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Submitted on 1 Jan 1978
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OBSERVATION OF SUBHARMONIC GAP
STRUCTURES IN SUPERCONDUCTING THIN
FILM TUNNEL JUNCTIONS FAVOURING
MULTIPARTICLE TUNNELING THEORY
P. Mukhopadhyay
To cite this version:
P. Mukhopadhyay.
OBSERVATION OF SUBHARMONIC GAP STRUCTURES IN
JOURNAL DE PHYSIQUE
Colloque C6, supplkment au no 8, Tome 39, aotit 1978, page C6-595
OBSERVATION
OF SUBHARMONIC GAP STRUCTURES I N SUPERCONDUCTING T H I N F I L M TUNNEL J U N C T I O N S
FAVOURING M U L T I P A R T I C L E TUNNELING THEORY
P. Mukhopadhyay
Department
of
Physics, ChaZmers University
of
TechnoZogy,
Fack, 5-402 20
Gothenburg, Sweden.
RGsumi5.- Les j o n c t i o n s tunnel composiSes a u s s i b i e n de supraconducteurs i d e n t i q u e s ou d i f f d r e n t s p r d s e n t e n t d e s s t r u c t u r e s subharmoniques q u i semblent pouvoir 8 t r e e x p l i q u e e s de fason s a t i s - f a i s a n t e p a r des e f f e t s t u n n e l s
1
p l u s i e u r s p a r t i c u l e s .A b s t r a c t . - Subharmonic gap s t r u c t u r e s a r e s e e n i n bothsymmetric and asymmetric superconducting t u n n e l j u n c t i o n s based on o x i d i z e d PbIn e l e c t r o d e s . The shape, amplitude and t h e r e l a t i v e magnitudes of t h e observed s t r u c t u r e s seem t o favour m u l t i p a r t i c l e t u n n e l i n g a s the p l a u s i b l e e x p l a n a t i o n .
This paper r e p o r t s t h e o b s e r v a t i o n of sub- harmonic gap s t r u c t u r e s (SGS) i n superconducting
tunnel j u n c t i o n s based on o x i d i s e d PbIn e l e c t r o d e s . The shape and t h e r e l a t i v e magnitudes of the s t r u c - t u r e s i n o u r non-shorted j u n c t i o n s s u p p o r t t h e m u l t i p a r t i c l e t u n n e l i n g t h e o r y I l l .
There a r e two main e x p l a n a t i o n s given f o r t h e SGS, namely : t h e m u l t i p a r t i c l e t u n n e l i n g
/ 11
and t h e non-linear Josephson type s e l f - c o u p l i n g / 2 / models. Both p r e d i c t t h e same l o c a t i o n i n v o l t a g e of t h e SGS (eV = 2A1/2n, 2A2/2n and A l + A2/(2n+I), where 2A1,2 a r e t h e energy gaps of t h e superconduc-
t o r s on e i t h e r s i d e of t h e j u n c t i o n ) and t h e same power dependence f o r r f r a d i a t i o n 131, / 4 / . However, t h e r e a r e d i f f e r e n c e s 131 : M u l t i p a r t i c l e t u n n e l i n g y i e l d s s t e p - l i k e s t r u c t u r e s , whereas t h e self-cou- p l i n g y i e l d s b o t h s t e p l i k e and resonance type s t r u c t u r e s
.
The m u l t i p a r t i c l e s t e p s (i ~ ]
)should f a l l o f f much more r a p i d l y than t h e s e l f - c o u p l i n g ones (M~), where M i s t h e t u n n e l i n g m a t r i x element and n i s t h e number of the SGS. I n t h e l a t t e r c a s e , t h e even (2n) and t h e odd (2n+
1) s e r i e s a r e due t o somewhat d i f f e r e n t mechanisms and may be expec- t e d t o show one d i f f e r e n c e i n t h e i r p r o p e r t i e s . Furthermore, t h e r e l a t i v e magnitudes of t h e s t r u c - t u r e s a t Al/e and A2/e should be p r o p o r t i o n a l t o A ~ / A ~ according t o m u l t i p a r t i c l e theory 131, andt d
A2/A1 a c c o r d i n g t o Werthamer's theory f o r t h e s e l f - c o u p l i n g 121.P r e v i o u s l y r e p o r t e d experiments have, i n some c a s e s , favoured m u l t i p a r t i c l e t u n n e l i n g /5/- 191, w h i l e t h e s e l f - c o u p l i n g mechanism h a s been t h e obvious c h o i c e i n o t h e r s / 4 / , / l o / ,
11 11.
For asymmetric j u n c t i o n s made o f . two d i f f e r e n t su- perconductors, most workers 151, 1111-1131 could n o t d i s t i n g u i s h between t h e two t h e o r i e s , e x c e p t f o r Giaever and Z e l l e r / l o / , who supported s e l f - coupling a s t h e s o l e mechanism. M e t a l l i c s h o r t s have been noted i n some c a s e s , when a l a r g e number of SGS were observed /12/-1141. PbIn was chosed a s a r e f e r e n c e e l e c t r o d e as i t was expected t o give a patchy oxide 1151 favouring m u l t i p a r t i c l e tunne- l i n g 111. As t h e top e l e c t r o d e of the j u n c t i o n , we e i t h e r chose Pb ( g i v i n g A1 % A2) o r a PbTl a l l o y 1161 (A1
#
A2). The p r e p a r a t i o n technique i s des- c r i b e d elsewhere 191. The absence of m e t a l l i c s h o r t s i n t h e j u n c t i o n s was i n d i c a t e d by the f a c t s t h a t t h e dc Josephson c u r r e n t could be suppressed t o zero by a small magnetic f i e l d a n t t h e I-V t r a c e s of t h e sample i n t h e superconducting s t a t e were always below t h a t of t h e normal s t a t e .The s t e p - l i k e c h a r a c t e r of t h e e x c e s s cur- r e n t i s shown i n f i g u r e l f o r a PbIn-Oxide-Pb j u n c t i o n , (I1(*) Q 5 m ) . S i m i l a r s t e p s were a l s o
o b t a i n e d i n most c a s e s f o r h i g h e r o r d e r s t r u c t u r e s . There was no d i f f e r e n c e i n shape between odd and even s e r i e s of s t e p s . I n low r e s i s t a n c e j u n c t i o n s
(with I1 t y p i c a l l y % 500 mA o r more), s e v e r a l s t e p s could be observed. I n no c a s e , howeyer, t h e r e l a t i - ve magnitudes of t h e h i g h e r o r d e r s t e p s exceded the maximum v a l u e s expected from m u l t i p a r t i c l e theory 131, i n c o n t r a s t t o some r e s u l t s observed by o t h e r s 141, 151, 1101, 1121.
I0
'
I 1 i s where IO i s t h e dc Josephson c u r r e n t .(For bad junctions, when the metallic shorts are
A d1/dV vs V trace, a more sensitive measurement,
present, we also observe a less rapid fall off for
SGS)
.
a E ' 5.0 C c2
3 0 0 0 1.o
2.0Voltage
( r n V )Fig.
1 :I-V curve for a PbIn-Oxide-Pb tunnel junc-
tion at 1.3 K.(I
,
the dc Josephson current %30
mAis suppressed inOthe figure). The dashed curve is
a theore
tical curve (calculated from reference /I/
)normalized to the current at eV
=A.
Figure 2 displays a typical I-V and dI/dV-
V characteristic for a PbIn-Oxide-PbT1 sample with
a relatively higher tunnel resistance (I
%300 mA).
1
Note that the structures are seen only at A and A
1
2
but not at
(A
+
A2)/3 (the first member of the odd
1
series).
The amplitude of higher order (n
= 1)structures falls off rapidly.
showed no structures for large n values. Also note
that the relative magnitudes of the structures at
A l and A2 (where A is the smaller PbTl gap) is
1
less than A /A2. This violates the prediction by
1
the self-coupling model and the previously reportEd
results 151, /lo/-/13/, where the ratio was A /A
2
1'but agrees with multiparticle tunneling.
Hence our results on the step-like structu-
res and on the relative magnitudes of the steps in
similar and dissimilar superconductor junctions
show that, unlike often stated, the multiparticle
tunneling model is a plausible explanation for the
SGS in case of the non-shorted tunnel junctions.
I would like to thank T. Claeson for his
help in dI/dV measurements and for supplying the
PbTl alloy.
References
/ I /
Schrieffer, J.R. and Wilkins, J.W., Phys. Rev.
Lett.
10
(1963) 17; Wilkins, J.W., Tunneling
phenomena in solids (ed. Burstein E. and Lund-
qvist S.), Plenum, N.Y. (1969).
/2/
