Evidence of surface superconductivity in the high-Tc oxides

Evidence of surface superconductivity in the high-Tc oxides

~ ) Solid State Communications, Vol. 81, No. 5, pp. 411-413, 1992. Printed in Great Britain. E V I D E N C 3 E ( ) 1,~" IN S [_) R T H E [~' A (~...

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Solid State Communications, Vol. 81, No. 5, pp. 411-413, 1992. Printed in Great Britain.

E V I D E N C 3 E

( ) 1,~" IN

S [_) R T H E

[~' A (~ E HIGH--T O.

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0038-1098/9255.00+.00 Pergamon Press plc

[ J P E R C O N D U C T I V ] O X I D E S c

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F. de Lima

I n s t i t u t o d e Fisica, Unicamp 13081 Campinas, SP, Brasil (Received

7 N o v e m b e r 1991 b y C.E.T. Goncralves d a Silva) ( i n r e v i s e d form 27 November, 1991)

For t h e f i r s t time ~ v i d e n c e of s u r f a c e s u p e r c o n d u c t i v i t y in t h e h i g h - T c s u p e r c o n d u c t o r s is s h o w n h e r e . The r o u n d e d p o r t i o n of t h e m a g n e t i z a t i o n vs. t e m p e r a t u r e c u r v e s n e a r t h e t r a n s i t i o n o n s e t is i n t e r p r e t e d a s a u n i v e r s a l r e s p o n s e e x h i b i t e d b y a n y g r a n u l a r s u p e r c o n d u c t i n g sample. A c l e a r c r i t e r i o n to locate H% a n d H% is p r e s e n t e d , t o g e t h e r with a p r e l i m i n a r y d i s c u s s i o n of t h e c o n s e q u e n c e s implied b y t h e s e b a s i c f a c t s .

Tc, w h i c h can be d e t e c t e d u p to T , 400K for BiSrCaCuO c o m p o u n d s 6. M o r e o v e r , t h i s e f f e c t o c c u r s in a s c a l e a b o u t 103 t i m e s s m a l l e r t h a n t h e one d i s c u s s e d in t h i s p a p e r . A n o t h e r t y p e of a p p r o a c h relies on the v o r t e x dynamics, w h e r e d i f f e r e n t p r o c e s s e s like a g i a n t flux c r e e p , t h e f l u x - l a t t i c e melting, a v o r t e x liquid s t a t e , or a v o r t e x g l a s s s t a t e , p l a y t h e c e n t r a l role. The main i d e a s in t h i s a r e a w e r e a l r e a d y r e v i e w e d v's b u t no c o n c l u s i v e p i c t u r e is a v a i l a b l e y e t . Here, I p r o p o s e t h a t s u r f a c e s u p e r c o n d u c t i v i t y could be t h e main c a u s e of t h e r o u n d e d region near the transition onset, and the p o i n t T% (Fig. 1) would be r e l a t e d to t h e s u r f a c e n u c l e a t i o n field H%. In o t h e r w o r d s , t h i s would be a u n i v e r s a l f e a t u r e e x p e c t e d to be o b s e r v a b l e in a n y g r a n u l a r sample 9, b e c a u s e t h e l a r g e s u r f a c e t o - v o l u m e r a t i o would imply in a l a r g e m a g n i f i c a tion of t h e s u r f a c e e f f e c t s . The d i a m a g n e t i c r e s p o n s e coming from t h e s u r f a c e s h e a t h s u p e r c o n d u c t i v i t y in t h e c r i t i c a l s t a t e 1° can be w r i t t e n , f o r d e c r e a s i n g f i e l d s H > HC2, as-"

S u r f a c e s u p e r c o n d u c t i v i t y is a v e r y b a s i c p h e n o m e n o n k n o w n to o c c u r in a r e g i o n of t h e H v s . T p h a s e d i a g r a m b e t w e e n t h e Hc2(T) a n d Hc3(T) l i n e s l ' z . A s s u m i n g a m a g n e t i c field, H, p a r a l l e l to a sample i n f i n i t e p l a n e s u r f a c e c o n t i g u o u s to an i n s u l a t i n g r e g i o n , it can be p r o v e d t h r o u g h G i n z b u r g - L a n d a u (GL) e q u a t i o n s , t o g e t h e r with t h e b o u n d a r y c o n d i t i o n ( d ~ / d x } x : 0 = 0, w h e r e ~ is t h e complex o r d e r p a r a m e t e r , that3: HC 3 -

1.695HC 2

(1)

The s u r f a c e n u c l e a t i o n field, w h i c h implies t h e e x i s t e n c e of a t h i n s u p e r c o n d u c t i n g s h e a t h n e a r t h e sample s u r f a c e , is t h e n a b o u t 70% h i g h e r t h a n t h e b u l k n u c l e a t i o n field. T h e r e is an a n g u l a r d e p e n d e n c e of He3({}) f o r f i n i t e s a m p l e s s u c h t h a t 1,4,s Hc 3({))~Hc2 ' w h e r e @ is t h e a n g l e b e t w e e n H a n d t h e sample s u r f a c e . When H is p e r p e n d i c u l a r to t h e s u r f a c e : H%(~/2)=Hc 2. For t h i n films 4, a n d n e a r sample edgesS: H% (0)/He2 >1.695. S t r o n g e x p e r i m e n t a l c o n f i r m a t i o n of t h e f o r e g o i n g f a c t s was f u l l y p r e s e n t e d for t h e c o n v e n t i o n a l low-Tc s u p e r c o n d u c t o r s 2, a n d s e v e r a l i m p o r t a n t c o n s e q u e n c e s w e r e e x p l o r e d in t h e l i t e r a t u r e p u b l i s h e d d u r i n g t h e 60's and 70's. In t h i s p a p e r , I p r e s e n t f o r t h e f i r s t time e v i d e n c e of s u r f a c e s u p e r c o n d u c t i v i t y o c c u r r i n g in h i g h - T c s u p e r c o n d u c t o r s (HTSC). A p r e l i m i n a r y d i s c u s s i o n of t h e c o n s e q u e n c e s implied b y t h i s f a c t is also i n t r o d u c e d . H u n d r e d s of p a p e r s p u b l i s h e d in t h e l a s t f i v e y e a r s , d e a l i n g with m a g n e t i z a t i o n (hi) or s u s c e p t i b i l i t y (X) d a t a f o r HTSC, show t r a n s i t i o n c u r v e s with a t y p i c a l s h a p e like t h a t d i s p l a y e d in Fig. 1. So f a r , t h e r e h a s b e e n no s a t i s f a c t o r y e x p l a n a t i o n for t h e r o u n d e d r e g i o n n e a r t h e o n s e t of t r a n s i t i o n , at T%. Some a u t h o r s h a v e p o i n t e d o u t t h e r m a l f l u c t u a t i o n a s t h e p o s s i b l e c a u s e for t h a t , while o t h e r s h a v e v a g u e l y a t t r i b u t e d t h i s e f f e c t to sample i n h o m o g e n e i t i e s . However, f l u c t u a tion e f f e c t s on M (or X) can o n l y a c c o u n t for a weak-monotonically-decreasing diamagnetism above

f

H

w h e r e He(T) is t h e t h e r m o d y n a m i c c r i t i c a l field, A(T) is t h e m a g n e t i c p e n e t r a t i o n d e p t h , K(T) is t h e GL f a c t o r , L is t h e l i n e a r d i m e n s i o n of t h e sample ( p e r p e n d i c u l a r to H), a n d f(H/Hc 2) is a s p e c i a l u n i v e r s a l f u n c t i o n ' ° , d e p e n d i n g o n l y on H/H% f o r h i g h - ~ s u p e r c o n d u c t o r s (~>3). U s i n g t h e a p p r o p r i a t e e x p r e s s i o n s 1'~ for all t h e s e b a s i c v a r i a b l e s I was a b l e to fit E q u a t i o n (2) to t h e r o u n d e d p o r t i o n (t:T/Tc~0.9} of M v s . T c u r v e s for YBaCuO s i n g l e c r y s t a l s * * , w i t h i n 3% of e r r o r 12. This r e q u i r e s t h e i n t e r p r e t a t i o n of t h e twin b o u n d a r y l ~ H a n e s , w h i c h o c c u r p a r a l l e l to t h e c r y s t a l c axis , a s e f f e c t i v e i n t e r f a c e s allowing s u r f a c e field n u c l e a t i o n n . The i n t e r p r e t a t i o n o u t l i n e d a b o v e could f o r instance explain why the b r o a d e n i n g associated with t h e r o u n d e d r e g i o n is (a) l a r g e r f o r HUc c o m p a r e d to H±c in s i n g l e c r y s t a l s 14, (b) smaller 411

SURFACE SUPERCONDUCTIVITY

412

or practically non-observable for untwinned single crystals, and is largest in polycrystalline s a m p l e s 7. T h e r e f o r e , t h e s e f a c t s w o u l d be s i m p l y revealing the different levels of granularity, which are probed directly by the surface bounda r y condition (d~/dx)x: 0 : 0, required for nucleating superconductivity at every type of interface (twin planes, grain boundaries, and external surfaces). The average bulk nucleation field, H%(T), can be defined through point Tc2 (Fig.l), which can be approximately determined by a linear extrapolation of the almost straight portion of M(T) (or x(T)), as has already been done by some authors 1.. Abrikosov's theory *~ for the bulk magnetization of type-II superconductors, based on the 3D-GL theory, gives support to this criterion. For high-~ materials it is a straightforward task to show that*~: 4~H--

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w h e r e Hco, '%.0 a n d ~o a r e r e s p e c t i v e l y t h e t h e r m o dynamic critical field, magnetic penetration depth a n d c o h e r e n c e l e n g t h a t t=0, ~ 0 is t h e f u n d a m e n tal quantum of f l u x , a n d j3 - 1.16 is a f a c t o r determined by the flux line lattice symmetry. E q u a t i o n (3) f i t s w i t h i n 2% o f e r r o r ~z t h e b u l k r e g i o n o f t h e p u b l i s h e d d a t a f o r YBaCuO s i n g l e c r y s t a l s a1 • The combined fitting of Equations (2) and (3) for the Y B a C u O data gives: ~0=15.6±0.2A', A0=I323±3A; Hc0=ll.32±0.05kOe; T%(H=0)=91.5K, in good agreement with previously d e t e r m i n e d T M ~ ' ~v values for these basic parameters. Furthermore, values for other basic parameters c a m e o u t in t h e e x p e c t e d ranges: T c 3 ( H : 0 ) = 9 4 . 5 K , {]=1.19±0.03, and L=1930±70A. This latter is indeed a very reasonable evaluation f o r t h e a v e r a g e t w i n p l a n e d i s t a n c e ~3, w h i c h p l a y s the role of a mesoscopic sample thickness, accordi n g to t h e n e w i n t e r p r e t a t i o n introduced here. The inset of Fig.] shows a typical plot of

Tc21

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IN T H E H I G H - T c O X I D E S

H% a n d Hc 3 a g a i n s t T, w h i c h in t h i s c a s e w e r e extracted from a collection of Mvs.T curves for s Ba0.6sKe. 3s Bi03 polycr ystalline sample, under a p p l i e d f i e l d s b e t w e e n 1 a n d 5 T e s l a .8. T h e r a t i o H % / H c 2 f o r t h a t s a m p l e is p l o t t e d in F i g . 2 , t o gether with other similar data for a granular sample of oxidized Nb p a r t i c l e s .9 and a • 20 YBa 2 Cu 3 O?-~ t a n g l e c r y s t a l The latter are measurements for the amplitude of the reflected signal from a resonant tank circuit at a fixed frequency. This is closely connected to t h e conventional ac method, thus allowing a clear d e f i n i t i o n o f Tc 2 a n d Tc 3, in a n a l o g y w i t h t h e a b o v e c r i t e r i o n s t a t e d f o r M(T) ( o r x ( T ) ) c u r v e s . I t is a l s o w o r t h m e n t i o n i n g t h a t a l t h o u g h i n t e r polation was used for determining Hca/H%, this c o u l d in p r i n c i p l e b e a v o i d e d if o n e h a s e n o u g h data of transition curves. T h e p e c u l i a r s h a p e o f t h e Hc3/Hc 2 v s . T curves ( F i g . 2 ) c a n b e v e r y well d e s c r i b e d 21 b y considering the temperature dependence of this ratio, together with the essential assumption that every magnetization ( o r X) s i g n a l is a r e s u l t o f the weighted contribution coming from the surface sheath, which should scale with V[~(T)/L], and from the bulk magnetization, which should scale w i t h V, w h e r e V is t h e s a m p l e v o l u m e . The inset of Fig.2 shows the same data plotted against the reduced temperature, t=T/Tc, suggesting a universal magnetic respOnse for a n y granular superconducting system 21, resembling the sample thickness influence on H % / H c 2 observed in thin films, m a n y years ago ~. In conclusion, evidence of surface superconductivity being manifested in the H T S C has for the first time been reported in the present communication. Due to the basic nature of this p h e n o m e n o n several consequences are expected to f o l l o w , h e l p i n g to c l a r i f y t h e c l o u d y a r e n a o f t h e • . ? magnettc propertms a s well a s t h e i n t r i g u i n g behavior of some transport properties {e.g. critical current and resistivity) under applied field, observed specially near the transition onset.

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Fig.1. Typical shape of the magnetization vs. temperature curve for a granular superconducting s a m p l e u n d e r a n a p p l i e d f i e l d H. P o i n t s Tc 2 a n d Tc 3 a r e e x p l a i n e d in t h e t e x t . I n s e t : S h o w s t h e Hc2(T) a n d Hc~(T) l i n e s w h i c h w e r e e x t r a c t e d f r o m M v s . T c u r v e s a~ f o r Ba0.6sK0.3sBiO 3.

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Fig.2. The ratio H%/Hc 2 obtained for different materials: D-Oxidized Nb p o w d e r (Tc=7.85K)19; A Ba 0 6sK0 3sBiO3 p o l y c r y s t a l (Tc=30.0K)'8; o - YBa-Cu-'Osingle crystal (Tc=88 8K) 2° ' z 3 7-~, " ' Inset: The same data plotted against the reduced temperature t=T/Tc.

Vol. 81, No. 5

SURFACE SUPERCONDUCTIVITY IN THE HIGH-T c OXIDES

A c k n o w l e d g e m e n t s - I am g r a t e f u l to t h e i n v a l u able a s s i s t a n c e g i v e n b y R. A n d r a d e J r . , M.A. Avila, S.P. d e Campos a n d O.P. F e r r e i r a . I wish also to t h a n k A.O. Caldeira, S. Moehlecke and C. R e t t o r i for a c r i t i c a l r e a d i n g of t h i s m a n u s c r i p t . The f i n a n c i a l s u p p o r t p r o v i d e d b y t h e Brazilian a g e n c i e s FAPESP a n d CNPq a r e also a c k n o w l e d g e d . REFERENCES

1. P.G. de G e n n e s , S u p e r c o n d u c t i v i t y of metals a n d alloys, (Benjamin, 1966). 2. D. S a i n t - J a m e s , G. Sarma, E.J.Thomas, T y p e II S u p e r c o n d u c t i v i t y , ( P e r g a m o n , 1969). 3. D. S a i n t - J a m e s a n d P.G. de G e n n e s , P h y s . L e t t e r s , _7, 306 (1963). 4. G. D e u t s c h e r , J. P h y s . Chem. Solids, 28, 74l (1967). 5. A.Yu. Simonov, A.S. Melnikov a n d S.V. S h a r o v , Mod. P h y s . Lett. B4, 1211 (1990). 6. W.C. Lee, J.H. Cho a n d D.C. J o h n s t o n , P h y s . Rev. B43, 457 (1991). 7. A.P. Malozemoff, in: P h y s i c a l P r o p e r t i e s of High T e m p e r a t u r e S u p e r c o n d u c t o r s , Ed. D.M. G i n s b e r g (World Sci.Publ., 1989), p.71. 8. D.S. F i s h e r , M.P.A. F i s h e r , a n d D.A. Huse, P h y s . Rev. B43, 130 (1991).

413

9. It is also r e q u i r e d t h a t x~0.42 (see Ref. 2). 10. H.J. Fink a n d L.J. B a r n e s , P h y s . Rev. Lett., 15, 792 {1965}. 11. U. Welp, W.K. Kwok, G.W. C r a b t r e e , E.G. V a n d e r v o o r t , a n d J.Z. Liu, P h y s . Rev. Lett., ~2, 1908 (1989). 12. O.F. d e Lima, (to be p u b l i s h e d ) . 13. M. S a r i k a y a , R. Kikuchi a n d I.A. Aksay, P h y s i c s C, 152, 161 (1988). 14. U. Welp, M. G r i m s d i t c h , H. You, W.K. Kwok, M.M. F a n g , G.W. C r a b t r e e a n d J.Z. Liu, P h y s i c a B163, 473 {1990). 15. A.A.Abrikosov, S o v i e t Phys.JETP5,1174 (1957). 16. L . K r u s i n - E l b a u m , R.L. G r e e n e , F. H o l t z b e r g , A.P. Malozemoff a n d Y. Y e s h u r u n , P h y s . Rev. Lett. 62, 217 {1989). 17. M.B. Salamon a n d J. B a r d e e n , P h y s . Rev. Lett. 59, 2615 (1987). 18. G.W. C r a b t r e e , W.K. Kwok, U. Welp, K.G. V a n d e r v o o r t , B. D a b r o w s k i a n d D. G. Hinks, P h y s i c a B163, 652 (1990). 19. R. A n d r a d e J r . , O.F. de Lima, M.A. Avila a n d S.P. de Campos, (to be p u b l i s h e d ) . 20. T.K. W o r t h i n g t o n , W.J. G a l l a g h e r a n d T.R. D i n g e r , P h y s . Rev. L e t t . , 59, 1160 (1987}. 21. O.F. de Lima, (to be p u b l i s h e d ) .