THE LAYER STRUCTURE ARISING AT THE SURFACE OF ALUMINIUM AFTER THE MAGNETIC FIELD H > Hc IS SWITCHED OFF

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THE LAYER STRUCTURE ARISING AT THE

SURFACE OF ALUMINIUM AFTER THE

MAGNETIC FIELD H > Hc IS SWITCHED OFF

V. Dolgopolov, S. Dorozhkin

To cite this version:

JOURNAL DE PHYSIQUE

Colloque C6, suppliment au no

8,

Tome 39, aotit

1978,

page

C6-685

THE LAYER STRUCTURE A R I S I N G AT THE SURFACE O F A L U M I N I U M AFTER THE MAGNETIC F I E U )

H

> H c I S SWITCHED OFF

V.T. Dolgopolov and S.I. ~ o r o z h k i n

Institut of Solid State Physics, USSR Academy of Sciences, ChernogoZovka, Moscow, USSR

Rbsum6.- On 6 t d d i e l a s t r u c t u r e d e l a couche au microscope. On t r o u v e que, s e l o n l a s t r u c t u r e , l a couche e s t semblable

B

1 ' Q t a t mixte

P

deux dimensions d e s supraconducteurs de premisre es- pSce.

A b s t r a c t . - A microscopic i n v e s t i g a t i o n of t h e l a y e r s t r u c t u r e i s c a r r i e d o u t . It i s found t h a t i n s t r u c t u r e t h e l a y e r i s c l o s e t o t h e two-dimensional mixed s t a t e of type-I superconductors 11-31.

The two-dimensional mixed s t a t e of type-I superconductors (tm-state) t h a t had been p r e d i c t e d by L.D. Landau was experimentally r e v e a l e d by I.L. Landau and Sharvin 141 i n studying voltampere cha- r a c t e r i s t i c s of hollow superconducting c y l i n d r i c a l samples. The tm-state can be t r e a t e d a s a l i m i t i n g case of t h e i n t e r m e d i a t e s t a t e a t a d e c r e a s e of t h e period o t t h e l a t t e r down t o thedimensions of t h e coherence l e n g t h / 1 , 3 / . It i s t o be hoped t h a t t h e tm-state l a y e r i s formed on type-I supercon- d u c t o r s u r f a c e when t h e e x t e r n a l magentic f i e l d H>Hc i s switched o f f

.

I n t h e p r e s e n t work an a t t e m p t h a s been made t o c a r r y o u t microscopic i n v e s t i g a t i o n s of t h e s t r u c t u r e t h a t a r i s e s on t h e sample on vanishing of t h e e x t e r n a l f i e l d a t a c h a r a c t e r i s t i c time ~ * 2 0 m s . Below t h e p r e l i m i n a r y i n v e s t i g a t i o n r e - s u l t s a r e given.

The measurements were c a r r i e d o u t on a p ~ r a l l e l e p i p e d th a t was c u t from a s i n g l e c r y s t a l of A 1 w i t h R3 00K/R4, 2K= 2x l o 4 , t h e c r y s t a l dimen- s i o n s were 1 x 1 ~ 2 cm. Roughness from t h e sample s u r - f a c e was removed by g r i n d i n g , a f t e r t h a t t h e defec- t i v e l a y e r was etched and t h e sample s u r f a c e e l e c t r o p o l i s h e d u n t i l a m i r r o r b r i l l i a n c e was obtained.

The s t a t e of t h e m e t a l c l o s e t o t h e s u r - f a c e i n one o f , t h e sample f a c e s t h a t i s p a r a l l e l t o a p l a n e (110) was s t u d i e d . The measurements were c a r r i e d o u t u s i n g p o i n t j u n c t i o n s produced by and e l e c t r i c breakdown of t u n n e l l i n g s t r u c t u r e s A l -

A1203-Ag. The t h i c k n e s s of t h e f i l m s A1203 and Ag

0 0

was of t h e o r d e r of 30 A , and 1000 A , r e s p e c t i v e l y . The r e s u l t s o b t a i n e d show t h a t each t u n n e l l i n g s t r u c t u r e accounted f o r only one j u n c t i o n . The

j u n c t i o n r e s i s t a n c e was w i t h i n t o 1 Ohm. The

*

e s t i m a t e of j u n c t i o n dimensions y i e l d s d * 3 x 1 0 ~ ~ t o 3x10-5cm. Since f o r A 1 t h e coherence l e n g t h

6

= I .36x10-4m / 5 / t h e n , i n t h e g i v e n experiments d<<co. Voltage on t h e j u n c t i o n s a f t e r switching o f f t h e magnetic f i e l d was e x p e r i m e n t a l l y determined a s a f u n c t i o n of time. The c u r r e n t through t h e junc- t i o n was f i x e d . Volt-ampere c h a r a c t e r i s t i c s of t h e j u n c t i o n a r e markedly d i f f e r e n t i n t h e normal and superconducting s t a t e s of A l . That allowed t o de- termine from t h e v o l t a g e magnitude i n what s t a t e t h e sample under t h e j u n c t i o n i s . The measurements were c a r r i e d o u t a t 0.4 K. he magnetic f l u x i n

t h e sample remained h i g h e r t h a n i t s c r i t i c a l v a l u e Oc=HcS ( S i s t h e sample c r o s s - s e c t i o n a r e a ) f o r approximately 1 s a f t e r t h e e x t e r n a l magnetic f i e l d H=2H was switched o f f . The s t a t e of t h e metal c l o s e t o t h e sample s u r f a c e was s t u d i e d i n t h e f i r s t 20 m s a f t e r t h e superconducting l a y e r was formed on t h e s u r f a c e . A f t e r amplifying ( t h e ampli- f i e r bandwith was 20 Hz t o 1 MHz) t h e v o l t a g e from t h e j u n c t i o n a r r i v e d i n one of t h e c a n a l s of a two- r a y memory o s c i l l o g r a p h . The s t a r t of t h e o s c i l l o - graph scanning was synchronised w i t h t h e magnetich f i e l d switching o f f .

F i g u r e 1,2 r e p r e s e n t s a s i g n a l recorded from t h e two j u n c t i o n s . (The d i s t a n c e between junc- t i o n s i s 1 = ( 0 . 8 + 2 . 3 ) ~ 1 0 - l cm)

.

Arrows show t h e mo- ment of s u p e r c o n d u c t i v i t y occurence t l o s e t o t h e s u r f a c e . Narrow p u l s e s correspond t o t h e normal s t a t e of t h e r e g i o n under t h e j u n c t i o n . By t u r n i n g t h e magnetic f i e l d i n t h e p l a n e of t h e sample one can f i n d such a p o s i t i o n (+=O) a t which t h e p u l s e s on t h e two j u n c t i o n s occur simultaneously (Figure

1). A change of t h e a n g l e $ r e s u l t s i n t h e f a c t

t h a t t h e p a l s e s a r e no l o n g e r synchronous ( F i g u r e t i n g r e g i o n dimensions a r e g r e a t e r by approximatel: 2 ) . a f a c t o r of 20. F i g . I :

6

= 0 , 0 = -1,2'. F i g . 2 :

6

= 2', O = 1,2' H o r i z o n t a l a x i s : 10 ms/ d i v . A time d e l a y A t between t h e c o r r e s p o n d i n g p u l s e s from t h e two j u n c t i o n s i s a l i n e a r f u n c t i o n ( s e e F i g u r e 3) of t h e a n g l e

6

a t low v a l u e s o f t h e l a t t e r . These r e s u l t s can e a s i l y be understood i n assuming t h a t t h e n o m a l r e g i o n s a r e t h e l i n e s of t h e o r d e r of Imn long. These l i n e s a r e e l o n g a t e d i n t h e magnetic f i e l d and move on t h e s u r f a c e nor-

m a l l y t o H. The motion v e l o c i t y , i n accordance w i t h F i g u r e 3 i s

2

=

V

= ( 1 , 3 t o 3,7) cm/s. E r r o r s i n

A t

t h e v e l o c i t y measurements a r i s e due t o t h e f a c t t h a t t h e s p a c i n g between t h e j u n c t i o n s i s ambiguous. The t r a n s v e r s e dimensions of t h e normal s t a t e r e g i o n s can be determined from t h e p u l s e d u r a t i o n

T, and t h e v e l o c i t y . At 0=1 ,20(-rl = 0 . 4 ~ 1 0 - ~ s ) t h e y a r e (5 t o 1 5 ) ~ 1 0 - ~ c r n , (4 t o 12)E0. The superconduc-

F i g . 3 : Dependence of t h e time d e l a y A t on t h e m a g n e t i c f i e l d d i r e c t i o n . 1 and 2 have r e v e r s e magnetic f i e l d .

They a r e (1 t o 3)xl0-*cm. C o r r e l a t i o n between t h e p u l s e s from t h e two j u n c t i o n s was observed o n l y i n t h e i n c l i n e d magnetic f i e l d ( 0 2 0 . 6 ~ ) . The p u l s e d u r a t i o n corresponding t o t h e normal s t a t e was s t r o n g l y depend on t h e a n g l e 0. It v a r i e d from 2 x 1 0 - ~ s a t O =2.4' t o 2 x 1 0 - ~ s a t O=0.

The l a y e r t h i c k n e s s e s t i m a t e d from t h e ma-

g n e t i c f l u x magnitude emerging a t occurence of t h e l a y e r was t h e o r d e r of coherence l e n g t h .

References

/ I / Landau, L.D., P r i v a t e Communcation, See D . Shoenberg S u p e r c o n d u c t i v i t y , Cambridge, Uni- v e r s i t y P r e s s , 1938, p. 59.

/ 2 / Gor'kov; L.P. and Dorochov, O.N., ZhETF,

67

(1974) -1925.

/ 3 / Andreev; A.F. and Yu.K.Dzhikaev, ZhETF, P i s . Red.,

26

(1977) 756.

/ 4 / Landau, I . L . and S h a r v i n , Yu.V., ZhETF, P i s . Red., 10 (1969) 192.