Heat capacity of suspended phonon cavities
  We present a detailed analysis of the vibrational spectrum and heat capacity of suspended mesoscopic dielectric plates, for various thickness-to-side ratios at sub-Kelvin temperatures. The vibrational modes of the suspended cavity are accurately obtained from the three-dimensional (3D) elastic equations in the small strain limit and their frequencies assigned to the cavity phonon modes. The calculations demonstrate that the heat capacity of realistic quasi-2D phonon cavities approach the linear dependence on T at sub-Kelvin temperatures. The behavior is more pronounced for the thinnest cavities, but takes place also for moderately thick structures, with thickness-to-side ratios $\gamma$=0.1 to 0.2. It is also demonstrated that the heat capacity of the suspended phonon cavities is invariant under the product of the temperature (T) with a characteristic lateral dimension (L) of the sample. The present results establish a lower bound for the heat capacity of suspended mesoscopic structures and indicate the emergence of the quantum mechanical regime in the dynamics of bounded phonon cavities. 
Andre Gusso, Luis G. C. Rego