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HYPERFINE INTERACTION STUDIES OF
CHEVREL PHASE SUPERCONDUCTORS
C. Kimball, van Landuyt, C. Barnett, G. Shenoy, B. Dunlap, F. Fradin
To cite this version:
JOURNAL DE PHYSIQUE Colloque C6, suppliment au no 8, Tome 39, aotir 1978, page C6-367
HYPERFINE INTERACTION STUDIES OF CHEVREL PHASE SUPERCONDUCTORS
*
*
C.W. Kimball, G. van Landuyt, C. Barnett, G.K. Shenoy
,
B.D. Dunlap,
and F.Y. Fradin*
Illinois University, De KaZb, Illinois, U.S.A.
*
Argonne National Laboratory, Argonne, Illinois60439, U. S. A.
RQsum6.- Nous avons effectus des Qtudes microscopiques des proprietds Qlectroniques de phases de Chevrel supraconductrices. Nous avons examine les anomalies de phonons au site de Sn dans SnMo6S8, SnMo,Se8 et La
.
8Sno.onM~6Se,. La polarisation des Qlectrons de conduction par les ions Eu a QtQ QtudiQe dans EQ 8nl-x~o,S8 et les consQquences de cette polarisation pour les propriQtQs supraconductrices sont dfscutees.
Abstract.- Microscopic studies of the electronic properties of Chevrel phase superconductors are reported. Phonon anomalies at the Sn site in SnMo6S8, SnMo,Se, and La, 99Sno,02M06Se, have been investigated. Studies of polarization of conduction electrons by the E; Ion m Eu Snl-xMo,S, have been performed, and the effects of Such polarization on superconducting propertiez are discussed.
Because of their relatively high transition temperatures, very high critical fields, and enhan- ced superconducting ropert ties with the addition of magnetic impurities, there has been great interest
recently in the Chevrel phase ternary compounds ba-
sed on the molybdenum sulphides and selenides. We have investigated a number of these materials utili-
zing the Mossbauer resonances in 'l9sn and 151Eu. The Mossbauer spectra of l19sn in SnMosSs show a well-resolved quadrupole doublet with a tem- perature dependent asymmetry due to anisotropy in the lattice vibrations (the Karyagin-Goldanskii effect). Careful measurements of the temperature de- pendencies of this asymmetry, the relative recoil- free fraction (£) and the line shift (6) were analy- sed / I / . It was shown that (i) the mean-square lat- tice displacements of the Sn atoms are strongly ani-
sotropic and anharmonic, (ii) there is an anomaly in the temperature dependences of 6 and f, indicative of a soft-mode transition in the Sn phonon spectrum and (iii) there is a possibly large contribution of the solft vibrational modes to the mass enhancement
1 and hence to the high Tc. Subsequent neutron dif-
fraction measurements / 2 / have shown that the major mode-softening occurs in modes associated with the Mo,S8 units and one presumes that a hybridization between the Sn-local modes and those of Mo6S8 is present. Such an effect appears necessary to explain both the Mossbauer and the neutron results 1 3 1 .
Data for the recoil-free fraction both paral- lel and perpendicular to the three-fold axis at the Sn site in SnMo,S, are shown in figure 1.
Fig. 1 : Temperature dependence of the recoil-free fraction, f, parallel and perpendicular to the three-fold axis at the Sn site in SnMo6S8. The das- hed and solid lines are calculated in the quasihar- monic approximation for the indicated values of
(a,, nu,
a;).
These quantities have been calculated in a quasi- harmonic approximation using the variational treat- ment of Hui and Allen 141. Two cases were conside-
red, with single particle potentials involving either fourth or sixth order anharmonicities, i.e.,
The results of the calculation for various values of Q2,Q6 and Q6 are shown in the figure. One sees that a large anharmonicity is required to explain the observed temperature dependencies.
In SnMo,Se,, the quadrupole splitting 1 s much smaller (0.52 mm/s, as compared to 0.81 mm/s. in SnMo6S,), possibly due to larger lattice spacing in this material. Because of the smaller splitting, de- tailed analysis of lattice anisotropy is not possi-
ble. Analysis of the area and shift data also shows a possible mode-softening in the vicinity of 80
-
100 K, although the phonon anomalies appear to be much smaller than observed in SnMo6S8. Data have al- so been obtained for LaMo,Se, with 2 % Sn substitu- ted for La. Again, a small quadrupole splitting is observed (0.32 m / s ) with less pronounced phonon anomalies.Measurements have also been made on compounds E U ~ S ~ , - ~ M O ~ S ~ (x = 0.25, 0.50). It is known that the incorporation of the E U ~ + ion into SnMo,S, has very little effect on the transition temperature for x
2
0.50 in spite of the large paramagnetic mo- ment of 7VB associated with Eu. In addition, thepresence of the paramagnetic ion has been observed to increase the value of the upper critical field
2
1.51.
By combining Mossbauer measurements on '"Eu, knight shift measurements on 'MO and bulk magnetization measurements, we have shown thatwhen an external field is applied to these materials, the primary effect of the paramagnetic EUZ+ ions is to cause a polarization of the conduction electrons. This polarization is parallel to the field at the Eu site, but shows a spatial dependence such thatit is negative at the Mo site. As a result, there is a partial compensation of the applied field at the Mo sites, inhibiting pair-breaking of the superconduc- ting Mo d-electrons, and causing an enhancement of the crltical field over that in the pure Sn /6/.
In the Mossbauer spectra, rather wide
lines are seen which are both temperature and con- centration dependent. This broadening is attributed
to the influence of paramagnetic relaxation effects.
The concentration dependence of the line-width in- dicates that spin-spin interactions are present, but the temperature dependence shows the presence of effects due to exchange coupling between the rare- earth and the conduction electrons (the Korringa process). Such effects are not seen in normal metal-
lic Eu systems becuase the coupling is too strong. This data thus provides a direct observation of the weak coupling of the Eu-conduction electron systems. In keeping with traditional analyses of Korringa in- teractions, this indicates a small density of states at the Eu site and/or a small exchange coupling J
sf' in agreement with the observed weak dependence of Tc on the rare-earth concentration.
References
11
/ Kimball, C.W., Weber, L., van Landuyt, G., Fradin, F.Y., Dunlap, B.D. and Shenoy, G.K., Phys. Rev. Lett.2
(1976) 412./2/ Bader, S.D., Knapp, G.S., Sinha, S.K., Schweiss, P. and Renker, B., Phys. Rev. Lett.
37
(1976) 344.131 Bader, S.D. and Sinha, S.K., to be published.
/ 4 / Hui,J.C.K.and Allen, P.B., J. Phys. C,
5
(1975) 2923.151 Fischer $, Delcroux, M., Roth, S., Chevrel, R. and Sergent, M., J. Phys. C
