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Submitted on 1 Jan 1978
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RESISTIVITY OF TERNARY CHEVREL-PHASE
SUPERCONDUCTORS
Roland Martin, K. Mountfield, L. Corruccini
To cite this version:
JOURNAL DE PHYSIQUE
Colloque C6, supple'ment au
no
8,
Tome 39,
aofit
1978, page
C6-371
RESISTIVITY
O F
TERNARY CHEVREL-PHASE SUPERCONDlJCTORSR. M a r t i n , K.R. M o u n t f i e l d , and L.R. C o r r u c c i n i
Physics Department University
of
California a t Davis, Davis, California
95616, USARdsum5.- Nous avons avons mesure l a r 6 s i s t i v i t 5 e n t r e 4.2 K e t 400 K d ' u n e s e r i e d e p h a s e s d e C h e v r e l q u i s o n t d e s composds s u p r a c o n d u c t e u r s . Tous m o n t r e n t u n e c o u r b u r e t r S s n 6 g a t i v e
( d 2 p / d ~ 2 ) au-dessus d ' e n v i r o n 60 K , s e m b l a b l e 1 c e q u ' o n o b s e r v e d a n s l e s composds A-15. Les r d s i s t i v i t 6 s B l a t e m p d r a t u r e ambiante s o n t 61evbes, d e l ' o r d r e d e Cl cm.
A b s t r a c t . W e have measured t h e r e s i s t i v i t i e s of a s e r i e s of s u p e r c o n d u c t i n g Chevrel-phase compounds from 4.2 K t o 400 K. A l l show s t r o n g n e g a t i v e c u r v a t u r e d 2 p / d ~ ' above about 60 K , s i m i l a r t o t h a t observed i n t h e A-15 compounds. Room t e m p e r a t u r e r e s i s t i v i t i e s a r e h i g h , i n t h e
Q
cm r a n g e .Recent i n t e r e s t h a s developed i n t h e d e v i a - t i o n s from l i n e a r i t y i n t h e r e s i s t i v i t y p(T) of t r a n s i t i o n - m e t a l s u p e r c o n d u c t o r s . I n p a r t i c u l a r a number o f t h e A-15 compounds e x h i b i t a n e g a t i v e
d2p
c u r v a t u r e dT a t h i g h t e m p e r a t u r e s , i n c l u d i n g Nbg Sn, N b g Sb,
Vj
S i , andV 3 Ge. The d a t a d o n o t
appear t o approach a l i n e a r t e m p e r a t u r e dependence a t h i g h T, a s e x p e c t e d from t h eloch-Griineisen
Theory. Woodard and Cody / I / and Williamson and M i l e w i t s / 2 / have p o i n t e d o u t t h a t a n e x p o n e n t i a l term -TO/T c a n be u s e d t o f i t t h e d a t a ; F i s k and Webb / 3 / and Wiesmann e t a l . / 4 / have s u g g e s t e d t h a t p(T) may " s a t u r a t e " a t h i g h T when t h e e l e c - t r o n mean f r e e p a t h becomes comparable t o t h e l a t - t i c e s p a c i n g . I n t h e l a t t e r c a s e t h e l a t t i c e spa- c i n g imposes a lower bound f o r t h e mean f r e e p a t h and a maximum f o r p.We have measured t h e r e s i s t i v i t y i n a s e r i e s o f s u p e r c o n d u c t i n g t e r n a r y C h e v r e l p h a s e compounds MlS2Mo6 X8, where M i s a m e t a l and X i s e i t h e r S o r S e , and f i n d a s i m i l a r n e s t i v e c u r v a t u - r e i n p(T) i n t h e s e m a t e r i a l s . Measurements were made on compressed, s i n t e r e d p e l l e t s r e a c t e d i n a n
i n e r t g a s atmosphere, u s i n g a s t a n d a r d f o u r probe DCmethod. To d a t e , s i n g l e c r y s t a l s of t h e s e com- pounds l a r g e enough f o r r e s i s t i v i t y measurements have n o t been s y n t h e s i z e d .
R e p r e s e n t a t i v e r e s u l t s of p v s . T t h e non- m a g n e t i c compound Pbl
.
Mo6 Se8 a r e shown i n f i g u r ev a t u r e above a b o u t 60 K . R e s i s t i v i t y r a t i o s p 300' /p15are t y p i c a l l y about 2 . 5 , b u t r a n g e a s h i g h a s 10. The d a t a can b e f i t above 40
K
w i t h i n a rms d e v i a t i o n of 1 t o 2 % by e i t h e r t h e s a t u r a t i o n r e s i s t i v i t y model of Wiesman e t a l . o r by a n expo- n e n t i a l dependence o n 1/T. The model of Wiesmann e t a l . assumes t h a t t h e t o t a l r e s i s t i v i t y of t h e system i s g i v e n by t h e electron-phonon r e s i s t i v i t y pideal(T) shunted by t h e l i m i t i n g r e s i s t i v i t y p max.
'
1 - 1
- -
-
+ - ,
1' i d e a l (T) 'max
Here pideal i s assumed t o have t h e form y + p l T , where y and p l a r e c o n s t a n t s . For t h e compounds s t u d i e d , pmax/p300 i s t y p i c a l l y e q u a l t o a b o u t 1.5, s u g g e s t i n g t h a t t h e r e i s r e l a t i v e l y l i t t l e f u r t h e r change i n r e s i s t i v i t y above room t e m p e r a t u r e .
T for
....
I . The r e s u l t s f o r PblV2Mo6 S8, S n l V 2 Mo6 S8, Sn1.2
F i g . j : R e s i s t i v i t y of P b l . 2M06 Se8(p = Mo S e a , La Mo S and L a l s 2 MO Se a r e q u a l i -
6 1 . 2 6 8 6 8 1.8 mS2 cm) r e p r e s e n t a t i v e of t h e c o m p o u n ~ ~ O s t u d i e d .
t a t i v e l y s i m i l a r , a s a r e compounds where M i s one c u r v a t u r e i s n e g a t i v e above a b o u t 60 K. of t h e r a r e e a r t h e l e m e n t s Gd, Dy, o r Sm. The r e s i s -
t i v i t i e s f o r a l l t h e s e compounds show n e g a t i v e c u r -
Fitted to an empirical exponential relation of the
form
p(T)
:a
+bT
+de -To/T
,
these compounds have
discussed various physical interpretations of this
thermally activated resistivity.
The room temperature resistivities of all
ternary compounds studied were high, typically
2-10
m.Q-
cm. This value is somewhat sensitive to
the method of sample preparation and grain size,
but in all cases it was found to exceed 1.8
mf2-
cm.
These values are roughly tem times higher than
those found in A-15 superconductors 131. It is not
clear how much of the measured resistivity is due
to contact resistance between grains in the compres-
sed samples. However, AC resistivity measurements
on two samples at
10 and 100kHz at 300 K and 77 K
were identical to the DC results, suggesting that
the impedance between grains is resistive in cha-
racter, and probably not large in comparison with
the bulk resistivity. Residual resistivity below T
was unobservably small. At low temperatures above
T ,the measured resistivities were in good agree-
ment with estimates obtained from electron mean
free paths derived from critical field measurements
151.
C.Y. Fong /6/ has suggested that the large
values of p300 are an intrinsic bulk property of the
Chevrel phase, due to the channeling effect of the
Mo-Mo bonds on the conduction electron motion. Be-
cause the conduction d orbitals of adjacent Mo
atoms are canted with respect to each other, the
electrons must scatter through an angle of about
70' to move from one atom to the next. This results
1
in a factor of cos (70")~
-
in the matrix element
3
used to calculate the collision term in the trans-
port equation. Aside from this, the situation is
similar to that found in A-15 compounds, where the
conduction is along the d-orbitals of the transition
metal ions, which are aligned in linear chains. In
this model the Chevrel phase conductivity would be
lower than A-15 compound conductivity by a factor
1
