  One of the most accepted models that describe the anomalous thermal behavior
of amorphous materials at temperatures below 1 K relies on the quantum
mechanical tunneling of atoms between two nearly equivalent potential energy
wells forming a two-level system (TLS). Indirect evidence for TLSs is widely
available. However, the atomistic structure of these TLSs remains an unsolved
topic in the physics of amorphous materials. Here, using classical molecular
dynamics, we found several hitherto unknown bistable structural motifs that may
be key to understanding the anomalous thermal properties of amorphous alumina
at low temperatures. We show through free energy profiles that the complex
potential energy surface can be reduced to canonical TLSs. The predicted tunnel
splittings from instanton theory, the number density, dipole moment, and
coupling to external strain of the discovered motifs are consistent with
experiments.

Materials Science;Other Condensed Matter