TUNNELLING EXPERIMENTS ON Cu1.8Mo6S8

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TUNNELLING EXPERIMENTS ON Cu1.8Mo6S8

U. Poppe, H. Wühl

To cite this version:

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JOURNAL DE PHYSIQUE

_{Colloque C6, supple'ment au}

_no

_8,

_Tome

_39,

_aoiit

_1978,

_page

_C6-36

₁

TUNNELLING EXPERIMENTS ON

U. Poppe and H. ~ C h l ,

Ins

titut

fur

Festktirperforschung,

KFA

JiiZieh,

0-51 70

JiiZich, Germany.

Rdsum6.- L ' d n e r g i e du gap A. e t des s t r u c t u r e s dans l a d e n s i t 6 d ' 6 t a t t u n n e l dues aux phonons o n t d t 6 mesurdes pour Cu Mo S 1 l ' a i d e de c o n t a c t s 1 p o i n t e . Le r a p p o r t 2A0/kTc exSde 4,0, c e q u i i n d i q u e un f o r t c o u p i i i e filzctron-phonon. S u i v a n t n o s r g s u l t a t s de dCriv6e seconde, l e s v i b r a t i o n s o p t i q u e s des atomes de Cu 1 b a s s e s Bnergie c o n t r i b u e n t 1 l a s u p r a c o n d u c t i v i t 6 .

A b s t r a c t . - The e n e r g y gap A, and phonon-induced s t r u c t u r e s i n t h e t u n n e l l i n g d e n s i t y of s t a t e s have been measured f o r C U ~ , ~ M O ~ S ~ , u s i n g p o i n t c o n t a c t s . The r a t i o 2A0/kT e x c e e d s 4.0, i n d i c a t i n g s t r o n g electron-phonon c o u p l ~ n g . From o u r second d e r i v a t i v e d a t a we i n f e r t f i a t t h e low l y i n g o p t i c v i b r a - t i o n s of t h e Cu atoms c o n t r i b u t e t o s u p e r c o n d u c t i v i t y .

The g e n e r a l i z e d phonon d e n s i t y of s t a t e s F(w) i s known f o r Chevrel phases MMo S from n e u t r o n

-

x 6 8

s c a t t e r i n g experiments. The compounds a r e r e g a r d e d a s m o l e c u l a r c r y s t a l s composed of two c o n s t i t u e n t s , m e t a l atoms M and Mo S u n i t s / I , 2 / . I n t h i s model

6 8

t h e v i b r a t i o n a l modes a r e d i v i d e d i n t o two g r o u p s : low-frequency e x t e r n a l modes of M and Mo S

,

and

6 8

high-frequency i n t e r n a l modes w i t h i n t h e Mo S u n i t s . 6 8 It i s commonly assumed t h a t t h e s u p e r c o n d u c t i v i t y i n t h e s e compounds i s mainly determined by t h e low- l y i n g e x t e r n a l modes, t h e t r a n s l a t i o n a l and t h e t o r - s i o n a l motion of t h e Mo S u n i t s b e i n g more impor-

6 8 t a n t t h a n t h e o p t i c motion of t h e M atoms / I , 3 / . Recent band s t r u c t u r e c a l c u l a t i o n s i n d i c a t e t h a t i t i s t h e i n t e r n a l c o n t r i b u t i o n of t h e u n i t s which i s e s s e n t i a l f o r the e l e c t r o n i c p a r t N(o)<J'> of t h e electron-phonon c o u p l i n g parameter h / 4 / . The c o n t r i b u t i o n of d i s t i n c t phonon f r e q u e n - c i e s t o t h e s u p e r c o n d u c t i n g p r o p e r t i e s i s d e s c r i b e d by t h e f u n c t i o n a 2 ~ ( w ) . T h i s f u n c t i o n can be e v a l u a -

t e d from t u n n e l l i n g e x p e r i m e n t s . The d i r e c t l y mea- s u r e d second d e r i v a t i v e d21/dv2 of t h e c u r r e n t - v o l - t a g e c h a r a c t e r i s t i c r e f l e c t s t h e s t r u c t u r e of a 2 ~ ( w ) We have done e l e c t r o n t u n n e l l i n g e x p e r i m e n t s on Cu Mo S

.

We chose t h e p o i n t c o n t a c t method 1'8 6 8 s i n c e i t i s v e r y d i f f i c u l t t o f a b r i c a t e t h i n f i l m s o r t o o b t a i n s u f f i c i e n t l y smooth s u r f a c e s on b u l k Chevrel-phase m a t e r i a l f o r vapour d e p o s i t i o n of a c o u n t e r e l e c t r o d e . The n a t u r a l l y grown o x i d e on A 1 t i p s and t h e S c h o t t k y b a r r i e r of GaAs t i p s a r e used a s t u n n e l l i n g b a r r i e r s . Both t y p e s of j u n c t i o n s y i e l d s i m i l a r r e s u l t s .

F i g u r e 1 shows t h e t u n n e l l i n g conductance dI/dV f o r Cu Mo S (Tc = 10.6 K). Values up t o

1.8 6 8

20 have been measured f o r t h e r a t i o of t h e z e r o b i a s t u n n e l l i n g conductance i n t h e normal s t a t e t o t h a t i n t h e s u p e r c o n d u c t i n g s t a t e . The smearing of t h e gap s t r u c t u r e presumably o r i g i n a t e s form inhomoge- n e i t i e s i n t r o d u c e d by t h e h i g h p r e s s u r e a t t h e p o i n t c o n t a c t a r e a and t h e s t r o n g p r e s s u r e dependence of Tc f o r t h e Chevrel phases

151.

V ImVl F i g . I : D i f f e r e n t i a l conductance dI/dV v e r s u s v o l - t a g e of an Al-oxide-Cul . 8 M ~ g S8 p o i n t c o n t a c t junc- t i o n a t 1.2 K. F o r t h e energy gap t h e peak p o s i t i o n minus 7 % i s chosen, t h u s t a k i n g i n t o a c c o u n t t h e r - mal smearing.

Depending on t h e a p p l i e d c o n t a c t p r e s s u r e , T mea- s u r e d by t h e t e m p e r a t u r e dependence of dI/dV a t ze- r o b i a s v a r i e s between 10.6 K and 13.0 K. Corres- pondingly, A, l i e s between 1.8 and 2.3 meV. The ra- t i o a O / k T c r a n g e s from 3 . 9 t o 4.2. These v a l u e s a r e s i g n i f i c a n t l y l a r g e r t h a n t h e weak-coupling BCS v a l u e 3.5. I n f i g u r e 2 a second d e r i v a t i v e of t h e tunne- l l i n g c h a r a c t e r i s t i c , d ? / d v 2 , i s p l o t t e d . Though F(w) e x t e n d s up t o 60 meV, r e l i a b l e t u n n e l l i n g d a t a have been s o f a r o b t a i n e d o n l y up t o 12 meV. At

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At higher energies the junctions have been too noisy. Instabilities in the tunnelling current have also affected the measurements below 12 meV.

Fig.

2

: d21/dv2 versus energy measured from the gap edge for an Al-oxide-Cul ,Mo,S8 point contact junction at 1.2 K; The ac modulation was 1 meV. The bars show the positions of the minima for different junctions. The intensity of the structure also varied from junction to junction.

The positions of minima in d21/dv2 pointing to maxi- ma in

c~~F(w)

are confirmed by the results for about

10 junctions. The variation of the positions for different junctions is marked by bars in figure 2 and is to some extent caused by different contact pres- sures. Minima at about 6 and 10 meV agree with maxi- ma in F(w) obtained from n-scattering experiments. Additional structures are observed at 2 and 3.5 meV, which have no counterpart in F(w) as measured in Ref.

121.

From our results we could like to conclude that the motion of the Cu atoms (6 meV) in Cu Mo

1.8 6

S contributes to superconductivity with about equal strength as the translational motion (10 meV) of the Mo S units. Since a direct coupling of the M atom

6 8

vibrations to the electronic system is assumed to be weak, an indirect coupling via the torsional motion of the units is considered /6/. TKe importance of the torsional motion for the superconductivity in Chevrel phases should be clarified by tunnelling experiments at higher energies. Such experiments are underway.

References

/I

/

Bader, S.D., Sinha, S.K. and Shelton, R.N., "Superconductivity in d-and f-band metals", ed. D.H. Douglass (1976) 209.

/2/ Schweiss, P., Renker, B. and Suck, J.

B.,

Pro- gress Report KFK 2538, KFK Karlsruhe (1977) 23 and to be published.

/3/ Fisher,

@.,

Proc. of the 14th 1nt.Conf. on Low Temperature Physics, Otaniemi, Finland (1975) Vol. 5, 172.

/4/

Andersen,

O.K.,

Klose,

W.

and Nohl,

H.,

to be published

in

Phys. Rev.

B.

/5/ Shelton, R.N., Lawson, A.C. and Johnston, D.C., Mater. Res. Bull.

10

(1975) 297.