The resistive critical field of niobium (columbium) wire
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The t r a n s v e r s e and l o n g i t u d i n a l r e s i s t i v e c r i t i c a l fields of n i o b i u m ( c o l u m b i u m ) w i r e s with d r a w i n g s t r a i n s up to e l e v e n were m e a s u r e d . The magnitude and a n i s o t r o p y of the r e s i s t i v e c r i t i c a l field can both be much g r e a t e r than the bulk G i n z b u r g - L a n d a u p a r a m e t e r s i n d i c a t e , even if " i n t e r n a l s u r f a c e " n u c l e a t i o n of s u p e r c o n d u c t i v i t y is a s s u m e d . It i s suggested that these effects a r e due to s u p e r c o n d u c t i v i t y in the d i s l o c a t i o n cell walls c r e a t e d by drawing or swaging. A s i m p l e m o d e l for this effect i s offered, and v a r i o u s e x p e r i m e n t s which test this model a r e d i s c u s s e d .
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effect of p l a s t i c d e f o r m a t i o n on the c r i t i c a l c u r r e n t d e n s i t y (Jc), of a Type II s u p e r c o n d u c t o r has been well e s t a b l i s h e d , 1-3 and i s g e n e r a l l y a t t r i b u t e d to fluxon pinning by the d e f o r m e d m i c r o s t r u c t u r e . The a n i s o tropy of Jc in c o l d - r o l l e d high field a l l o y s 4 and in n i o b i u m (columbium), with r e s p e c t to the d i r e c t i o n of the m a g n e t i c field, has been a s s o c i a t e d with d i s l o c a tion s u b s t r u c t u r e and g r a i n b o u n d a r y effects r a t h e r than t e x t u r i n g . The r e s i s t i v e c r i t i c a l field (Hr) also a p p e a r s to be a n i s o t r o p i c , at least for drawn w i r e s . 6,7 This effect was accounted for by n u c l e a t i o n of s u r f a c e sheath s u p e r c o n d u c t i v i t y at the wire s u r f a c e . 7 However, l a t e r e x p e r i m e n t s 8 on N b - Z r a l l o y s by W i l l i a m s o n and F u r d y n a d e m o n s t r a t e d that Hr was well above the bulk c r i t i c a l field but was not in t h e i r case a s s o c i a t e d with the wire s u r f a c e ; s i n c e the a n g u l a r dependence of Hr was close to T i n k h a m ' s f o r m u l a g for s u r f a c e n u c l e a t i o n , they concluded that " i n t e r n a l s u r f a c e " n u c l e a t i o n , p r o b a b l y at f i b e r b o u n d a r i e s , had o c c u r r e d . R a l l s ~g has c o n f i r m e d this point of view by c o m p a r ing m i c r o w a v e , u l t r a s o n i c and r e s i s t i v e m e a s u r e m e n t s and points out that Hr of Nb-25 pct a l l o y wire may d e c r e a s e from the " i n t e r n a l s u r f a c e " value to the " b u l k " value if it is heavily a n n e a l e d . Van Gurp, H working with n i o b i u m foils, a t t r i b u t e d a n o m a l o u s l y high v a l u e s for Hr to " f i l a m e n t a r y r e g i o n s " having h i g h e r values of ,~ ( G i n z b u r g - L a n d a u p a r a m e t e r ) than the bulk m a t e r i a l . Hence, the m a g n i t u d e and a n i s o tropy of Hr appear to be r e l a t e d to the d e f o r m a t i o n structure. I) THE BCC
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