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Title: What atoms tunnel in the two-level systems of amorphous silica?
Plain: The tunneling model assumes small groups of atoms that flip between two nearly equal positions. Which atoms move, and how many, has never been identified in any glass.
Precise: For amorphous SiO2 (and a-Si, a-Al2O3), identify the atomic configurations that form tunneling two-level systems with energies below $\sim 1e-4\,\mathrm{eV}$ ($\sim 1\,\mathrm{K}$), and reproduce their measured density of states $P$, deformation potential $\gamma \sim 0.5-1\,\mathrm{eV}$, electric dipole moments and isotope dependence.
Settled by: Atomistic simulation of a realistically quenched glass that finds tunneling double wells with the measured $P$, $\gamma$ and dipole moments, confirmed by an isotope-substitution or strain-spectroscopy experiment.
Kind: well-posed
Answer type: identification
References: (empty)
Literature status: open
Status note: (empty)
Posed since: (empty)