The low temperature thermal properties of amorphous solids are known to be markedly different from their crystalline counterparts due to low energy excitations not typically found in crystalline materials. These excitations result in an increase in the low temperature heat capacity and cause strong scattering of phonons and elastic waves. Many of these properties are explained by the two-level systems (TLS) which explains a wide range of experimental results but it offers little insight into the physical nature of the tunneling entities themselves. Despite 40 years of active research several open questions remain: Why are TLS found with roughly the same density in most amorphous solids? Are these excitations intrinsic to the amorphous state? Do TLS form in tetrahedral bonded materials like a-Si and a-Ge? Interest has been renewed in understanding the origin of TLS as they cause decoherence in solid state qubits and noise in superconducting resonators.

We are addressing these questions using a-Si as model system. We have found that the TLS density in e-beam evaporated a-Si decrease by several orders of magnitude as the density of the amorphous film approaches the crystalline value. This result suggest that TLS form in low density regions of the film that are distinct from the average sited in disordered amorphous matrix.

This work is performed in collaboration with the Lawrence Berkeley National Lab, the National Renewable Energy Lab, and the Naval Research Lab.

D.R.Queen, X. Liu, J. Karel, T.H. Metcalf, and F. Hellman Phys. Rev. Lett. 110 135901 (2013) link.