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Submitted on 1 Jan 1981
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ACOUSTIC ATTENUATION OF SURFACE
ACOUSTIC WAVES IN SUPERCONDUCTING THIN
FILMS OF AMORPHOUS Bi
M. Toguchi, F. Akao
To cite this version:
JOURNAL DE PHYSIQUE
LblLoqus C5, suppldment au nO1O, Tome 82, octobro 1981 page C5-701
ACOUSTIC ATTENUATION OF SURFACE ACOUSTIC WAVES IN SUPERCONDUCTING THIN F I L M S OF AMORPHOUS B i
M. T o g u c h i and F. Akao
TIiz Iizstitutt' of Scient i f i e and Industrial Rclsenrch, Osaka University, Sui ta Osaka 565, upan an
Abstract.- The surface acoustic wave ( S A W ) propagating on a
piezoelectric substrate is accoinpanied by electric fields whicn can interact with charge carriers in an adjacent metal film.
This interaction causes the attenuation of the SAW. We studied
the interaction between the SAM and the normal electrons in the amorphous Bi film which is evaporated on a LiNbOj substrate.
The product of the mobility ( u ) and the Ferml energy ( E F ) of tne
amorphous Bi was obtained from the experimental results and was
evaluated to be 2.4x103(cm2/~s)e~. And the steep change of the
attenuation due to the acoustoelectric ef:ect has been observed
at the superconducting temperature Tc.
Since the superconducting amorphous Bi was discovered by Buckel
and Hilsch /1/, the many authors studied about the properties of this
material / 2 / . knorphous Bi is prepared by the method of general con-
densation. Bi was evaporated in a high vacuum and condensed onto a
substrate cooled with liquid He. On the other hand, Bierbaum /3/
has measured the SAW attenuation as a result of the interaction between the electric fields accompanying the wave and the charge carriers in the film which deposited on a piezoelectric substrate.
The work of Hutson and White /4/ and Adler /5/ yielded a theoretical
forlnula for the attenuation of SAW due to the interaction. Bierbaan
snowed that the interaction is most intense under a certain range of
tile rLil1~1 tnickness. In this paper, we report the temperature depend-
ence of the SXI attenuation in tile amorphous Bi films in the intense interaction range.of the film thickness.
The schematic diagram of the evaporation vacuum chamber for the
condensation at the He-temperature isillustrated in Fig. 1. The
chamber is i!,mlersed in liquid helium. The experiments were carried
out using a L i N b 0 3 (12S0Y-cut, X-direction) substrate with trans-
mitting and receiving transducers. Bi was deposited onto the sub-
strate wnich mounted o n the copper block by means of Apiezon L grease
During the evaporation (background pressure 1x10-* Torr), the film showed a superonducting state which has been checked by the measure-
ment of d . ~ . electrical conductivity. The variation of the attenu-
C5-702 JOURNAL DE PHYSIQUE
ationin the film was measured by standard tichnique between 2.0 and 300 K at frequencies of 190, 330, and 570 MIiz.
Fig. 2 shows the changes dfthe SAW attenuation and film resis- tivity asa function of the evaporation time (i.e. film thickness) during deposition of the Ei film. The change of the attenuation is similar to the result of Bierbaum. By using the theoretical formula of llutson and White / 4 / and Bierbaum / 3 / , we can obtain the following condition for the maximum attenuation am considering the interaction between the electric fields accompanying the wave and the charge carriers in the film:
with A = 4.34k2wv02q2, B = v~~~~ ( E
+
1 ) - ' ~ ~ - ~ , C = (E,, + 1)c0q2 ( v O b+
Yp 2 ~ F 2 w 2 ) , and D = 2 w v o q 2 p ~ F , where k i s t h e effective electromechanical coupling coefficient; w and v o (cm/sec) are the radian frequency and velocity of SAW, respectively; q (As) is electron charge; c y is rela- tive dielectric constant of the piezoelecric substrate; (As/Vcm) is the permittivity of free space; p (cm2/vs) is the electron mobility in the film; EF (eV) is the Permi energy. From the graph shown in Fig. 2,the product ~ E F can be calculated from Eq. (l), using the values of am = 10 dB/cm [from Pig. 1) and following constants: k 2 = 5 . 5 ~ 1 0 - ~ , E~ = 38.5, v o = 4x10' cm/sec, w = 2nx190 MHz, q = 1.6x10.-" As/Vcm.
From our experimental data we obtained ~.~x~o~(cI$/vs)~v as the value of uEF. This value is compared with the experimental data of Bierbaum performed at room temperature. In Fig. 3 the temperature dependence of the attenuation (after substruction of the residual attenuation) in
0
the very thin film (about 50 A) obtained at the maximum point (A) of the attenuation in Fig. 2 is shown. In this film the remarkable change was not observed at the transition temperature. However just below the crystallization temperature T x , the attenuation in the film decreases rapidly, and the attenuation rapidly increased again at the T,. Fig. 4 shows the temperature dependence of the attenuation in the thin film which the thickness corresponds to the point B in Pig. 2
The SALJ attenuation is very steep change at the Tc. It is shown that the change of the attenuation observed in Fig. 4 is due to the inter- action between the a.c. electrical fields accompanying the wave and
the normal electrons in the film. Fig. 5 shows tne
temperature dependence of the attenuation of the same film after annealed. Repeating the annealing,.the steep chage of the attenuation
Fig. 6 shows the SAW attenuation in the suerconducting and normal statesasa function of temperature for three frequencies of the wave. From the results, it is shown that in the normal state the attenu- ation is proportional to the frequency of the wave.
Prom the results of Fig. 2 and 4, we suppose that the thin Bi film evaporated at the He-temperature consists of the coexistent of the non-superconducting disorder phase and the superconducting
amorphous phase. The variation of the attenuation in Fig. 2 reflects the interaction with normal electrons in the parts of the non-super- conducting phase. The steep change of the attenuation in Fig. 4 represents the interaction with the normal electrons in the supercon- ducting amorphous domains when the superconducting shield effect is vanished at Tc.
References
/1/ W. Buckel and R. Hilsch, 2. Physik
138,
109 (1954). / 2 / See, for example, G. Bergmann, Phys. Rep.27,
159 (1976)./3/ P. Bierbaum, Appl. Phys. Lett. 21, 595 (1972).
P. Bierbaum, J. Acoust. Soc.
Am.55,
766 (1974)./4/ A. R. Iiutson and D. L. White, J. Appl. Phys.
33,
40 (1962). / 5 / R. Adler, IEEE Trans. Sonics Ultrasonics SU-18, 115 (1971).JOURNAL DE PHYSIQUE I ( A )
,
EVAPOilRNT: Bi T E Z I P E R A ' C U R E : 6 K : ATTENUATION 0 : R E S s I T I V l T Y- . _
EVAPORATION TIICE (sec)0
-1.0
-2.0
-3.0
Fig. 2. changes of
the SAW attenuation
and resistivitv against evaporation
time (i .e. thickness)
r B i FILM VERY THIN F l L M THCKNESS*SOA
-
-
,,*-
-
1 1 9 1 2 4 6 8 1 0 0.2-
m D-
r C - 0-
-
< I U A-
-
4 5 - 0 . 2 * '5..I
Pig. 3. Temperature dependence of the SAX attenuation in the very tQin Bi film(about 5 0 A). 81 FlLM F I L M THICKNCSS s 500 i
-
-
VIRGIN F I L M Pig. 4. Temperature dependence of the SAM attenuation in the amorphous Blfilm (about 5 0 0
