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TT: Tiefe Temperaturen
TT 7: Postersitzung I: Amorphe- und Tunnelsysteme (1-8), Mesoskopische Systeme (9-21), Schwere Fermionen (22-32), Kernmagnetismus (33-34), Josephson-Kontakte und SQUIDs (35-45), TT-Detektoren und Kryotechnik (46-49)
TT 7.6: Poster
Montag, 27. März 2000, 14:30–18:00, A
Vibrating Reed Studies on Compacted Metal Powders at Very Low Temperatures — •I. Usherov-Marshak and R. König — Physikalisches Institut, Universität Bayreuth, 95440 Bayreuth
Compacted metal powders are widely used in low temperature experiments to improve the thermal coupling between solids and quantum liquids. In order to obtain new information about their acoustic properties, and the possible influence of the granular structure of such compacts on the thermal coupling to liquid 3He, we have started with measurements of sound velocity variation Δ v/v and internal friction Q−1 in samples made of compacted Ag (grain size 70 nm) and Pt (grain size 2 µm) powders with packing fractions of ∼ 60 %.
Using the vibrating reed technique, we have performed acoustic measurements in the temperature range 0.1 ≤ T ≤ 500 mK at frequencies 320 ≤ ν ≤ 1050 Hz. We observe a pronounced maximum at Tco in the temperature dependence of Δ v/v which could be interpreted in terms of the Tunneling Model as a crossover between different interaction regimes of phonons with tunneling systems. At temperatures above Tco, Δ v/v of powder compacts and bulk samples show the same temperature dependence. At T < Tco, however, we observe a strong deviation in the acoustic behaviour of the compacted powder from that of the bulk material.