational dynamics of colloidal glasses and (defected) crystals. The experimental determination of the vibrational density of states allows us to directly observe the Boson peak as a strong enhancement of low frequency modes. Using a novel method [Zargar et al., Phys. Rev. Lett. 110, 258301 (2013)] to determine the free energy, we also determine the entropy and the specific heat experimentally. It follows that the emergence of the Boson peak and high values of the specific heat are directly related and are specific to the glass: for a very defected crystal with a disorder that is only slightly smaller than for the glass, both the low-frequency density of states and the specific heat are significantly smaller than in the glass.