New effect at superconducting contacts

New effect at superconducting contacts

Volume 21, n u m b e r 4 PHYSICS r e n t s c a n b e r e p r e s e n t e d b y t h e l o o p s s h o w n in f i g . 2. T h e h o r i z o n t a l p a...

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Volume 21, n u m b e r 4

PHYSICS

r e n t s c a n b e r e p r e s e n t e d b y t h e l o o p s s h o w n in f i g . 2. T h e h o r i z o n t a l p a r t of t h e c u r r e n t l o o p s a t t h e e n d of t h e c y l i n d e r n e a r e s t to t h e m a g n e t , r e p e l t h e m a g n e t in m u c h t h e s a m e w a y a s t h e c i r c u l a r c u r r e n t l o o p s of a p e r f e c t c y l i n d e r .

NEW

EFFECT

AT

LETTERS

1 J u n e 1966

References 1. B.Daniels a n d P . W . M a t t h e w s , Rev. Sci. Instr. (to be published, June 1966).

SUPERCONDUCTING

CONTACTS

J . I. P A N K O V E RCA Laboratories, Princeton, New Jersey Received 7 May 1966

When two oxidized superconductors a r e in p r e s s u r e contact, a c u r r e n t in excess of the n o r m a l singlep a r t i c l e tunnel c u r r e n t is obtained over a range of biases extending m o r e than a decade beyond 2A.

A p r e s s u r e c o n t a c t b e t w e e n two s u p e r c o n d u c tors appears superconducting [1-3]. Since most metals are coated with a thin insulating oxide l a y e r , t h e s u p e r c o n d u c t i n g c u r r e n t t r a n s p o r t at t h e c o n t a c t m u s t b e i n t h e f o r m of J o s e p h s o n c u r r e n t [4]. A l t h o u g h r e c e n t e x p e r i m e n t s [5] w i t h contacts involving anodized superconductors have b e e n u s e d to m e a s u r e t h e e n e r g y gap of s u p e r c o n ductors by tunneling spectroscopy, the following e f f e c t s e e m s to h a v e e s c a p e d n o t i c e ( o r b e e n d i s m i s s e d a s e r r a t i c in t h e c a s e of e v a p o r a t e d d i o d e s [6, 7 ] ) . T h e V - [ c h a r a c t e r i s t i c s of p r e s s u r e c o n t a c t s (as p e r r e f . 2) b e t w e e n o x i d i z e d Nb w i r e s o r b e t w e e n P b - c o a t e d n i c h r o m e w i r e s at 4.2OK s h o w t h e p r e s e n c e of a l a r g e s u p e r c u r r e n t a t b i a s e s c o n s i d e r a b l y in e x c e s s of 2A. T h i s s u p e r c u r r e n t is evidenced by the fact that the socalled ohmic r e g i o n e x t r a p o l a t e s to s o m e f i n i t e c u r r e n t at z e r o voltage. The V - I characteristic breaks near the maximum zero-voltage-current and follows a displaced ohmic straight line. Raising the contact t e m p e r a t u r e by l i f t i n g t h e s p e c i m e n a b o v e t h e h e lium bath gives the expected ohmic characterist i c (fig. l a ) . At 4.2OK, t h e s u p e r c u r r e n t h a s b e e n f o l l o w e d f o r b i a s e s of t e n s of m i l l i v o l t s . A magnetic field H = I'/5r can be induced t r a n s v e r s e l y to t h e c o n t a c t c u r r e n t b y p a s s i n g a c o n t r o l c u r r e n t I ' a l o n g o n e of t h e c r o s s e d s u p e r c o n d u c t o r s [2] ( r i s t h e w i r e r a d i u s ) . A s 1' i s varied, the d.c. Josephson current undergoes the characteristic oscillatory behavior. However, a s s h o w n in fig. l b , t h e s u p e r c u r r e n t r e m a i n s 406

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Fig. 1. V-I c h a r a c t e r i s t i c s of contact between two c r o s s e d wires. (a) Contact betweeb Nb wires, Upper curve: contact in liquid He; lower c u r v e with contact above bath (Nb normal). (b) Effect of magnetic field on contact between wires. Upper curves: contact at 4.2°K; lower curve: T> Tc (c) Contact between Nb and A1 wires at 4.2°K. (All these 5 rail d i a m e t e r Nb wires were anodized at 6 volts).

Volume 21, number 4

PHYSICS LETTERS

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Fig. 2. Speculative model for the new effect. (a) Equivalent circuit for contact between 2 superconductors. (b) Equivalent circuit for contact between a superconductor and a normal metal: S - saturable superconductor; RT - normal tunneling resistance: R s spreading resistance in normal metal. (c) Tunneling between broadened energy gap interfaces. p r a c t i c a l l y unchanged until the c o n t r o l w i r e is quenched at 3.4 A ( t h e r m a l quenching nucleated at the connections). This phenomenon has not y e t b e e n c l e a r l y und e r s t o o d . H o w e v e r , it is e v i d e n t that the contact c o n s i s t s of two c o m p o n e n t s : (1) a superconducting t r a n s p o r t and (2) an o h m i c t r a n s p o r t which app e a r s when the f i r s t p r o c e s s is s a t u r a t e d . This s t a t e m e n t can be s y m b o l i z e d by the c i r c u i t d i a g r a m of fig. 2a: the contact c o n s i s t s of a s a t u r a b l e (but not c u r r e n t quenchable) e l e m e n t in p a r a l l e l with a r e s i s t o r . P r e s u m a b l y , the r e s i s t a n c e is the tunneling e f f e c t i v e r e s i s t a n c e . The s a t u r a ble s u p e r c o n d u c t o r b r i d g e r e p r e s e n t s the new e f fe c t. When a co n t act is made b e tw e e n a n o r m a l m e t al and a s u p e r c o n d u c t o r , the V-I c h a r a c t e r i s t i c of the contact shows an i n i t i a l r e g i o n of high conductance with an abrupt change to a r e g i o n of l o w e r conductance (fig. l c ) . The change s o m e t i m e s is an abrupt bend and s o m e t i m e s a b r e a k with a s h o r t r e g i o n of n e g a t i v e r e s i s t a n c e e x hibiting h y s t e r e s i s . The l o w e r conductance mode e x t r a p o l a t e s to s o m e finite c u r r e n t at z e r o v o l t -

1 June 1966

age and a p p e a r s p a r a l l e l to the o h m i c c h a r a c t e r i s . tic obtained at T > Tc. Such a b e h a v i o r can be s y m b o l i z e d in fig. 2b w h e r e R s is the s p r e a d i n g r e s i s t a n c e of the co n t act i n s i d e the a l u m i n u m . In the h i g h - c o n d u c t a n c e mode S is connected s e r i a l l y to R s (finite initial slope of V - f c h a r a c t e r i s t i c ) . When S s a t u r a t e s , the e x c e s s c u r r e n t flows through R T and R s in s e r i e s . T y p i c a l l y , R s is of the o r d e r of 1 ohm. F r o m this s p r e a d i n g r e s i s t a n c e one c a l c u l a t e s an e f f e c t i v e contact d i a m e t e r of 50 ~ . The " s a t u r a t i o n " c u r r e n t (typi c a l l y of the o r d e r of 10 -3 A) c o r r e s p o n d s to a c u r r e n t density of the o r d e r of 5 × 109 A / c m 2. This is a m i n i m u m c u r r e n t density s i n c e it is b a s e d on the a s s u m p t i o n of a single c u r r e n t - c a r rying spot. Actually, in the c a s e of a contact b e tween two s u p e r c o n d u c t o r s , the undamped o s c i l l a t o r y b e h a v i o r of the d . c . J o s e p h s o n c u r r e n t in a m a g n e t i c field i n d i cat es that the contact cons i s t s of at l e a s t two c u r r e n t c a r r y i n g spots. T h e r e f o r e , on the b a s i s of this model, one would p r e d i c t that the c u r r e n t density is well above 109 A / c m 2. Such high c u r r e n t d e n s i t i e s b r i n g to mind the work of P a r m e n t e r [8] who s u g g e s t e d the p o s s i bility of i n c r e a s i n g the s u p e r c o n d u c t i n g e n e r g y gap by m e a n s of high c u r r e n t d e n s i t i e s . It is conc e i v a b l e that the p r e s e n t high density i n c r e a s e s the e n e r g y gap of the s u p e r c o n d u c t o r at the c o n tact i n t e r f a c e s and p e r m i t s a su p er co n d u ct i n g tunneling c u r r e n t to flow (with o r without photon o r phonon a s s i s t a n c e ) while the F e r m i l e v e l s a r e at d i f f e r e n t p o t e n t i a l s on e i t h e r side of the contact (fig. 2c). The author is g r a t e f u l to G . D . Cody, R . H . P a r m e n t e r , and B. Ro sen b l u m f o r helpful d i s c u s s i o n s .

References 1. H.Meissner, Phys. Rev.109 (1958) 686. 2. J . I . Pankove, IRE Transactions PGED-7 (1960) 137. 3. J . E . Z i m m e r m a n and A.H.Silver, Physics Letters 10 (1964) 47. 4. B.J. Josephson, Physics Letters 1 (1962) 261. 5. H.J. Levinstein and J. E. Kunzler, Bull. Am. Phys. S o c . l l (1966) 88. ~. T.Seidel and A.W.Wieklund, VIII Intern. Conf. on Low temperature physics, London (Butterworths, 1962) p. 176. 7. G.D. Cody, Y. Goldstein and R. Cohen, RCA Review 25 (1964) 433. 8. R.H.Parmenter, Phys.Rev. l l 6 (1959) 1390; Phys. Rev. 140 (1965) A1952.

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