Superconducting contacts in high magnetic fields

Superconducting contacts in high magnetic fields

Vohune 22, number 5 PHYSICS LETTERS SUPERCONDUCTING CONTACTS 15 September 1966 IN HIGH MAGNETIC FIELDS J. I. PANKOVE RCA L a b o r a t o r i e...

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Vohune 22, number 5

PHYSICS LETTERS

SUPERCONDUCTING

CONTACTS

15 September 1966

IN HIGH

MAGNETIC

FIELDS

J. I. PANKOVE RCA L a b o r a t o r i e s , P r i n c e t o n , N e w J e r s e y

Received 26 July 1966

"Supercurrents" through-contacts between oxidized superconductors have previously been observed at biases in excess of 2A at H = 0 and now they have also been observed at magnetic fields greater than Hc2. An anomalous magnetoresistance occurs in Pb-PbO-Pb contacts.

In a p r e v i o u s l e t t e r [1] I have r e p o r t e d the o b s e r v a t i o n of a new e f f e c t at the contact between two s u p e r c o n d u c t o r s : the 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 at 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. Ref. 1 m e n t i o n e d that including a m a g n e t i c f i e ld t r a n s v e r s e l y to the contact c u r r e n t by p a s sing a n o t h er c u r r e n t along one of the s u p e r c o n d u c t o r s (Silsbee effect) a f f e c t s the d . c . J o s e p h s o n c u r r e n t (causing a f i e ld dependent o s c i l l a t i o n ) but does not affect the s u p e r c u r r e n t at n o n - z e r o b i a s a c r o s s the contact. The p u r p o s e of the p r e s e n t l e t t e r is to r e p o r t the b e h a v i o u r of the contact at still l a r g e r m a g n e t i c f i e l d s obtained by m e a n s of an e x t e r n a l m a g n e t . A p r e s s u r e contact was made between two c r o s s e d s u p e r c o n d u c t i n g w i r e s 0.005" in d i a m e t e r . Niobium or P b - c o a t e d n i c h r o m e was used. A companion sample of the s a m e w i r e with four connections, f o r m o n i t o r i n g the state of the w i r e , was p l a c e d in c l o s e p r o x i m i t y to the c r o s s e d wires contact. The V-I c h a r a c t e r i s t i c s of the contact and the potential drop a c r o s s the inner connections of the companion sample (carrying 10mA) were measured at 4.2°K in a magnetic field that could be varied from 0 to 10 kgauss. The Pb-coated nichrome wire becomes r e s i s tive at about 500 gauss. Its resistmnce r i s e s more abruptly at about 2000 gauss (Hc2) and remains constant above 3000 gauss, indicating that the Pb Table 1 Characteristics of a Pb-PbO-Pb contact at 4.2°K. H

Is

d V / d l at V = 0

(kgauss)

(mA)

(m~)

0 1.7 2.7 5.1 7.6

65 35 20 10.5 7

0 0.78 1.2 4.3 5.9

d V / d I at V > 1 mV

(m~ 16 11 9.3 7.5 7.3

ll|l

HI1

xIln

.................

, 0

I

:,,., '

rl i

/I

~,,

i

i i i

I 20mV

I0

_

tOOmA~ O'H:

,,

~

(

V;'

~

o_ --

~""

0

2

4mV

Fig. 1. V-/characteristics of two Pb-PbO-Pb contacts at 4.2°K. (H in kgauss). i s c o m p l e t e l y quenched above 3 k g au ss. The field i s t r a n s v e r s e to the c u r r e n t . Niobium h as a m o r e g r a d u a l quenching c h a r a c t e r i s t i c which begins at about 5.5 k g a u s s g/c2). Fig. 1. shows the V - [ c h a r a c t e r i s t i c s of two P b - P b O - P b c o n t a c t s at v a r i o u s m a g n e t i c f i e l d s . The z e r o f i el d c u r v e s exhibit a d . c . J o s e p h s o n c u r r e n t - a c h a r a c t e r i s t i c e x p e c t e d of a contact between two s u p e r c o n d u c t o r s s e p a r a t e d by a thin oxide l a y e r . The 1.5 k g a u s s c u r v e and those taken at h i g h e r f i e l d s , show a finite contact r e s i s t a n c e at low b i a s e s (a c h a r a c t e r i s t i c of a contact b e tween n o r m a l m e t a l s ) indicating that the s u p e r 557

Volume 22, n u m b e r 5

PHYSICS

conductors have been magnetically quenched. The resistance at low bias increases with increasing f i e l d s . B e y o n d a b i a s of a b o u t 1 m V , t h e V-[ characteristic forms a straight line which does n o t e x t r a p o l a t e to t h e o r i g i n b u t r a t h e r to a n o n z e r o c u r r e n t a t z e r o b i a s . T h i s w i l l be r e f e r r e d to a s the s u p e r c u r r e n t . A s s h o w n in t a b l e 1, t h i s s u p e r c u r r e n t (/s) i s a d e c r e a s i n g f u n c t i o n of t h e magnetic field. However, the remarkable feature i s t h a t the s u p e r c u r r e n t t h r o u g h t h e c o n t a c t c a n be observed even when the bulk superconductor i s c o m p l e t e l y q u e n c h e d by t h e m a g n e t i c f i e l d . T h i s e f f e c t d o e s n o t d e p e n d on t h e o r i e n t a t i o n of t h e f i e l d w i t h r e s p e c t to t h e c u r r e n t . Another interesting observation with Pb contacts - another new effect which needs an explan a t i o n - i s a d e c r e a s e of t h e d y n a m i c r e s i s t a n c e dV/d[ above 1 mV as the magnetic field is increased. The supercurrent has also been observed at c o n t a c t s b e t w e e n Nb w i r e s a b o v e t h e c r i t i c a l f i e l d f o r t r a n s p o r t a l o n g t h e N b w i r e (fig. 2). T h e f a c t that Nb is magnetically quenched is evident from t h e a b s e n c e of d . c . J o s e p h s o n c u r r e n t a b o v e 6 k g a u s s . Y e t the V-[ c h a r a c t e r i s t i c a b o v e 1 m V e x t r a p o l a t e s to a f i n i t e c u r r e n t a t z e r o b i a s . In c o n t r a s t to t h e a n o m a l o u s m a g n e t o r e s i s t a n c e o b s e r v e d w i t h l e a d , t h e d y n a m i c r e s i s t a n c e of Nb-NbO-Nb contacts above 1 mV is nearly indep e n d e n t of H. The present data indicate that at a sufficiently h i g h c u r r e n t t h r o u g h t h e c o n t a c t t h e i n t e r r a c i a l r e g i o n s of t h e m e t a l s a t t h e c o n t a c t b e h a v e a s s u p e r c o n d u c t o r s . A c c o r d i n g to p r e v i o u s w o r k [1],

COLD

NEUTRON

SCATTERING

LETTERS

15 September 1966

I.OmA

,,,i, '"1 ....

,,o 2

I

,""

! ....

I i

///''i

2

4 mY

Fig. 2. V-I c h a r a c t e r i s t i c s of N-NbO-Nb contact at

4.2°K. ( H i n kgauss). t h i s b e h a v i o u r s e e m s to o c c u r w h e n t h e l o c a l c u r r e n t d e n s i t y throug.h m i e r o s p o t s of t h e c o n t a c t i s of t h e o r d e r of 10 u A / c m 2. T h e s e n e w r e s u l t s s u p p o r t t h e i n t e r f a c i a l m o d e l of r e f . 1 b a s e d on Parmenter's current-induced superconductivity

[2]. I a m g r a t e f u l to J . E . B e r k e y h e i s e r f o r h i s a s s i s t a n c e a n d to W. H i c i n b o t h e m , R . D . L a r r a b e e , a n d G. W. C u l l e n f o r t h e u s e of t h e i r m a g n e t s .

References 1. J . I . Pankove, Physics L e t t e r s 21 (1966) 406. 2. R . H . P a r m e n t e r , Phys. Rev. 116 (1959) 1390; Phys. Rev. 140 (1965) A1952.

BY

LIQUID

NITROGEN

H. T E U T S C H , N. M A T E E S C U , V. N A H O R N I A K a n d A. D I A C O N E S C U Institute of Atomic Physics, Bucharest, Rumania Received 26 July 1966

The d i s p e r s i o n law for the quasi-phonons in liquid nitrogen is derived from the m e a s u r e m e n t s of neutron s c a t t e r i n g on liquid nitrogen.

By m e a n s of c o l d n e u t r o n s c a t t e r i n g a n u m b e r of d y n a m i c a l m o d e l s of t h e b e h a v i o u r of t h e a t o m s in l i q u i d s h a v e b e e n v e r f i e d [ 1 - 8 ] . O u t s t a n d i n g importance was attached to coherently scattering 558

liquids, especially the monoatomic liquids such a s l i q u i d a r g o n [9,10] a n d l i q u i d m e t a l s [11,12]. We s t u d i e d c o l d n e u t r o n s c a t t e r i n g by l i q u i d nitrogen, which is essentially coherent. The dis-