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Submitted on 1 Jan 1978
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SUPERCONDUCTING PROPERTIES OF (La, Nd
)Sn3ALLOYS
W. Schmid, E. Umlauf, C. Bredl, F. Steglich
To cite this version:
JOURNAL DE PHYSIQUE Colloque C6, supplément au n° 8, Tome 39, août 1978, page C6-880
SUPERCONDUCTING PROPERTIES OF ( L a , N d ) S «
3ALLOYS
W. Schmid, E. Umlauf, C D . Bredl
+and F . Steglich
+Zentralinst. f. Tieftemperaturforschung, D-8046 Garohing, Germany
+
II. Phys. Institut Vniversitat Koln u. SFB 126, D-5000 Koln, Germany
Résumé.- Des mesures de la température de transition à l'état supraconducteur Tc en fonction de la concentration en Nd et le saut de la chaleur spécifique à T montrent que Nd se comporte dans LaSn3
comme un ion trivalent stable dont la dégénérescence est levée par le champ cristallin. Comme on n'a pas détecté d'effet de mélange, les résultats confirment l'hypothèse que la diminution initiale re-lativement forte de Tc ne peut être expliquée par une interaction d'échange isotropique des spins.
Abstract.- Measurements of the superconduction transition temperature Tc vs Nd concentration and the
jump of the specific heat at Tc show that Nd in LaSn3 behaves as a stable crystal-field split tri-valent ion. Since no hybridization effects have been found, the results support the assumption that the relatively large initial depression of Tc cannot be explained by isotropic spin exchange
inter-action.
It is well known that in dilute alloys contai-ning Rare Earth (RE) impurities (e.g. La(RE)Al2) /l/
certain quantities, e.g., the superconducting tran-sition temperature T , scale with the so-called de Gennes factor (g-1)2 J(J+1), where g is the Lande factor and J is the quantum number of the total an-gular momentum. Two of the authors have found pre-viously that in La(RE)Sn this scaling behaviour is not satisfied /2/. Especially for the light RE im-purities the depression of T is much larger than expected from the de Gennes factor. One could think of two reasons for this striking observation, namely hybridization effects and/or anisotropic exchange scattering of the conduction electrons on the iso-lated RE-ions. Several experiments performed recen-tly on (La, Pr)Sn3 /3/, Ikl and (La, Sm)Sn3 151, Id I
reveal the existence of Kondo anomalies and there-fore support the relevance of hybridization effects in these two systems. We have chosen Nd as an impu-rity which, from its position in the periodic table, is expected to be a stable trivalent ion. Here, we report first experiments on T and the specific heat jump at T .
In figure 1 the reduced transition tempera-ture, obtained by different experimental methods, is plotted vs. Nd-concentration x. It shows a dis-tinct influence of the crystal field (CF) splitting, namely a deviation from the Abrikosov-Gorkov (AG) theory to higher values. This results from the ther-mal depopulation of excited CF-levels 111 and has been observed in various alloys containing RE ions with nonmagnetic groundstates /8/, /9/. Figure 1 de-monstrates this phenomenon for the first time for
t. K —1 1 1 1 1 1 1 1 1 1 — .9- V "V . % La,.,NdxSn3 T V*. Tco . N^. .6 - N X * S - A G - " \ X%
\x
•3 - * : X \ •> 0: 9 \ \v. •
c
\ v
1 1 1 1 I • • I 1] I 1 1 2 3 4 5 6 xUf/.Nd) ,0Fig. I : Reduced transition temperature Tc/ Tc o as a
function of Nd - concentration (Tc o = 6.40 K ) . Solid line : Abrikosov - Gorkov theory. Dashed - dotted line : numerical calculations /10/ based on the CF theory of Keller and Fulde /7/) (without inelastic Coulomb scattering)
a Kramers ion. Our data can be well fitted by the CF-theory /10/ if one employs a realistic level scheme /11/.
The influence of the CF is less pronounced in the experimental data of AC/AC vs T ll (figure 2). We note, that these are the first results for a CF-split Kramers-ion. Besides the high temperature re-gion all data are in excellent agreement with the AG-theory. The theoretical calculations (dashed-dotted line) for the CF split Nd in LaSn3, performed by Keller /10/, deviate considerably from the AG theory and our data. However, it was found earlier I Ml,
that the experimental data of the system (La Pr) In lie on or even below the AG-curve, thus deviating
0 0.5 T, T,, 1.
Fig. 2
:Reduced specific heat jump AC/AC versus
reduced transition temperature
O(ACo =
135
$9
r n T / ~ ' mole). Solid and dash-dotted lines
:as in Fig.
I
appreciably from the CF calculations. In reference
/lo/, it was pointed out that the theory might not
fit well the data for a strong coupling superconduc-
tor. Possibly we are faced with the same problem in
La Sn3 which has a value of ACo/Tco
=1.78 (compared
to the BCS value 1.43).
To summarize, we find thevsxdependence of
(La, Nd)Sn, in good agreement with the predictionof
CF-theory applied to stable ~
d
ions. In the jump
~
+
of the specific heat at Tc there is a considerable
discrepancy between experimental and theoretical
results.
A
possible influence of hybridization
e f -fects on this quantity is rather unlikely. This we
conclude from the observation that the experimental
data AC/ACo vs Tc/Tco follow the universal AG curve.
In order to explain the relatively high initial de-
pression of T in
(La,
Nd)Sn3 the isotropic spin-
exchange interaction model seems to be not suffi-
cient. We think that anisotropic exchange scattering
/ I 3 1
should be taken into account.
References
/ I /
Maple, M.B., Solid State Commun.
8
(1970) 1915
/2/ Schmid, W. and Umlauf, E., Commun. Physics
1
(1976) 67
131 Lethuillier, P. and Haen, P., Phys. Rev. Lett.
35 (1975) 1391
-
/4/ Abou-Aly, A.I., Bakanowski, S., Berk, N.F.,
Crow, J.E. and Mihailisin,T., Phys. Rev. Lett.
35 (1975) 1387
-
/ 5 /
