ISSN:
1573-7357
Source:
Springer Online Journal Archives 1860-2000
Topics:
Physics
Notes:
Abstract Quench-condensed lead-based alloy films were investigated by the superconductive tunneling method to study how structural order affects the phonon distribution, and in particular the appearance of localized impurity modes, As solute, 10 at. % of either Na, Mg, Zn, Ga, Ge, Ag, Cd, In, Sn, Sb, or Te was used. The films were condensed upon a substrate at 0.4 K and studied as deposited and after anneals to 30, 100, and 300 K. It was found that the shape of the phonon-induced tunneling structure could be used as a means to classify the state of structural order in the films. Temperatures of transformations from one state to another could be identified with those where steps occurred in curves of film resistance vs. annealing temperature. The most notable transformation was the one that took the film from an amorphous to a finely grained crystalline state. The transformation temperature scales with the melting point of the pure impurity element of the binary alloy. As the annealing temperature was increased further another temperature region was reached where the crystallites grew in size, causing notable changes in the phonon spectra. A third transformation occurred at low temperatures within the amorphous state for a few of the alloys. Localized impurity modes could be observed only inPbIn and only when the films had been annealed so that relatively large crystallites had formed. The absence of these high-frequency modes in most of the alloy films can be explained as being due to a lack of order or solubility in the ordered state. Such arguments cannot, however, explain why no localized modes are seen inPbSn. In disordered films the phonon distribution is shifted toward lower frequencies compared to the case of a single crystal. For this type of lead-based alloy the shift implies lower values of the superconducting transition temperatureT c and gap Δ0 (0), but a higher2Δ 0 (0O)/kB T C ratio. The dependence of these values and of the tunnel resistance upon the structural state is studied. The effect of fluctuations on the superconducting transition is noted for the thin, amorphous films.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00654394
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