Superconductivity and dislocation cell structure in niobium: Stress effects
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e x p e r i m e n t a l e v i d e n c e x-~ has i n d ic a t e d that the d e v e l o p m e n t of a d i s l o c a t i o n c e l l s t r u c t u r e in niobium as the r e s u l t of m e c h a n i c a l d e f o r m a t i o n can enhance the magnitude and the a n i s o t r o p y of the r e s i s t ive c r i t i c a l field, HE . In the c a s e of s e v e r e d e f o r m a tion by d r a w i n g ~ o r swaging, a the magnitude of HR m e a s u r e d with the m a g n e t i c field p a r a l l e l to the w i r e s u r f a c e , which we c a l l Hll, can r a n g e up to 6Hc2 9 In addition, the r at i o of HII to H i , which is H R m e a s u r e d with the m a g n e t i c field p e r p e n d i c u l a r to the s u r f a c e , ls a function of d e f o r m a t i o n . Initially the a n i s o t r o p y r a t i o , Htl/H'~, is about 1.20. It r i s e s slowly with i n c r e a s i n g d e f o r m a t i o n until e ~ 8 and then r i s e s r ap i d l y to a peak of 1.90 at ~ -~ 9.80. T h e r e a f t e r HII/Hj. d e c r e a s e s r a p i d l y . 3 It has been shown that a n i s o t r o p i c H R in v a r i o u s niobium a l i o y s is not s u r f a c e - r e l a t e d , but is not a t r u e bulk phenomenon e i t h e r . 4 Hence it was s u g g e s t e d that h e t e r o g e n e o u s nuc i e a t i o n , i . e . " i n t e r n a l s u r f a c e s " , is r e s p o n s i b l e . Although the o p t i m a l v a lu e of HII/Hx c o i n d i d e s with the p r e d i c t e d l o w - t e m p e r a t u r e t h e o r e t i c a l c a l c u l a t i o n of the ra t i o HcJHca by Hu and K o r e n m a n , 5 it is e s s e n t i a l to note that both H N and H t f o r d e f o r m e d niobium a r e f a r in e x c e s s of Hc2 as c a l c u l a t e d f r o m the GLAG t h e o r y J -~ T i l l e y has o f f e r e d a p a r t i a l a n s w e r in the f o r m of c a l c u l a t i o n s for n u c l e a t i o n f i e l d s at c l o s e l y spaced m u l t i p l e p l a n a r d e f e c t s which do in fact p r e d i c t much higher v a l u e s of HR , H o w e v e r , as o u r p r e v i o u s e x p e r i m e n t s have shown, the t e m p e r a t u r e d e p e n d e n c e of Hll/H• d i s a g r e e s with the p r e d i c t i o n s of such a model.3 We have i n t e r p r e t e d the a n i s o t r o p y of H R as a shape effect, i , e . , as the H R of long, thin s u p e r c o n d u c ting c y l i n d e r s which the d i s l o c a t i o n c e l l s roughly ap p r o x i m a t e . 2'3 The b e h a v i o r of the a n i s o t r o p y is i n t e r p r e t e d as r e f l e c t i n g , in the d e c r e a s e of the a n i s o t r o p y for s t r a i n s beyond 9.80, the d e c r e a s e of the s c a l e of the d i s l o c a t i o n celI s t r u c t u r e to that of the c o h e r e n c e
length, ~ (4.2~ A similar disappearance of the anisotropy is observed when ~ (T) reaches the expected scale of the c e l l s t r u c t u r e , s The m e c h a n i s m c o n t r o l l i n g the e n h a n c e m e n t of HR , i . e . , HIt and H• in e x c e s s of 4Hca and 4He.a r e
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