PROXIMITY EFFECT OF Cu CLAD Nb AND Cu CLAD NbTi WIRES

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PROXIMITY EFFECT OF Cu CLAD Nb AND Cu

CLAD NbTi WIRES

Y. Oda, G. Fujii, H. Nagano

To cite this version:

Y. Oda, G. Fujii, H. Nagano.

PROXIMITY EFFECT OF Cu CLAD Nb AND Cu

PROXIMITY EFFECT OF Cu CLAD Nb AND Cu CLAD NbTi WIRES

Y. Oda, G. Fujii and H. Nagano

Institute for Solid State- Physios, University of Tokyo, Roppongi, Minato-ku, Tokyo 106, Japan

Abstract.- The Meissner screening length in Cu of Cu clad Nb and Cu clad NbTi wire has been measured over temperature range from T down to 25 mK. The temperature dependence of the screening length is expressed by (T+T )- T/2 from about 1.5 K to the minimum temperature. The length becomes 12 urn for Cu clad Nb and 7 ym for Cu clad NbTi wire at 25 mK. The measurement has been also done for Cu clad fine multiple NbTi wire. In this case, the screening length shows a drastic increase above 1 K, and the slope becomes remarkably small below 1 K. Some comments for application are also presented.

Normal metal N contacted with a superconduc-tor S shows superconducting properties. In low ma-gnetic field, Meissner effect is observed in the normal metal /1-2/. We have measured the temperatu-re dependence of the sctemperatu-reening length in Cu of Cu clad Nb and Cu clad NbTi wire, which are available for the superconducting magnet./3/. The mutual in-ductance due to the wires were measured by mounting them parallel to AC magnetic field in measuring coil /4/. The mutual inductance due to the wire changes suddenly at superconducting transition tem-perature T of Nb or NbTi core. As temtem-perature is lowered, the mutual inductance increases. This is attributed to the increase of the screening length p in Cu, which is the distance, from the N/S inter-face, to which magnetic field penetrates freely in N. Experimental p was calculated by

p = r{(l + AM(T)/AM (T ) ) "1/2 - 1} ( 0 o c

where r is the radius of the core, AM(T) is the mutual inductance change below Tc > AM dc) is the inductance change due to the transition of the co-re /4/. The measuco-rements weco-re made at several fco-re- fre-quencies from 20 to 700 Hz. The features of the temperature dependence did not change in this re-gion. The temperature dependence at 70 Hz is given in figure 1 against T- 1/2. The temperature depen-dence of p is expressed by (T + T ) a'2. The values of p become 12 ym for Cu clad Nb wire and 7 ym for Cu clad NbTi wire at 25 mK, respectively.

A theoretical calculation of the screening length was made by Deutscher and de Gennes / I- 2 1 .

In the dirty limit, where electron mean free path

1 is smaller than the coherence length L in N, p

N a

is given by

Where K(0) is the Ginzburg-Landau parameter in N at the interface. In the dirty limit, £N for T » TC N (transition temperature of N) is given by

V

T )

• <*

v F

V

3 k T ) I / 2 ( 3 )

Because K(0) is independent of T at low temperatures, T- 1/2 dependence is derived from this equation 111.

Equations (2) and (3) mean that p does not include any parameters depending on S. This equation gives a close value to the experimental result. But the

( T + T )- 1/2 dependence can not be obtained by this 0

Fig. 1 : Temperature dependences of the screening length p, which were measured at 70 Hz for Cu (0.35mm<j>) clad Nb (0.25mm<}>) and Cu (O.lmmijO clad NbTi (0.07mm<|>) wires.

JOURNAL DE PHYSIQUE Colloque C6, supplément au n° 8, Tome 39, août 1978, page C6-693

Résumé.- La longueur d'écrantage Meissner dans le Cu du fil de Nb recouvert de Cu et du fil de NbTi recouvert de Cu a été mesurée dans une gamme de température comprise entre Tc et 25 mK. La dépendan-ce en température de la longueur d'écrantage s'exprime (T+XD) ^ à partir d'environ 1,5 K jusqu'au minimum de température.NLa longueur à 25 mK sera de 12 ym pour le fil de Nb recouvert de Cu et de __ 7 ym pour le fil de NbTi recouvert de Cu. La mesure a été également effectuée pour le fil de NbTi a filament fin multiple recouvert de Cu. Dans ce cas, la longueur de blindage présente une augmenta-tion soudaine entre Tc et 1 K, et la pente devient considérablement réduite au-dessous de 1 K.

e q u a t i o n . T has been found t o be 73 mK f o r Cu c l a d

0

Nb and 57 mK f o r Cu c l a d NbTi. E l e c t r o n - e l e c t r o n i n t e r a c t i o n i n

N

may be needed t o g i v e T term.

0

The measurement., was a l s o made f o r Cu c l a d f i n e m u l t i p l e NbTi w i r e 151. I n t h i s c a s e , t h e mu- t u a l inductance change below Tc was very d r a s t i c above 1 K. The temperature dependence of t h e mutual inductance i s given i n f i g u r e 2 a g a i n s t 1 / T .

Fig. 2 : Temperature dependence of t h e mutual in- ductance below T due t o t h e Cu (0.12 mm$) c l a d f i n e m u l t i p l e ~ b f i (0.009mm$x78) wire.

One p o s s i b l e e x p l a n a t i o n of t h i s temperature depen- dence above 1 K i s t h e overlapping of t h e s c r e e n i n g range from each i n t e r f a c e . From f i g u r e 1, p should be about 1 pm a t 1 K . This i s about same v a l u e a s t h e d i s t a n c e between t h e c o r e s . Then a l l c o r e s may be included i n a superconducting sheath below 1 K.

One a p p l i c a t i o n i s t o use t h e s e w i r e s f o r thermometry down t o mK region. It g i v e s much l a r g e r inductance change t h a n t h a t of CMN. Moreover, bet- t e r thermal c o n t a c t w i t h m e t a l can be e a s i l y achie- ved.

On

t h e o t h e r hand, our r e s u l t s imply t h a t Cu c l a d superconductor a r e n o t s u i t a b l e a s a f l u x transformer o r a magnetic p i c k up c o i l , f o r i n s - tance, f o r SQUID measurements.

References

/ I / Deutscher, G. and de Gennes, P.G. : Superconduc-

t i v i t y , e d i t e d by R.D. Parks, Marcel Dekker, I n c . , New York 1969, v o l 2 , page 1005.

/2/ Deutscher G. and V a l e t t e C. : Proc. of LT 13,

3

(1972) 603.

/3/ Vacuum M e t a l l e r g i c a l Co., LTD., Japan.