In this talk I review the phenomenological picture of tunneling defects in low-temperature glasses. Despite the successes of this model, it has been very difficult to verify its microscopic foundations. Leveraging the power of a novel Monte Carlo method, we have prepared in silico glasses annealed in a manner that corresponds to the range of rates found in real experiments. Via a detailed search of the energy landscape of our model glasses, we verify that tunneling defects are the dominant excitations in glasses at ultra-low temperatures, and that their distribution is consistent with the standard picture. We show that more slowly quenched glasses have fewer tunneling systems, in harmony with recent experiments. Our approach enables the detailed microscopic investigation of the nature of the tunneling events themselves. We find that most tunneling states correspond to simple local vacancy motion, but that rare tunneling systems can be found that involve highly collective motion. I also discuss connections between tunneling defects and soft harmonic modes in our computer generated glasses.
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