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STABILITY AND ELECTRONIC PROPERTIES OF VACUUM-EVAPORATED AMORPHOUS METALLIC
Au-Ge ALLOY FILMS
V. Nguyen Van, M. Thèye, Serge Fisson
To cite this version:
V. Nguyen Van, M. Thèye, Serge Fisson. STABILITY AND ELECTRONIC PROPERTIES OF
VACUUM-EVAPORATED AMORPHOUS METALLIC Au-Ge ALLOY FILMS. Journal de Physique
Colloques, 1980, 41 (C8), pp.C8-489-C8-492. �10.1051/jphyscol:19808122�. �jpa-00220219�
JOURNAL DE PHYSIQUE CoZZoque C8, suppZdment au n 0 8 , Tome 41, aoct 1980, page C8-489
STABILITY AND ELECTRONIC PROPERTIES OF VACUUM-EVAPORATED AMORPHOUS METALLIC Au-Ge
ALLOY FILMS
V. Nguyen Van, M.L. ThSye and S. F i s s o n
Laboratoire d l @ t i q u e des SoZides i-
,
Universite' Pierre e t Marie Curie, 4 , place Jussieu, 75230 Paris Ce'dex 05, fiance.RQsum6.
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On d i s c u t e l e s c o n d i t i o n s d ' e x i s t e n c e e t l a s t a b i l i t 6 d ' a l l i a g e s m d t a l l i q u e s amorphes Au-Ge prSpar6s p a r co-dvaporation s o u s u l t r a - v i d e . Leurs p r o p r i 6 t b s o p t i q u e s s o n t a n a l y s c e s en c o n t r i b u t i o n de Drude e t c o n t r i b u t i o n i n t e r b a n d e . On e n d g d u i t l a p r 6 s e n c e d ' h y b r i d a t i o n sp-d dans c e s a l l i a g e s .A b s t r a c t .
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The e x i s t e n c e and s t a b i l i t y o f amorphous m e t a l l i c Au-Ge a l l o y s p r e p a r e d by c o - e v a p o r a t i o n under u l t r a - h i g h vacuum a r e d i s c u s s e d . T h e i r o p t i c a l p r o p e r t i e s a r e a n a l y s e d i n terms o f a Drude and a n i n t e r b a n d c o n t r i b u t i o n . 0 c c u r r e n c e o f sp-d h y b r i d i z a t i o n i s i n f e r r e d .1 . I n t r o d u c t i o n . - Amorphous m e t a l l i c a l l o y s b e t - ween a n o b l e m e t a l and S i o r Se have been o b t a i n e d by d i f f e r e n t methods (1-5). T h e i r main i n t e r e s t l i e s i n t h e r e l a t i v e s i m n l i c i t y o f t h e e l e c t r o n i c s t r u c t u r e o f e i t h e r component. The complex d i e l e c - t r i c c o n s t a n t o f pure n o b l e m e t a l s f o l l o w s a f r e e - e l e c t r o n b e h s v i o r up t o 2-4 eV, where a s t r o n p i n - t e r b a n d c o n t r i b u t i o n , due e s s e n t i a l l y t o t r a n s i - - t i o n s between t h e f i l l e d d-band and t h e Fermi l e v e l , s e t s i n . These p r o p e r t i e s a r e l i t t l e modified i n t h e l i q u i d s t a t e ( 6 ) . S i and Ge a r e c o v a l e n t i n t h e i r s o l i d ( c r y s t a l l i n e and amorphous) s t a t e , due t o sp3 h y b r i d i z a t i o n , b u t m e t a l l i c i n t h e i r l i q u i d s t a t e , w i t h a b o u t f o u r c o n d u c t i o n e l e c t r o n s p e r atom. These a l l o y s a r e t h e r e f o r e good c a n d i d a t e s f o r t e s t i n g t h e v a l i d i t y o f a r i g i d - b a n d model, i n which S i o r Ge i m p u r i t i e s would j u s t a d d t h e i r f o u r v a l e n c e e l e c t r o n s t o t h e h o s t c o n d u c t i o n band. The o p t i c a l p r o p e r t i e s o f s p u t t e r e d amorphous Au-Si a l l o y s f o r S i c o n c e n t r a t i o n s between 13 and 50 a t . X have r e c e n t l y been i n t e r p r e t e d a s c o n f i r m i n e such a model ( 7 ) . However, t h e s i t u a t i o n m a y n o t b e s o sim- p l e , b e c a u s e o f t h e s t r o n g i n t e r a c t i o n which can b e e x p e c t e d between t h e i m p u r i t y s-p s t a t e s and t h e h o s t d s t a t e s ( 8 ) . Moreover, t h e type o f bonding i s l i k e l y t o change, a t l e a s t l o c a l l y , when i n c r e a s i n ~ t h e S i o r Ge c o n c e n t r a t i o n , which w i l l a l t e r t h e m e t a l l i c c h a r a c t e r o f t h e a l l o y s .
We p r e s e n t h e r e t h e r e s u l t s o f o p t i c a l measure- ments on amorphous Au-Ge a l l o y s w i t h Ge c o n c e n t r a -
t i o n s between 20 and 40 a t . % . These a l l o y s were p r e p a r e d by co-evaporation under u l t r a - h i g h vacuum
i n o r d e r t o a v o i d s p u r i o u s e f f e c t s due t o b u i l t - i n gazeous i m p u r i t i e s . T h e i r s t r u c t u r e was c o n t r o l l e d by e l e c t r i c a l r e s i s t a n c e measurements and e l e c t r o n microscope i n v e s t i g a t i o n s . We d i s c u s s b r i e f l y t h e
e x i s t e n c e and s t a b i l i t y of amorphous m e t a l l i c a l l o y s and we a n a l y s e t h e i r o p t i c a l p r o p e r t i e s i n r e l a t i o n w i t h d i f f e r e n t models f o r t h e i r e l e c t r o n i c s t r u c t u r e
2. E x ~ e r i m e n t . - The samples a r e t h i n (200-400 8 ) s e m i - t r a n s p a r e n t f i l m s d e p o s i t e d by co-evaporation under u l t r a - h i g h vacuum o n t o g l a s s s u b s t r a t e s main- t a i n e d a t low t e m p e r a t u r e (15-20 K ) . The d e p o s i t c o m p o s i t i o n and t h i c k n e s s a r e monitored by two ca- l i b r a t e d q u a r t z m i c r o b a l a n c e s . The f i l m r e f l e c t a n c e and t r a n s m i t t a n c e a r e measured i n s i t u j u s t a f t e r d e p o s i t i o n , between 0.6 and 4.5 eV, w i t h a s p e c i a l - l y b u i l t s p e c t r o m e t e r ( 9 ) . E l e c t r i c a l d . c . r e s i s - t a n c e measurements performed i n s i t u by a f o u r - probe t e c h n i q u e a s a f u n c t i o n o f t e m p e r a t u r e a l l o w t o c h a r a c t e r i z e t h e a s - d e p o s i t e d sample, t o d e t e r - mine i t s s t a b i l i t y range and t o f o l l o w p o s s i b l e s t r u c t u r a l changes on a n n e a l i n g . O p t i c a l measure- ments a r e r e n e a t e d i n s i t u a t any d e s i r e d a n n e a l i n g s t a g e . A t room t e m n e r a t u r e , t h e y a r e performed b o t h under vacuum and i n a i r w i t h a Cary 14 s p e c t r o m e t e r , a s a check of t h e o p t i c a l d a t a a c c u r a c y . The s t r u c - t u r e o f t h e a s - d e p o s i t e d f i l m and i t s changes upon a n n e a l i n g a r e t r a c e d back from room t e m p e r a t u r e e l e c t r o n - d i f f r a c t i o n and e l e c t r o n microscopy r e s u l t s , sunplemented by a d e t a i l e d a n a l y s i s o f t h e a n n e a l i n g c u r v e . The f i l m t h i c k n e s s i s determined by a n X-ray i n t e r f e r e n c e method ( l o ) , i t s composi-
t i o n bv a n e l e c t r o n microprobe t e c h n i q u e (11).
f' Equipe de Recherche Associde a u CNRS n o 462
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19808122
JOURNAL DE PHYSIQUE
C8-490
3 . Existence and s t a b i l i t y of amorphous a l l o y s .
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Only t h e main r e s u l t s a r e given h e r e . The d e t a i l s of e l e c t r i c a l r e s i s t a n c e measurements and s t r u c t u r e i n v e s t i g a t i o n s w i l l b e r e p o r t e d elsewhere ( 12,)
.
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For small Ge c o n c e n t r a t i o n s ( 5 , 11 a t . % ) t h e f i l m s always c o n s i s t i n a mixture of pure Au and pure Ge c r y s t a l l i t e s .- For Ge c o n c e n t r a t i o n s c l o s e r t o the e u t e c t i c , 27 a t . % Ge (13) (24, 28, 30 a t . l ) , we o b t a i n e d amorphous a l l o y s whose s t r u c t u r e i s c h a r a c t e r i z e d by two u n s p l i t broad d i f f r a c t i o n r i n g s c e n t r e d a t s = sinB/X = 0.217 and 0.380 r e s p e c t i v e l y . This d i f f r a c t i o n p a t t e r n i s very s i m i l a r t o t h e one re- p o r t e d f o r a l i q u i d Au 75Ge25 a l l o y (14) and i n d i c a -
t e s a simple
,
close-packed s t r u c t u r e w i t h no Ge-Ge bond. These amorphous a l l o y s have i n i t i a l r e s i s t i - v i t i e s between 124 and 138 pSlcm, w i t h very small n e g a t i v e temperature c o e f f i c i e n t s : -4 t o -7 xC r y s t a l l i z a t i o n s t a r t s above 250 K with t h e ~ r e c i - p i t a t i o n of pure Au m i c r o c r y s t a l l i t e s and, i n some c a s e s , t h e formation of a m e t a s t a b l e complex c r y s - t a l l i n e a l l o y phase.
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For h i p h e r Ce c o n c e n t r a t i o n s (34 a t . ? ) , we s t i l l o b t a i n e d amorphous a l l o y s , b u t w i t h a d i f f e r e n t s t r u c t u r e . Their d i f f r a c t i o n diagrams p r e s e n t an a d d i t i o n a l broad r i n g c e n t r e d a t s = 0.155 A-I r e m i n i s c e n t of t h e f i r s t d i f f r a c t i o n r i n g of pure amorphous Ge (15). These amorphous a l l o y s have s l i g h t l y h i g h e r r e s i s t i v i t i e s , w i t h more n e g a t i v e temperature c o e f f i c i e n t s , of t h e o r d e r of -5 x 10 -4.
A m e t a s t a b l e complex c r y s t a l l i n e a l l o y phase, d i f - f e r e n t from the previous one, appears a b r u ~ t l y j u s t bklow room temperature.
4. Analysis of o p t i c a l d a t a .
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Yeasuring b o t h the r e f l e c t a n c e R and t h e t r a n s m i t t a n c e T of a t h i n f i l m with known t h i c k n e s s allows t o determine t h e complex d i e l e c t r i c c o n s t a n t E" = + i c 2 a c c u r a t e l y a t each frequency (16). Unfortunately, i n t h e pre- s e n t c a s e , t h e computed E" v a l u e s a r e extremely s e n s i t i v e t o even very small experimental uncer- t a i n t i e s on R and T, which, b e s i d e s , vary very l i t t l e w i t h frequency, e s p e c i a l l y i n the n e a r i n f r a - r e d .-
For Ge compositions between 20 and 3C a t . % the E~ and E~ curves f o r t h e as-deposited a ~ o r p h o u s a l l o y s a r e s i g n i f i c a n t l y d i f f e r e n t from those f o r pure Au ( f i g u r e 1 ) . A m e t a l l i c behavior i s s t i l l observed a t low e n e r g i e s b u t t h e o p t i c a l a b s o r p t i o n i s considerably enhanced; an a b s o r p t i o n edgeFigure 1. - O p t i c a l a b s o r p t i o n E / A f o r a Au70ce30 a l l o y : amorphous (a) and p a r t i a l ? y c r y s t a l l i z e d (b)
,
and f o r pure Au ( c ) .reminiscent of t h e o n s e t of i n t e r b a n d t r a n s i t i o n s i n pure Au i s d i f f i c u l t t o d e t e c t a t h i g h e r ener- g i e s (such a n edge shows up immediately when pure Au c r y s t a l l i t e s appear on annealing, a s shown by
curve (b) i n f i g u r e 1 ) .
We t e n t a t i v e l y analysed t h e low energy d a t a i n terms of a Drude model which, even i n a d i s o r d e r e d system, should r e p r e s e n t a f i r s t approximation t o the o o t i c a l p r o p e r t i e s a s s o c i a t e d w i t h conduction e l e c t r o n s :
? = l - w / i o ( w + i / ~ ) 2 + z C
P 1
2 1/2
where w = (4rNeffe /mo) (Neff t h e e f f e c t i v e P
number of e l e c t r o n s per u n i t volume and m t h e o p t i c a l e f f e c t i v e mass) i s t h e plasma frequency and T t h e o p t i c a l r e l a x a t i o n time of t h e conduc- t i o n e l e c t r o n s ; t h e c o n s t a n t r e a l term zC accounts
1 f o r i n t e r b a n d t r a n s i t i o n s o c c u r r i n g a t h i g h e r e n e r g i e s . Computer-fitting procedures on ( E l Y &2) o r d i r e c t l y on (R, T), a s w e l l a s g r a p h i c a l methods, were used t o determine w T and &:. Reasonable
P ' 0
agreement w i t h a simple Drude model was found from 0.6 uo t o 2.5 eV. Allowing t h e r e l a x a t i o n time t o b e frequency-de~endent did n o t improve t h e f i t s i g n i f i c a n t l y . We found f o r xGe = 30 a t . ? (with
2.5): fiio = 14.20 eV; E/T = 2.85 eV.
P
Above 2.5 eV, t h e r e i s an a d d i t i o n a l contribu- t i o n t o the o p t i c a l a b s o r p t i o n which can b e a t t r i - buted t o i n t e r b a n d t r a n s i t i o n s ( f i g u r e 2 ) . I t i s much s m a l l e r than i n pure Au and p r e s e n t s a very smooth edge, which makes a n y , o n s e t energy very d i f f i c u l t t o determine.
F i g u r e 2. - Decomposition o f E / A f o r an amorphous A u ~ ~ G ~ 30 a l l o y ( a ) i n t o a Drude term (b) and a n 2 i n t e r b a n d term ( c ) .
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For l a r g e r Ge c o n c e n t r a t i o n s , t h e o p t i c a l s p e c t r a c o r r e s p o n d i n g t o t h e a s - d e p o s i t e d amorohous a l l o y s a r e c o m p l e t e l y d i f f e r e n t from t h e p r e v i o u s ones ( f i g u r e 3).
There i s a s t r o n g a b s o r o t i o n maxi- mum i n t h e v i s i b l e , c e n t r e d a t a b o u t 2.4 eV, and no m e t a l l i c b e h a v i o r can b e d e t e c t e d o v e r t h e i n v e s - t i g a t e d s p e c t r a l r a n g e . L i t t l e change i s observed through c r y s t a l l i z a t i o n .F i g u r e 3 . Au Ge taPPiz2Ld
. -
O p t i c a l a b s o r p t i o n E / A f o r aa l l o y : amorqhous ( a ) i n 3 p a r t i a l l y c r y s - i n t o a complex c r y s t a l l i n e phase
(y-AuGe) (b)
.
5 . D i s c u s s i o n .
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By c o - e v a p o r a t i o n under u l t r a - h i g h vacuum, amorphous m e t a l l i c Au-Ge a l l o y s c o u l d b e o b t a i n e d o v e r a s m a l l composition range o n l y and t h e i r s t a b i l i t y was v e r y l i m i t e d . For h i g h e r Ge c o n t e n t s , a l t h o u g h t h e f i l m s were s t i l l amorphous, l o c a l changes i n bonding, p e r h a p s f a v o u r e d by com- p o s i t i o n f l u c t u a t i o n s , m o d i f i e d t h e e l e c t r o n i c p r o p e r t i e s o f t h e a l l o y s ; o p t i c a l l y t h e y w e r e no morem e t a l l i c . D i f f e r e n t r e s u l t s were o b t a i n e d on s p u t - t e r e d amorphous Au-Si a l l o y s which, i n s p i t e o f a s t r u c t u r e change above 4C a t . Z S i (5), had a metal- l i c o p t i c a l b e h a v i o r up t o 50 at.8, S i ( 7 ) . --
The ~ ~ t i c a l ~ r o - o e r t i e s o f t h e amorphous m e t a l l i c Au-Ge a l l o y s a r e n e v e r t h e l e s s v e r y s i m i l a r t o t h o s e o f amorphous ( 7 ) and l i q u i d (17) m e t a l l i c A u - S i a l l o y s . A l l d a t a f o l l o w a Drude law i n t h e n e a r i n f r a - r e d , w i t h comparable v a l u e s o f t h e p a r a m e t e r s c h a r a c t e r i z i n g t h e c o n d u c t i o n e l e c t r o n s b e h a v i o r . Extremely s h o r t r e l a x a t i o n times T a r e deduced, about a n o r d e r o f magnitude s m a l l e r t h a n i n p u r e l i q u i d n o b l e m e t a l s (6) b u t o f t h e same o r d e r of magnitude a s
i n
p u r e l i q u i d Ge ( 1 8 ) . The e f f e c t i v e number o f c o n d u c t i o n e l e c t r o n s which c a n b e deduced from w assuming t h a t t h e o n t i c a l mass i s e q u a l t oP '
t h e f r e e - e l e c t r o n mass, i s q u i t e h i g h : a b o u t 2.5 e l e c t r o n p e r atom on a v e r a g e f o r x = 30 a t .% i f
Ge
we t a k e t h e same a t o m i c d e n s i t y a s i n p u r e Au ( 1 9 ) . T h i s v a l u e i s much l a r g e r t h a n t h e one computed i n a r i g i d band model, about 1.9. I n a l l c a s e s , t h e r e i s a n a d d i t i o n a l c o n t r i b u t i o n t o t h e o p t i c a l absorp- t i o n a t e n e r g i e s where i n t e r b a n d t r a n s i t i o n s from t h e d hand o c c u r i n p u r e Au, b u t i t i s c o n s i d e r a b l y smoothed o u t .
A l l t h e s e r e s u l t s s u g g e s t t h a t a s i m p l e r i g i d band model cannot b e a p p l i e d , and t h a t t h e Au c o n d u c t i o n band and d band a r e s i g n i f i c a n t l y a l - t e r e d when a l l o y i n g w i t h Ge ( S i ) . I t h a s been shown t h e o r e t i c a l l y (8) t h a t t h e e l e c t r o n i c s t r u c - t u r e o f a S i i m p u r i t y i n a Cu m a t r i x i s d e e u l y m o d i f i e d by sp-d h y b r i d i z a t i o n . T h i s t h e o r y e x p l a i n s
t h e c h a r a c t e r i s t i c a n t i - r e s o n a n t s t r u c t u r e obserwed i n t h e p - p a r t i a l d e n s i t y o f s t a t e s around S i atoms i n d i l u t e Cu-Si a l l o y s by s o f t X-ray e m i s s i o n ( 2 0 ) . A c o n d u c t i o n band b e i n g f a r from f r e e - e l e c t r o n - l i k e i s t h e r e f o r e e x p e c t e d i n t h e a l l o y s . T h i s c o n c l u s i o n , which was a l s o i n f e r r e d from magnetic s u s c e ? t i b i l i t y measurements on l i q u i d Au 81Si19 a l l o y ( 2 1 ) , i s more c o m p a t i b l e w i t h t h e v a l u e s of t h e p a r a m e t e r s c h a r a c t e r i z i n g t h e c o n d u c t i o n e l e c - t r o n b e h a v i o r deduced from t h e o p t i c a l d a t a . The sp-d h y b r i d i z a t i o n i s a l s o l i k e l y t o modify t h e d - p a r t i a l d e n s i t y o f s t a t e s . A s h i f t o f t h e upper peak o f t h e Au d band towards lower e n e r g i e s , accompanied by a smoothing o f t h e upper edge, h a s been o b s e r v e d f o r e v a p o r a t e d amorphous Au-Ge a l l o y s (20 and 40 a t . % G e ) by p h o t o e m i s s i o n ( 3 ) . Such a n e f f e c t would e x p l a i n , b e t t e r t h a n d i s o r d e r a l o n e , t h e e x p e r i m e n t a l r e d u c t i o n o f t h e i n t e r b a n d
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a b s o r p t i o n .
Acknowledgement.
-
The r e s e a r c h r e n o r t e d h e r e i n h a s b e e n s p o n s o r e d i n p a r t by t h e European O f f i c e o f A e r o s p a c e R e s e a r c h and Development.R e f e r e n c e s (1) KLEMENT, W., WILLENS, R.H. and DUIJEZ, P.,
N a t u r e 187 (1960) 8 6 9 .
(2) CHEN, B.S. and TURNBULL, D., J . A ~ p l . P h y s . 38 (1967) 3646.
( 3 ) FUKUSEIYA, J . , TAMJR.4, K . , ENDO, I?.
,
KISHI, K., IKEDA, S. a n d MIXOMURA, S . , J. P h y s . 35( 1974) C4-26 1 .
( 4 ) HIXAKI, A., IWAMI, M., SHIMIZU, A . and SHUTO, K., ? d a t e r . S c i . Eng. 2 3 (1976) 289.
(5) HAUSER, J . J . and TAUC, J . , P h y s . Rev. B17 ( 1978) 3371
.
(6) HODGSON, J . N . , P h i l . Map. 5 (1960) 272.
(7) HAUSER, E., ZIRKE, R . J . , TAUC, J., HAUSER, J . J . and NAGEL, S.R., P h y s . Rev. L e t t . 40 (197P) 1733; ~ h y s . Rev. B19 (1979) 6331.
( 8 ) TERAKURA, K., J. Phys. Soc. J a p a n 4 0 (1976) 450.
( 9 ) NGUYEN VAN, V. a n d FISSON, S . , Revue P h y s . Appl. 13 (1978) 155.
( 1 0 ) KIESSIG, H., Ann. P h y s i k 10 (1931) 769;
UMRATH, W . , 2 . Angew. P h y s i k 22 ( l a 6 7 ) 406.
(11) FHILIBERT, J . , RIVORY, J.; BRYCFAERT, D. a n d TIXIER, R., J . P h y s . D: Appl. P h y s . 3 (1970) L70.
( 1 2 ) NGUYEN VAV, V., FISSON, S. and THEYE, M.L., t o b e p u b l i s h e d i n t h e P r o c e e d i n g s o f t h e 8 t h I n t . Vacuum C o n g r e s s (Cannes, F r a n c e , 1980).
(13) HAYSEN, K., C o n s t i t u t i o n o f b i n a r y a l l o y s (McGraw H i l l , New-York) 1958.
( 1 4 ) !,JAGMORNE, R.Y., RIVLIN, V.G. and WILLIUS, G . I . , 3 . P h y s . F: M e t a l P h y s . 6 (1976) 147.
(15) GAYDAIS, M., TFEYE, M.L., FISSON, S. and BOISSONADE, J . , P h y s . S t a t . S o l . ( b ) 5 8 (1973) 601.
(16) ABELES, F. and THEYE, M.L., S u r f . S c i . 5 (1966) 325.
( 1 7) ACKERMAVN
,
K.
F. ,
LIARD,
Y. and GUNTHERODT,
H . J . , t o b e p u b l i s h e d i n J. P h y s . F: M e t a l Phys
.
( 1 8 ) HODGSON, J . N . , P h i l . Map. 6 (1961) 509.
( 1 4 ) ?flAVGIN, P., MARCEAL, G., MOUREY, C. a n d JANOT, C., t o b e p u b l i s h e d i n P h y s . Rev. B.
(20) TANAKA, K . , WATSW.OT0, Y. and MARUNO, S . , A ~ p l . ? h p . L e t t . 27 (1975) 529.
( 2 1 ) . BAGLEV, B.G. and DI SALVO, F . J . , B u l l . Am.
Phys. Soc. 21 (1976) 385.
