SUPERCONDUCTIVITY IN METALLIC GLASSES

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SUPERCONDUCTIVITY IN METALLIC GLASSES

W. Johnson

To cite this version:

W. Johnson. SUPERCONDUCTIVITY IN METALLIC GLASSES. Journal de Physique Colloques, 1980, 41 (C8), pp.C8-731-C8-741. �10.1051/jphyscol:19808183�. �jpa-00220286�

JOURNAL DE PHYSIQUE CoZZoque C8, suppZBment au n08, Tome 4 1 , aoCt 1980, page ~ 8 - 7 3 1

SUPERCONDUCTIVITY IN METALLIC GLASSES

W.L. Johnson

W. M. Keck Laboratory o f Engineering Materials, CaZifornia I n s t i t u t e o f TechnoZogy, Pasadem, C a l i f o r n i a 91125, U.S.A.

I. INTRODUCTION

The e a r l i e s t studies of amorphous superconduc- t o r s were c a r r i e d out twenty-five years ago on t h i n f i lms of simple metals prepared by vapor deposition on a cryogenic substrate(''*). ~ e r ~ m a n n ( ~ ) , and the author(4) have surveyed some of the properties of such materials in recent reviews. In the past ten years, studies of amorphous superconductors have been extended t o t r a n s i t i o n and most recently t o metal1 i c glasses (4,697). rhe

where D(0) = electron density pf s t a t e s a t the Fermi level

<I 2 > = average squared electron-ion matrix element

M = ionic mass

<w 2 > = mean square phonon frequency (defined by FlcMi 1 1 an) l a t t e r materials a r e prepared by rapid quenching of

liquid metal1 i c alloys using techni.ques o r i g i n a l l y developed by ~ u w e z ( ~ ) .

Although the phonon spectrum and Eliashberg func- t i o n a 2 (w)F(w) of amorphous metals may d i f f e r from t h a t used by IlcMillan t o obtain eqn. ( I ) , one would s t i 11 expect t h a t the corresponding expression f o r T, i n amorphous alloys will have a similar

form (4y10). Therefore, eqns. ( 1 ) and (2) s t i l l provide a convenient framework in which t o analyze the systematics of superconductivity in amorphous metals

.

I t would be both d i f f i c u l t and r e p e t i t i v e t o completely review t h i s f i e l d in the limited space available here. The interested reader i s referred t o the above mentioned a r t i c l e s f o r a morecomplete survey. In the present a r t i c l e , a t t e n t i o n will be focused on recent developments. The main r e s u l t s of e a r l i e r work will be summarized where conve- nient.

A. Simple Metals 11. ELECTRONIC STRUCTURE AND SUPERCONDUCTIVITY

The occurrence of superconductivity i n amor- phous simple metals can be understood by using an extension of the nearly f r e e electron model.

Ziman has applied t h i s model t o 1 iquid metals('' )

.

~ e r ~ m a n n ' ~ ) has given an extensive account of the experimental evidence which supports such a picture i n the case of superconducting amorphous simple metals. In p a r t i c u l a r , the model correctly predicts the Hall coefficient(12) RH, e l e c t r i c a l r e s i s t i v i t y p(T), and temperature dependence of p(T) f o r the majority of simple metals(13).

Rainer and ~ e r g m a n n " ~ ) have a l s o discussed the

2 2

calculation of <I > and <w >. They argue t h a t disorder r e s u l t s in an enhancement of low fre- quency contributions t o electron-phonon s c a t t e r - ing and consequently t o enhanced values of X.

They conclude t h a t amorphous metal s should tend t o

~ c11 an(') has given a solution t o the strong- ~ i coupling equations of Eliashberg which he obtained using numerical techniques together with the mea- sured phonon spectrum of niobium. Using his solu- tion, the superconducti ng t r a n s i t i o n temperature i s given by

In t h i s expression, eD i s t h e Debye temperature ( o r suitably averaged phonon frequency), ].I* the e f f e c t i v e Coulomb coupling constant, and X the dimensionless electron phonon coupling constant.

I.lcbli 11 an expresses A in terms of other microscopic parameters as

be strong-coupling (A > 1) superconductors.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19808183

(28-732 JOURNAL DE PHYSIQUE

Experimental evidence obtained from superconductive

t u n n e l i n g indeed supports t h i s c o n ~ l u s i o n ( ~ ~ ~ ~ ~ ~ ~ ) . The author(4) has attempted t o e x p l a i n t h e

systematics o f s u p e r c o n d u c t i v i t y i n simple amor- phous metals by assuming t h a t t h e s t r o n g d i s o r d e r leads t o a n e a r l y s p h e r i c a l Fermi surface. A simple j e l l i u m model was used t o determine t h e v a r i a t i o n o f X w i t h a few simple parameters such as valence Z, atomic volume va, and i o n i c mass M. I n t h e j e l l i u m model, t h e i o n i c plasma frequency Q P =

(*)'

These r e s u l t s a r e i l l u s t r a t e d i n F i g . 2 .

Valence Z 1.2-

1.0

0.8

0.6

0.4

0.2

0

Fig. 1. V a r i a t i o n w i t h valence o f t h e supercon- d u c t i n g t r a n s i t i o n temperature o f simple amorphous metals. The values o f Tc a r e normalized t o t h e c h a r a c t e r i s t i c bare phonon temperature ChS2 /K ] o f each

m a t e r i a l . P

Amorphous -

Simple ^I

Metals Pb.9 Cu.~ _{* I}

:

- ^J,

:

I I

Te9Te.l

- ^{GO*:*. .S%Cu.,}

/ ^'?ES?z

- I I

I I I

- Be I/

0 r

- .//*cd,9~e.,

,'I f Mg., Z~II (melt-quenched)

---:

0 2 4

I n c r y s t a l l i n e metals, t h e i n f l u e n c e o f l o n g range o r d e r on e l e c t r o n i c band s t r u c t u r e r e s u l t s i n l a r g e d e v i a t i o n s of t h e e l e c t r o n i c d e n s i t y o f s t a t e s D(E) from t h e f r e e e l e c t r o n model. These d e v i a t i o n s i n t u r n i n f l u e n c e electron-phonon s c a t t e r i n g , and e l e c t r o n screening. The absence o f l o n g r a n g e o r d e r i n amorphous metals tends t o e l i m i n a t e these band s t r u c t u r e e f f e c t s and leads t o a f r e e e l e c t r o n - l i ke Fermi surface. As a r e s u l t , t h e j e l l i u m model g i v e s a much b e t t e r d e s c r i p t i o n o f amorphous as compared t o c r y s t a l l i n e simple metals which a r e n o t we1 1 described by t h e j e l l i u m model(4). The micro- scopic o r i g i n and systematics o f s u p e r c o n d u c t i v i t y a r e correspondingly e a s i e r t o understand i n amor- phous metals

.

6. T r a n s i t i o n Metals

The superconducting p r o p e r t i e s o f amorphous t r a n s i t i o n metals are governed mainly by t h e p a r t i a l l y occupied d-states. I n c o n t r a s t w i t h t h e case o f simple metals, one cannot use t h e f r e e e l e c t r o n approach t o understand these m a t e r i a l s . The d-electrons a r e perhaps b e s t described as being

(19) t i g h t l y bound. F r i e d e l ( 1 8 ) , Cyrot-Lackmann ,

and o t h e r s have discussed t h e t i g h t - b i n d i n g approx- i m a t i o n TBA as i t a p p l i e d t o l i q u i d t r a n s i t i o n metals. Recently, Varma and ~ y n e s ' ~ ' ) have ana- l y z e d s u p e r c o n d u c t i v i t y i n c r y s t a l l i n e t r a n s i t i o n metals using an extension o f t h e TBA method t o i n c l u d e t h e nonorthogonal i ty o f atomic d - o r b i t a l s centered on neighboring atoms. They f i n d t h a t t h e d-band c o n t r i b u t i o n D ~ ( E , ~ ) t o D ( E ~ ) i s t h e most i m p o r t a n t microscopic parameter which determines X.

2 2

T h e i r argument shows t h a t t h e r a t i o [<I >/<w >] i n eqn. ( 2 ) should be roughly c o n s t a n t w i t h i n a g i v e n d-band ( i . e . t h e 4d o r 5d band).

Applying t h e Varma-Dynes a n a l y s i s t o t h e case o f amorphous t r a n s i t i o n metals r e q u i r e s i n f o r m a t i o n r e g a r d i n g t h e behavior o f Dd(c). Several e x p e r i - mental techniques have been used t o o b t a i n such i n f o r m a t i o n . These i n c l u d e measurements of mag- n e t i c s u s c e p t i b i l i t y , low temperature s p e c i f i c heat, x-ray and u l t r a v i o l e t photoemission spectra, and a l s o i n d i r e c t deduction o f D ( E ~ ) from upper c r i t i c a l f i e l d , Hc2(T), measurements. Magnetic s u s c e p t i b i l i t y measurements on metal 1 i c glasses o f t h e s e r i e s (Mol-xRux)80P20 have been r e p o r t e d (21 1,

The temperature independent c o n t r i b u t i o n t o t h e s u s c e p t i b i l i t y , G, was found t o be l a r g e and t o decrease w i t h x as s b ~ w n in F i g . 3 . ^{I f one}

VALENCE Z

assumes t h a t t h e Paul i paramagnetic c o n t r i b u t i o n associated w i t h D ( E ~ ) dominates x0, then t h i s can be i n t e r p r e t e d t o i n d i c a t e a smooth decrease o f D(cF) w i t h i n c r e a s i n g x as shown i n t h e f i g u r e . Also shown i n t h e f i g u r e i s t h e v a r i a t i o n of Tc w i t h x f o r these m e t a l l i c glasses and t h e v a r i a - t i o n o f Tc vrith x f o r amorphous Mol-xRux t h i n f i lms p r e a r e d by cryoquenching by, C o l l v e r and H a m m ~ n d ( ~ ~ . These r e s u l t s suggest a d i r e c t r e 1 a- t i o n s h i p between D(E) and Tc. Recent low tempera- t u r e s p e c i f i c h e a t m e a s ~ r e m e n t s ( ~ ) on t h e same s e r i e s o f m e t a l l i c glasses have confirmed t h e de- crease o f D(E) w i t h x, The absolute values o f D(E) obtained from t h e c o e f f i c i e n t o f t h e l i n e a r c o n t r i b u t i o n t o c a r e i n remarkably good agree-

P ( 4 )

ment w i t h D(E) values estimated from xo

.

Schroeder e t a1(22) and Amamou and t h e a u t h o r (23)

F i g . 2. Values of t h e e l e c t r o n - phonon c o u p l i n g constant A calcu- l a t e d using a simple j e l l i u m m d e l w i t h Z, M, and v, as i n p u t para- meters. See r e f . 4 f o r d e t a i l s .

have used XPS and UPS measurements t o g a i n i n s i g h t i n t o t h e v a r i a t i o n o f D(E) f o r E < cF. Typical r e s u l t s a r e shown i n Fig. 4 f o r t h e a l l o y

(f.100~6R~0~4)80B20. The data suggest t h a t a general l o s s o f s t r u c t u r e i n D(E) Occurs on going from t h e c r y s t a l l i n e t o t h e amorphous s t a t e . ^' I t a l s o sug- gests t h a t D(E) i s c h a r a c t e r i z e d by a broad maxi- mum w i t h t h e peak o c c u r r i n g somewhat below t h e Fermi l e v e l o f t h e a l l o y . I n o t h e r words, D d ( ~ ) appears t o vary r a t h e r smoothly w i t h d-band occupation e x h i b i t i n g a maximum f o r a r o u g h l y h a l f f i l l e d d - s h e l l . The v a r i a t f o n o f Tc observed by Col 1 v e r and ~ a m n o n d ' ~ ) f o r cryoqueiched f i l m s of t r a n s i t i o n metals and a l l o y s ofaneighboring metals o f t h e 4d s e r i e s i s shown i n Fig. 5. The v a r i a t i o n o f Tc through a s i m i 1 a r broad maximum j s consistent w i t h t h e Varma-Dynes a n a l y s i s i f t h e d e n s i t y o f

JOURNAL DE PHYSIQUE

Fig. 3. (top) The temperature inde- pendent contribution

xo

x0

(bottom) The variation of Tc f o r the same alloys along with t h a t observed by Collver and Hammond f o r ~~lol-xRux thin films.

s t a t e s Dd(&) has the above c h a r a c t e r i s t i c s . In electrons. This i s due t o the highly disordered contrast, t h e Matthias curve f o r c r y s t a l l i n e tran- atomic arrangement. In the case of simple metals, s i t i o n metals i s shown in the same figure. In the the atomic potential i s comparatively weak. The c r y s t a l l i n e case, a rapid variation of Dd(cF) w i t h Ziman theory of l i q u i d metals(11) i s premised on d-band occupation i s predicted by band s t r u c t u r e

calculations(24y25) and confirmed by s p e c i f i c heat measurements ( 9 y 2 0 y 2 6 ) . Those c r y s t a l l i n e metals and alloys f o r which cF f a l l s near a sharp maximum in D(E) have high Tc values as expected i n t h e Varma-Dynes model. The c r y s t a l l i n e t o amorphous t r a n s i t i o n washes out the d-band s t r u c t u r e and thus leads t o a smoother variation of both Dd(g) and consequently Tc(4).

C. Effects of Disorder on Superconductivity

the assumption of weak s c a t t e r i n g and as such neglects multiple s c a t t e r i n g e f f e c t s . Using the simplest solution t o the Boltzmann equation, one can estimate the e l e c t r o n i c mean f r e e path f o r amorphous simple metals d i r e c t l y from measurements of p(T). A t cryogenic temperatures, p(T ^-+ 0 ) ranges from 25-100 pQcm f o r typical simple metals.

From

In amorphous metals, e s s e n t i a l l y every atom

can a c t a s a s c a t t e r i n g center f o r conduction and f r e e electron values f o r the Femi velocity,

C8-735

-

I-

Sputter Cleaned

Amorphous (MqsRu.,)80B20

- X-ray Photoemission

Spectrum

I 24

Electron Energy (eV) -

Fig. 4. T k x-ray photoemission spectrum of (Mo0~6Ru0~4)80B20 shows the width and shape of the d-band.

UPS spectra a r e given i n r e f . 22.

vFY one obtains an electron mean f r e e path Re which ranges from several times the interatomic distance d up t o roughly 10 d. The Ziman model i s expected tobreak down when Re = d. Mott has termed t h i s t h e electron diffusion(27) o r strong s c a t t e r i n g regime and predicts t h a t t h i s regime i s approached when

p ? 300 yQcm. R e s i s t i v i t i e s of t h i s order a r e i n f a c t observed in amorphous t r a n s i t i o n metals where eqn. (3) gives values of a, = d. I t should be noted t h a t s t i l l larger values of p ( i . e .

p 3000 v ~ c m ) are expected t o r e s u l t in Anderson (28)

localization of e l e c t r o n i c s t a t e s

.

The small value of Re typically found f o r amorphous metals ( 2 7 y 2 8 ) has important consequences f o r superconductivity. The purity parameter A = (cO/te) where to i s the zero temperature coherence length in the clean l i m i t P (to = ' 3 ( 2 9 )

ITA

where t i s the reduced tempertature t =

r;:

^)!

while the penetration depth i s given by (29)

% -%

Ad(t) = 0.615 AL(0) h t

P ⁽⁵⁾

where AL(0) i s the London penetration depth. For

0

amorphous metals ~ ~ ( 0 ) i s i n the range 30-10'OA while Ad(0) i s of the order of 1 vm. The Ginzburg- Landau parameter K = (A/<) i s of order 10-100 (30,311.

As a consequence of the above f a c t s , amor- phous superconductors have a large upper c r i t i c a l f i e l d H ~ ~ ( T ) ( ~ ~ ' ~ ~ ) , small lower c r i t i c a l f i e l d HC, ( T ) ( ~ ~ ) , and e x h i b i t large c r i t i c a l fluctua- t i o n ~ ' ~ ~ ) . The upper c r i t i c a l f i e l d behavior i s

-

i s exceptionally large, A a 10-100, f o r amorphous p a r t i c u l a r l y i n t e r e s t i n g and has recently been metals. Thus, amorphous metals a r e a case of P studied over an extended range of t f o r amorphous

(29)

superconductivity i n the extreme d i r t y l i m i t

.

JOURNAL DE PHYSIQUE

(GROUP NUMBER)

Fig. 5. The variation of Tc with average group number f o r thin amorphous films of 4d t r a n s i t i o n metals and a l l o y s . For comparison, the variation of Tc in corresponding c r y s t a l l i n e metals and alloys i s shown.

in Fig. 6. The theoretical calculation of Hc2(T) in t h e d i r t y l i m i t has been carried out by several authors(33y34y35). Corrections t o t h e basic theory f o r spin o r b i t s c a t t e r i n g ( 3 5 ) , paramagnetic 1 imi t- i ng (36) s t r o n g - c o u p ~ i n ~ ( ~ ~ ) , an anisotropy effectsi38) have been successful in explaining most data on known superconductors. Exceptioos.to t h i s included recent r e s u l t s on u l t r a t h i n films(39), and the data in Fig. 6. The figure shows the theoretical curves f o r A ( t ) = (Hc2(T)/Hc2(0)) where L Hc2(0) i s obtained by l i n e a r extrapolation. By L l e t t i n g t h e spin-orbi t coupling parameter As, -t a,

one obtains the highest appropriate theoretical prediction f o r f i ( t ) . The theory i s c l e a r l y inade- quate t o explain the p e r s i s t e n t l i n e a r i t y of K ( t ) a t low t. Combining the data of Fig. 6 with a l l other available data on amorphous re,veals t h a t t h e discrepancy w i t h theory systematically increases with increasing normal s t a t e r e s i s t i v i t y p (31

.

Values of p of t h e order of 200 @cm a r e observed

f o r the alloys i n Fig. 6. On t h i s basis, i t i s suggested t h a t t h i s behavior i s related t o strong- electron s c a t t e r i n g e f f e c t s and the tendency toward electron localization. Such e f f e c t s would a l s o be

(39) expected f o r the case of u l t r a t h i n films

.

The topological and chemical disorder in amorphous t r a n s i t i o n metal a1 loys should r e s u l t i n large fluctuations in the atomic environment. In the TBA model of the d-band, one would expect large fluctuations i n the two center overlap integral (17)

where and F2 a r e the position vectors of neighboring atoms, Va the atomic p o t e n t i a l , and

ad an atomic d-orbital. Roughly J depends on

1x1

^1F2-F1 1

Reduced Temperature

T/ T,

Fig. 6. The reduced upper c r i t i c a l f i e l d h ( t ) = Hc2(T)/Hc2(0) vs. t f o r several transition-metal-base L

m e t a l l i c glasses. Dashed l i n e s a r e t h e o r e t i c a l curves.

where qo i s t h e S l a t e r c o e f f i c i e n t f o r decay o f t h e d - o r b i t a l . The d-band w i d t h i s o f order

where z i s t h e f i r s t neighbor c o o r d i n a t i o n number.

Frem t h e atomic p a i r c o r r e l a t i o n f u n c t i o n o f metal- I i c glasses(41 ) (e. g. (M0.5~u0. 5 ) 8 0 ~ 2 0 we e s t i m a t e t h e mean value of !L t o be Z = 3A w h i l e f l u c t u a t i o n s i n !L a r e o f order At = 1 . 0 i . Taking ~ ( 1 ) = l e v , and qo = 1 i - I (40) g i v e s Jo s 10-30 eV. Thus t h e r e l a t i v e f l u c t u a t i o n s i n J(!L) a r e t y p i c a l l y

Chemical d i s o r d e r (e.g. f l u c t u a t i o n s i n Flo-Mo, Mo-Ru, and Ru-Ru c o o r d i n a t i o n ) wi 11 produce addi- t i o n a l diagonal d i s o r d e r . One m i g h t expect t o approach t h e Anderson l o c a l i z a t i o n c r i t e r i a f o r t h e d-band of a t r a n s i t i o n metal glass.

The above d i s c u s s i o n ignores s-d (p-d) h y b r i d - i z a t i o n which can be parametrized by a parameter Jsd. I f Jsd ⁺ 0, then t h e d-electrons would be decoupled from t h e s s p - e l e c t r o n s and c o u l d e x h i b i t l o c a l i z a t i o n f o r s u f f i c i e n t l y h i g h d i s o r d e r . For f i n i t e Jsd. t h e l o c a l i z a t i o n would tend t o be suppressed by t h e g r e a t e r s p a t i a l e x t e n t o f s,p o r i b t a l s. This i n t e r e s t i n g problem has i m p o r t a n t consequences f o r s u p e r c o n d u c t i v i t y s i n c e t h e qOA%

($)=(+) I%l!L=i

Thus we estimate

The s i m p l i f i e d p i c t u r e of t h e d-band d e n s i t y (AJ/Wd) % J5 = 4 ⁽¹⁰⁾ ~ f s t a t e s D ( ~ ) d e ~ c r i b e d i n s e c t i o n I I B . ignores

d

t h e i n f l u e n c e of m e t a l l o i d elements (e.g. B, p, (42) We see t h a t t o p o l o g i c a l d i s o r d e r alone can produce e t c . 1 on e l e c t r o n i c p r o p e r t i e s . Recent s t u d i e s s u b s t a n t i a l f l u c t u a t i o n s i n l o c a l environment. suggest t h e m e t a l l o i d elements p l a y a s i g n i f i c a n t

C8-738 ^JOURNAL DE PHYSIQUE

0 -from HC2 X - ^{sp. heat}

Y

(mi)

mole-K

0 0.12 016 0.20 0.24

X Boron Concentration

x ( A t o m i c Percent M e t a l l o i d )

F i g . 9. L i n e a r c o e f f i c i e n t f o r low temperature

s p e c i f i c heat vs. composition f o r ( I l o 0 ~ 6 R ~ 0 ~ 4 ) 1 - ~ x Fig. 7. liearest neighbor d i s t a n c e d vs.composition

m e t a l l i c glasses. The open c i r c l e s were deduced f o r (~lo0.6R~0.4)1-xBx,~ (~100.6R~0.4)1-xSi,, and

from c r i t i c a l f i e l d data and eqn. (11). The (Pd0.5Ni0.5)l-xPx m e t a l l i c glasses. Distances

crosses a r e taken d i r e c t l y from s p e c i f i c h e a t data.

were determined from t h e Debye formula. (see ref.42)

E l e c t r i c a l R e s i s t i v i t y vs.

M e t a l l o i d C o n t e n t 300

L I10 I ! l ~ ~ ~ l l l l l ~ l ~ l ~ ~ , , , , l l l

20 30

x ( A t o m i c P e r c e n t M e t a l l o i d )

Fig. 8. E l e c t r i c a l r e s i s t i v i t y vs. composition f o r ( M o 0 ~ 6 R ~ 0 ~ 4 ) 1 - x B x and ( H O ~ . ~ R U , , . ~ ) ~ - ~ S ~ ~ m e t a l l i c glasses.

r o l e in both the atomic s c a l e s t r u c t u r e and elec- t r o n i c properties of metallic glasses. This i s best i l l u s t r a t e d by the example of

( M o ~ . ~ R u ~ . ~ ) ~ -xBx glasses which can be obtained over the composition range 0.10 < x < 0.24. Several properties of these alloys have been found tochange discontinuously with composition near x = 0.18.

Some examples shown i n Figs. 7 and 8 a r e the aver- age nearest neighbor distance and the e l e c t r i c a l r e s i s t i v i t y . The former is found t o increase w i t h x f o r x ^< 0.18 and decrease f o r x > 0.18. The

l a t t e r i s nearly independent f o r x f o r x < 0.18 and increases rapidly f o r x > 0.18 (from % 120 ~Qcm t o 250 ~ Q c m ) . Another example of such behavior is a l s o shown f o r the metal-metalloid glasses

(Pd0.5Ni0, 5)1-xPx. The metal1 i c glasses

( ~ o ~ ~ ~ R u ~ ~ ~ ) ~ - ~ S ~ ~ a r e similar but show no dis- c o n t i n u i t i e s . The c r i t i c a l f i e l d gradient

(dHc2/dT)T and e l e c t r i c a l r e s i s t i v i t y can be used

C

together t o determine the e l e c t r o n i c density of s t a t e s D(eF) using the relation (43)

where y i s the equivalent l i n e a r coefficient in t h e e l e c t r o n i c heat capacity ( i n erg and 6 i s i n units of ( ~ c m ) . Values of y so obtained a r e plotted f o r ( ~ 1 0 0 ~ 6 R ~ 0 ~ 4 ) 1 - x B x glasses i n Fig. 9.

Specific heat data(44), available f o r two of t h e alloys, i s included in the figure and agrees extremely we1 1 with the y deduced from Hc2. A precipitious drop i n y vs. x occurs near x = 0.18 w i t h y ^c 4 (m~/mole-K 2 ) f o r x < 0.18 and

y = 2 (mJ/mole-K 2 ) f o r x > 0.18. We conclude t h a t D ( E ~ ) drops by roughly a f a c t o r of 2 over t h i s narrow composition i n t e r v a l .

All of the above f a c t s suggest some type phase t r a n s i t i o n or dramatic s t r u c t u r a l change near x = 0.18. The above observations,are explained by assuming two d i f f e r e n t amorphous phases with com- positions x < 0.18 and x > 0.18. Near x = 0.18, the a l l o y would contain both phase in comparable proportions. The technique of small angle x-ray s c a t t e r i n g SAS was used t o obtain evidence of the proposed phase separation ( 4 5 ) . Typical small angle s c a t t e r i n g spectra a r e shown in Fig. 10. Curves A,B,C, and D show spectra f o r as quenched f o i l s w i t h x ⁼ 0.12, 0.14, 0.18, and 0.20 respectively.

A c l e a r f e a t u r e i n the SAS i n t e n s i t y i s observed f o r s c a t t e r i n g vector K % 0.3. This feature i s most pronounced f o r x = 0.18 an can be interpreted by assuming a phase segregation i n t o domains of

0

c h a r a c t e r i s t i c s i z e ^% 10-15A with a typical i n t e r -

0

domain distance of 20-30A.

Results of neutron i r r a d i a t i o n studies on (Moo . 6 ~ u 0 . 4)82B1 have previously been reported (46) The SAS spectrum of a neutron i r r a d i a t e d sample i s shown by curve E. The sample was i r r a d i a t e d t o a t o t a l fluence of 10'' n/cm2 of f a s t neutrons

(1 !lev) over a period of several days. The overall increase in small angle s c a t t e r i n g r e f l e c t s an increase of point defects during i r r a d i a t i o n . The l a r g e increase i n the broad maximum near K ^% 0.3 suggests enhanced phase separation. This can be understood since the increased point defect concen- t r a t i o n should enhance diffusion a t room tempera-

(47) t u r e

.

I t i s c l e a r t h a t both the e l e c t r o n i c s t r u c t u r e and atomic s c a l e s t r u c t u r e of these metal 1 i c glasses a r e strongly influenced by t h e phase separation discussed above. On the other hand, the supercon- ducting properties of these materials should r e f l e c t only the average properties of the material since

c.

the s c a l e of phase separation 10-252\ i s substan- t i a l l y smaller than the coherence length of the

0

superconductor ~ ~ ( 0 ) = 50A. However, i t may be possible t o produce a coarser phase separation by thermal aging of t h e glass. This would r e s u l t in an i n t r i n s i c a l l y inhomogeneous s u ~ e r c o n d u c t i ~ d material. An example of c l e a r phase separation on a much larger s c a l e has been observed i n a novel family of superconducting metal 1 i c glasses

( ~ b ~ - ~ ~ b ~ ) ~ - ~ ~ u ~ ( ~ ~ ) . Here, phase separation on a s c a l e of ^% 1000 A leads t o an inhomogeneous super- conducting' material in which both phases percolate throughout the sample.

IV. SUblNARY

This a r t i c l e , as mentioned in the introduction, was not intended t o be a comprehensive review. The questions raised i n the a r t i c l e serve t o emphasize 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 of superconducting metallic glasses i s f a r from being well understood.

Rore generally, the e l e c t r o n i c and atomic s c a l e s t r u c t u r e of metallic glasses may be considerably more complex than previous1 y recognized. Emphasis has been given t o several current problems in the

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