Rectangular flux line lattice in type II superconductors

Volume 28A, n u m b e r 9

RECTANGULAR

PHYSICS LETTERS

FLUX

LINE

LATTICE

IN

10 F e b r u a r y 1969

TYPE

II

SUPERCONDUCTORS

B. O B S T Institut f'~r P h y s i k am M a x - P l a n c k - I n s t i t u t f l i t Metallforschung, Stuttgart and Institut f ~ r theoretische und angewandte P h y s i k d e r Universit~it Stuttgart. Germany Received 8 J a n u a r y 1969

Square flux line lattice and t r i a n g u l a r lattices were o b s e r v e d on Pb - 1.6wt%T1 c r y s t a l s depending on the c r y s t a l orientation.

A c c o r d i n g to A b r i k o s o v [1] t h e m a g n e t i c s t r u c t u r e of t h e m i x e d s t a t e of a t y p e II s u p e r c o n d u c t o r c o n s i s t s of a p e r i o d i c a r r a n g e m e n t of flux l i n e s o r i e n t a t e d p a r a l l e l to t h e a p p l i e d f i e l d . K l e i n e r e t al. [2] a n d M a t r i c o n [3] h a v e s h o w n t h a t f o r > 1 the triangular lattice is slightly more stable t h a n t h e s q u a r e l a t t i c e . E s s m a n n a n d T r O u b l e [4] observed the triangular lattice in Pb-In alloys a n d Nb. F o r s m a l l ~ K r a m e r [5] f o u n d t h e f r e e e n e r g y of t h e t r i a n g u l a r l a t t i c e to b e h i g h e r t h a n t h a t of t h e s q u a r e l a t t i c e f o r a p e n e t r a t e d f l u x B >/0.3 H c.

In t h i s n o t e w e r e p o r t t h e m a g n e t i c s t r u c t u r e s of P b - T 1 r o d s (1.6wt%, ~ ~ 0.72 + 0.02 a t 1.1OK) made visible by the Tr~iuble and Essmann techn i q u e [6]. F o r f l u x d e n s i t i e s B > 0.3 H c w e f r e q u e n t l y o b s e r v e d a s q u a r e flux l i n e l a t t i c e . T h i s confirms Kramer's predictions. Fig. 1 shows p a r t of a g r a i n t h r o u g h o u t w h i c h a s q u a r e l a t t i c e has developed. Our results furthermore indicate that the cryst a l a n i s o t r o p y i n f l u e n c e s t h e s y m m e t r y of t h e f l u x - l i n e l a t t i c e . In s i n g l e c r y s t a l s w i t h t h e f o u r f o l d s y m m e t r y a x i s p a r a l l e l to t h e a p p l i e d m a g -

~ ~I ~gc~b,~

Fig. 1. Grain boundary of the s p e c i m e n (dotted). Above: t r i a n g u l a r lattice. Below: s q u a r e lattice. B ~ 350 G, T = 1.1°K.

662

~. e ~ d a * *

iI~ ~ $ ~

~ ' ~

-~$

Fig. 2. Well developed flux line lattice as observed on {100} s u r f a c e s of Pb-T1 single c r y s t a l s . B ~ 350 G, T = 1.1°K.

Volume 28A. n u m b e r 9

PHYSICS

netic field found square lattices throughout (see fig. 2). In c a s e of t h e f i e l d p a r a l l e l to t h e t h r e e fold crystal symmetry axis most regions exhibited a t r i a n g u l a r flux l i n e l a t t i c e , s o m e r e g i o n s , h o wever, were observed where the triangular lattice changed continuously into a square lattice. T h e i n f l u e n c e of f i e l d s t r e n g t h a n d a l l o y c o m p o s i t i o n o n t h e s e r e s u l t s a r e now s t u d i e d a n d w i l l be published later.

LETTERS

10 F e b r u a r y 1969

References 1. A.A. Abrikosov, Zh. Eksp. i Teor. Fiz. 32 (1957) 1442: Soviet Phys. J E T P 5 (1957) 1174. 2. W.H. K l e i n e r , L. M. Roth and S. H. Autler, Phys. Rev. 133 (1964), A 1226. 3. J. M a t r i c o n , Phys. L e t t e r s 9 (1964) 289. 4. U. E s s m a n n and H. Tr~iuble, Phys. L e t t e r s 24A (1967) 526. 5. L. K r a m e r , Phys. L e t t e r s 23 (1966) 619. 6. H. Tr~iuble and U. E s s m a n , Phys. Stat. Sol. 18 (1966) 813.

I a m v e r y i n d e b t e d to P r o f e s s o r D r . A. S e e g e r a n d D r . U. E s s m a n n f o r t h e i r e n c o u r a g e m e n t a n d helpful suggestions. . . . . .

THE

SWITCH TYPE

II

FROM THIN TO BULK SUPERCONDUCTING

BEHAVIOR FILMS

OF

R. W. B R O W N *

U.S. Naval Ordnance Laboratory. Silver Spring, Maryland, USA Received 7 J a n u a r y 1969

M e a s u r e m e n t s a r e r e p o r t e d of the upper c r i t i c a l field for d i s o r d e r e d and annealed superconducting tin films. F o r each film. the switch from thin to bulk behavior is found at a field value independent of s t a te of anneal.

T h e s t r u c t u r e of t h e m i x e d s t a t e of b u l k t y p e 17 s u p e r c o n d u c t o r s h a s b e e n s t r i k i n g l y d e m o n strated by Essman and Trauble using magnetic r e p l i c a t i o n [1,2]. T h e t e c h n i q u e w o u l d f a i l n e a r the upper critical field where the flux is uniform. H e r e a l e s s d i r e c t e s t i m a t e of t h e s p a t i a l r a n g e of e n e r g y g a p s t r u c t u r e i s p r o v i d e d b y t h e b e h a v i o r of t y p e II f i l m s i n a p a r a l l e l m a g n e t i c f i e l d . For example, anomalies in tunneling current i n t o s u c h f i l m s h a v e b e e n i n t e r p r e t e d [ 3 - 5 ] to a r i s e f r o m t h e a c c o m o d a t i o n of a s t r u c t u r e d g a p function within the film thickness. Changes in t h e t e m p e r a t u r e d e p e n d e n c e of t h e u p p e r c r i t i cal field have been recognized since the early w o r k of Z a v a r i t s k y [6] to i n d i c a t e t h e f i r s t a p p e a r a n c e of s t r u c t u r e a c r o s s t h e f i l m , a s h a s s i m i l a r r e c e n t w o r k [4, 5] . T h e b a s i c v a l i d i t y of t h e i d e a of " g a p n u c l e a t i o n " i n s u c h f i l m s i s b e y o n d d i s p u t e . H o w e v e r , e s t i m a t e s of t h e s i z e of t h i s g a p s t r u c t u r e a t f i r s t e n t r y i n t o t h e f i l m s u f f e r f r o m t h e f a c t t h a t t h i c k n e s s (d) a r e n o t g e n e r a l l y k n o w n to b e t t e r t h a n t h e +10% e s t i m a t e in the present work. T h e a d v a n t a g e of t h e e x p e r i m e n t r e p o r t e d h e r e

lies in thickness stability. Measurements are presented on each film, as deposited and after e a c h of a s e q u e n c e of a n n e a l i n g s . L e a d f i l m s s o t r e a t e d m a i n t a i n t h i c k n e s s to +3% i n t h e r a n g e of interest here, as shown by Bassewitz and Minn i g e r o d e [7]. A s i m i l a r c o n s t a n c y f o r low t e m perature deposited tin films is indicated by the w o r k of M S n c h [8]. T h i s s t a b i l i t y m a k e s t h e p r e s e n t r e s u l t s a c o m p e l l i n g a d d i t i o n to p r e v i o u s d e m o n s t r a t i o n s of g a p n u c l e a t i o n . Tin films were evaporated on substrates held n e a r 4.2OK. S u c h f i l m s a r e h i g h l y d i s o r d e r e d [9]; t h i s r e s u l t s i n r e s i s t i v i t i e s (p) a n d u p p e r c r i t i c a l f i e l d s [6] h i g h e r t h a n f o r t h e s a m e f i l m s w h e n a n n e a l e d . M e a s u r e m e n t s of t h e f i e l d f o r r e s i s t i v e o n s e t (Hu) w e r e m a d e a g a i n s t t e m p e r a t u r e (T) f o r t h e f i l m s a s d e p o s i t e d a n a f t e r a n n e a l i n g to t e m p e r a t u r e s f r o m 75 to 300°K. T h e * E x p e r i m e n t p e r f o r m e d at the University of Maryland. Extensive use was made of equipment received under ARPA support. The r e s u l t s f o r m p a r t of thesis submitted in partial fulfillment of the r e q u i r e m e n t s for the Ph.D. in physics. 663