Two-Level Systems in Amorphous Silicon Frances Hellman, University of California-Berkeley, DMR 0907724.
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Low temperature heat capacity and internal friction measurements were made on evaporated amorphous silicon a-Si films, for various thickness and growth temperature TS which cause different film density nSi.The density of quantum mechanical two-level systems (TLS) are extracted from each measurement. TLS are found (only) in films of lower density. Increasing TS or film thickness results in denser films and greatly reduced low temperature heat capacity and internal friction, suggesting that TLS occur in voids in the film and not in the fully coordinated a-Si network. This result suggests that TLS are not intrinsic to the amorphous state but rather due to localized states in under coordinated sites.
TLS densities no from low T heat capacity and from internal friction Q-1 versus film density nSiof evaporated a-Si grown at various Ts and for various film thicknesses. no measures all TLS while measures TLS in a narrow spectral region. This figure shows that no and scale with each other, and both depend strongly and inversely on the film density. The proportionality constant no/ ~100, much higher than found in other amorphous materials.
The low temperature properties of non-crystalline materials are dominated by low energy excitations described as quantum mechanical two-level systems (TLS). The physical nature of these TLS is however not understood. This work suggests an extrinsic rather than intrinsic origin and provides a prescription for preparing a-Si without TLS. This result is important for solid state quantum computing applications where TLS in the dielectric layers leads to dephasing of the qubits.
Education and Outreach:
This grant supported a graduate student and 3-4 undergraduates each year, a significant fraction of whom are women and/or come from diverse backgrounds.
PI Hellman continues to be involved in public outreach and educational activities, including the Elementary Institute of Science in San Diego, the SF Exploratorium, and the UC Berkeley society of women physics students.