Probing phonon surface scattering in nanostructures

In insulating materials, heat is transmitted by atomic vibrations (``phonons''). In nanostructured materials such as nanowires and nanosheets, the characteristic length scale of a material can be less than the mean free path of a phonon. The phonon transport is then drastically altered and becomes dominated by scattering from surfaces. We demonstrate a method to assess the scattering rate and transmission factor of phonons traversing a silicon nanosheet. Generation and detection of phonons is accomplished by a superconducting tunnel junction attached to the silicon nanostructure and operated at a temperature of 0.3K. Decay of excited states in the superconductor is employed as a tunable narrow-band source of phonons [1,2]. This tunable source enables investigation of the phonon mean free path as a function of phonon frequency and surface roughness, for frequencies from $\sim$100 GHz to $\sim$500 GHz in nanosheets 100 to 200 nm thick. This work is supported by DOE (DE-SC0001086). \\[4pt] [1] H. Kinder. Phys. Rev. Lett. 28, 1564 (1972)\\[0pt] [2] J. B. Hertzberg et al, Rev. Sci. Inst. 82, 104905 (2011).