ISOTOPE EFFECT IN THE SUPERCONDUCTING CHEVREL PHASE Mo6Se8

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ISOTOPE EFFECT IN THE SUPERCONDUCTING

CHEVREL PHASE Mo6Se8

F. Culetto, F. Pobell

To cite this version:

JOURNAL DE PHYSIQUE

Colloque C6, supplPment au no 8, Tome 39, aotit 1978, page

C6-354

ISOTOPE EFFECT

IN THE

SUPERCONDUCTING CHEVREL

PHASE

Mo6Se8

F.J. Culetto and F. Pobell

I n s t i t u t fur Festkdmer.?orscbW&7, KFA JiiZich, 0-51 70 JtlZich, Germany

RQsum6.- Des mesures de Tc des composds Mosses contenant les isotopes stables de Mo et Se donnent respectivement pour l'exposant de l'ef fet isotope en Tc a ~ 6

BM0

, = (0,27 ? 0,04) et

Bse

= (0,27 +0,05). Nous concluons que seuls des modes auxquels les 6 atomes de Mo et les 8 atomes de Se con- tribuent, dQterminent Tc dans ce compos8. I1 s'agit des modes acoustiques de translation des com- plexes moldculaires Mo6Se8et des modes internes optiques, les modes de torsion du complexe moldcw laire Qtant d'une importance moindre.

Abstract.- Measurements of Tc of Mosses compounds containing the stable isotopes of Mo and Se, give for the isotope effect exponent in Tc a M-B,

b0

= (0.27

+

0.04) and

BSe

= (0.27 f 0.05), respectively. We conclude that only modes to which the 6 Mo and 8 Se atoms contribute about equal- ly are determining Tc in this compound. These are acoustic translational modes of the MosSesclus- ter, and its internal optical modes, whereas torsional modes of the cluster must be of minor importance.

We have measured the influence of the isoto- pic mass on the transition temperature to the super- conducting state of Mosses. This experiment has been performed to understand which of the various phonon modes of this compound are important for its super- conductivity. The binary system Mosses is believed to be essential for the properties of ternary Chevrel superconductors / I / . It is well suited for isotope effect measurements because its two atoms both have a substantial number of stable isotopes.

The main experimental problem has been the synthesis of compounds with a reproducible and sharp transition temperature. The optimal preparation conditions have been found empirically using natural Mo and Se powders as starting materials 121. To avoid contamination with MoSe2 we choose the under- stoichiometric composition Mo6Se7.6. Our synthesis resulted in samples with a maximum T scattering of f 0.015 K for samples prepared in one production cycle. X-ray examinations showed the samples to be single phase.

Figure 1 shows the transition curves

-

mea- m

sured inductively

-

of Moa Se7.s and of %os~e7.6, respectively. Plots of log Tc versus log MM and versus log M are shown in figure 2. Fitting these

Se-

data to Tc a M we obtain

Within the molecular crystal model 141 for the Chevrel phases, the 42 modes of Mosses can be separated into 3 acoustic translational plus 3

Fig. 1 : Inductively measured superconductive tran- sitions of MosmSe7.6 and nMosSe7.6

.

The nominal isotopic masses are m = 82, 80, 78, 76, and n = 100; 98, 96, 94, 92 from left to right .I2

torsional modes of the Mosses cluster, and 36 inter- nal modes of it. The Eliashberg electron-phonon coupling function can then be written as

Each of the three modes is connected with an effec- tive mass Meff. The isotope effect exponent calcula- ted from the linearlized Eliashberg equations is according to ref. 151,

0 810 References

6 L5

I

/ I

/

For a review see Fischer, .

0..

. Proc. of the 14th 1nt.Conf.on Low Temp.Phys. Vol.V, Helsinki (1975) p. 172

/2/ Culetto, F.J. and Pobell, F., to be published /3/ Besides the direct variation of Tc as a function

of M (via the variation of phonon frequencies), there is an indirect change of T, due to a change of the unit cell volume if M is changed. This

-

contribution can well account for the fact we > 0.5 ; see Nakajima, T., et al., ;s^: 27 (1977) 245

.

-

-

Fig. 2 : Logarithmic plot of Tc versus isotopic mass

M of Mo and Se, respectively. The lines /4/ Bader, S.D., Knapp, G.S., Sinha, S.K., Schweiss, have slopes of -0.27. P. and Renker, B., Phys.Rev.Lett.

37

(1976) 344

/ 5 / Rainer, D., to be published

d log Tc= lmdm R(W)CX~F(U) d log Meff(w)

,

with 0

d w 6Tc

~ ( w ) = -(-

-

)

,

and 2Tc 6a2F(w)

The coefficient cMo and cSe describe how much the 6 Mo or the 8 Se atoms contribute to a particular mode. Assuming as an approximation, discrete phonon frequencies for the various modes, we can write

d log Tc =

-

1

~(w)a'F(w) d log Mef 'a 3w

trans tors int

Using Meff = 650+8MSe, Meff a (%o+4MSe). and

Meff = %oMSe/(so+MSe)

,

for typical effective mas- ses of the translational torsional, and internal modes, respectively, we find cMo/cSe = 0.9, 0.3, and 0.8 for the ratio of the coefficients of the three mentioned modes. These ratios have to be compared with our experimental result BMo/BSe = 1.0. Contrary to former suggestions /4/, we can conclude that tor- sional modes do not contribute significantly to @

and Tc of MosSee

.

For example, we could exclude an equal coupling of torsional and translational modes if these two modes are largely governing T 121. Unfortunately, the ratios of the translational and of the internal modes are both rather close to our experimental result, so we can at present not dis- tinguish between them.