Acoustic measurements of two-level systems in the ultrastable organic glass 2TNATA

It has recently been discovered that vapor deposition of amorphous silicon onto a substrate held at an optimal temperature reduces the concentration of low-energy excitations that dominate the low-temperature thermal, acoustic, and dielectric properties of almost all amorphous dielectric materials by almost three orders of magnitude. Using a similar film deposition process, it has also been shown that numerous organic glasses formed directly from the vapor phase at an optimal substrate temperature show ``ultrastable'' properties of enhanced kinetic stability, high density, and low entropy. In order to investigate a proposed connection between ultrastability and a strong reduction of these excitations in such vapor-deposited glasses, we deposit ultrastable thin films of the organic glass 2TNATA---4,4\ensuremath{'},4\ensuremath{'}\ensuremath{'}-Tris(N-(2-naphthyl)-N-phenyl-amino)triphenylamine---on ultra-high-$Q$ mechanical resonator substrates known as double-paddle oscillators (DPOs). The low-energy excitations, commonly and successfully modeled as a distribution of two-level tunneling systems, TLSs, are measured through internal friction and characterized by a tunneling strength $C$. The ultrastable character of the films is confirmed through conventional and flash differential scanning calorimetry, and after initial internal friction measurements of the ultrastable state, the films were annealed into the ordinary state to quantify the difference in $C$. We find the ultrastable glasses to have $C\ensuremath{\approx}6.8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$, still within the glassy range, and representing a modest 25% reduction compared to the ordinary glass.