ULTRASONIC SURFACE WAVE ATTENUATION BY INDUCED SUPERCONDUCTIVITY IN PbCu AND PbAg "SANDWICHES"

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Submitted on 1 Jan 1978

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ULTRASONIC SURFACE WAVE ATTENUATION BY

INDUCED SUPERCONDUCTIVITY IN PbCu AND

PbAg ”SANDWICHES”

J. Park, P. Tai, S. Freake

To cite this version:

ULTRASONIC SURFACE WAVE ATTENUATION BY INDUCED SUPERCONDUCTIVITY IN PbCu AND PbAg "SANDWICHES"

J.G. Park, P. Tai and S. Freake*

Physics Department, Imperial College, London SW7 2BZ, U.K. The Open University, Boar's Bill, Oxford, U.K.

Abstract.- We have been making measurements between 1.4 and 4.2 K on the attenuation of ultrasonic surface waves produced by proximity effect "sandwiches", in which a su-perconducting Pb film is coated with a layer of Cu or Ag, as a function of layer thickness and temperature. A regular pattern of behaviour is observed for the two types of specimen which does not seem to be explicable by any of the existing theories.

We present here some results for the atte-nuation of ultrasonic surface waves at 100 MHz in PbCu and PbAg "sandwiches" made from rather "clean" films. This technique has been pioneered by Kratzig /!/ who worked with "dirty" films in which H <<?

n c where 5 i s the coherence length in ideal pure mate-rial hv_/(2irkT)'. There i s as yet no theory of ultrasonic

r

attenuation that can be applied to the circumstan-ces of our experiments. A pattern of behaviour is revealed in our experiments which is sufficiently simple and reproducible to suggest that a theore-tical interpretation might well be found for -it eventually.

Our results for a , the increase in atte-n

nuation at the low magnetic field transition when the superconductivity induced in the N layer (Cu or Ag) is destroyed, are shown in figures 1 and

2 : they have been reduced to a common relative scale by making use of the further increase as that takes place at a higher field when the S film

(Pb) becomes normal. The reduced N layer attenua-tion A = a * /a*, where a* = a./(l.d.). It is

evi-r n s l i l l

dent that for any given specimen, except for the thinner layers at the low temperature end of our range, the points cluster around a straight line through the origin when a (or A ) is plotted against 1/T. The PbAg data is consistent with the assumption that the deviations from 1/T behaviour take place tan going over from the "clean" to the

"dirty" regime as 1/T increases, but for PbCu they

do not. Thus it may be that in general the range of the 1/T regime is determined by parameter? other than £ /l .

c n

The closeness of the points plotted in fi-gure 3 to the straight lines we have drawn suggests Fig. 1 : Variation of the reduced attenuation A for the PbCu specimens. The origins have been displaced in order to separate the data for the different specimens. They have been arranged in order of decreasing values of d /ln> starting from the bottom of the page. On each of the straight lines the point at which Sc = In has been marked by a large square. For each set of points

the scale factor indicates the amount by which the Ar scale has been expanded : a factor x 2 for instance, indicates that the scale is 0.25 per di-vision rather than 0.5.

JOURNAL DE PHYSIQUE Colloque C6, supplément au n" 8, Tome 39, août 1978, page C6-681

Résumé.- Nous avons mesuré entre 1,4 et 4,2 K l'amortissement des ondes ultrasoniques de surface causé par effet de proximité dans des "sandwiches" de plomb surmontés d'une couche de cuivre ou d'argent, en fonction de l'épaisseur et de la température. Un com-portement systématique est observé dans les deux types d'échantillon qui ne paraît pas explicable en fonction des théories existantes.

that, in the limit of

I/T

proportional to a function of the form a. exp Reference Eb(dn/ln)l

]IT.

/1/ Kr'ritzig, E., Solid State Comun

2

for Cu, and a = 24.0 and b = 5.29 for Ag.

Fig. 2 : The same as figure I but for the PbAg specimens.

Fig. 3 : The slopes A: for each of the lines in figures 1 and 2, plotted on a logarithmic scale against (dn/ In) >, Each specimen is represented

by the same symbol used to represent it in figure

1 or 2.

Table I

The thickness dn and Mean free path I n in the N