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On the anomalous dependence of the superconducting transition temperature Tc of CeRu2 on magnetic ion concentration
Volume 4 1A, number 5
ON THE ANOMALOUS TEMPERATURE
PHYSICS LETTERS
23 October 1972
DEPENDENCE OF THE SUPERCONDUCTING
TRANSITION
Tc OF CeRu2 ON MAGNETIC ION CONCENTRATION B. HILLENBRAND
and M. WILHELM
Siemens Research Laboratories Erlangen, Germany
Received 23 August 1972 It is shown that the anomalous concentration dependence of the superconducting transition temperature of CeRu2 on gadolinium concentration is not a simple superposition of the pair-breaking effect of magnetic ions and an alloying effect.
Generally is found that the superconducting transition temperature T, decreases linearly with the concentration c of magnetic ions at least in the low concentration range. This decrease is often very large, dTJdc being of the order 10 to 100 K/mole-%, and is attributed to a pair-breaking mechanism resulting from exchange interaction between local spins and conduction electron spins [l] . Strictly speaking there is a linear dependence of T, on a pair-breaking parameter p, p being proportional to cN(0)Jzfl, where N(0) is the density of states and Jeff the effective exchange interaction constant. Apparently CeRu2 and its mixed crystals with magnetic rare earth elements are an exception from this rule [2,3]. In this case dT,/dc is zero or even positive for c + 0. Cesium in CeRu, is tetravalent, the other rare-earth elements of interest here are trivalent. Therefore one can make the assumption that the observed T, versus c-curve is produced by superposition of an alloying effect because of different valency of cesium and its substitutes and of the usual pair-breaking effect. If we assume that the real Tco (transition temperature without local spins) of a Gd,Cel_,Ru2mixed crystal is obtained when replacing gadolinium by yttrium, then we can take into account the alloying effect in T,, but not in the pair-breaking parameter. Such a procedure, however, did not result in a linear concentration dependence of T, [4]. On the other hand, mixed crystals of the very similar intermetallic compound LaRu2 with GdRu, gave the linear concentration dependence [S] . Therefore we prepared mixed crystals of the composition Ce,,Y0.16_,GdXRu2 with a constant part of trivalent ions. Thus we intended to eliminate the alloy-
0
0
I
I
2
4
I
/
/I
I
I
8 12 6 10 COrlCerltr2tlUrlOf Gd RUZ ---*
I 14mole-X16
Fig. 1. The dependence of transition temperature T, and Curie temperature Bc for the isoelectronic mixed crystals Ceo&4(Yo_l,j_x~dx)Ru2 on GdRu2-concentration.
ing effect and expected the normal linear concentration dependence of T,. Preparation and measurements were done as described in [6] . The result is to be seen in fig. 1. There are plotted transition temperatures T, and Curie temperatures Bc against the concentration of GdRu,. Bc depends linearly on the concentration the straight line being the same as for alloys without yttrium. T, also shows no remarkable difference to the yttrium-free alloy [3]. Because of the yttrium content, T, amounts to 4.9 K in accordance with already published results [6], but the general feature has not changed, that is a nonlin-
419
Volume
41A, number
5
PHYSICS
dependence of T,_ on concentration, starting horizontally at the origin. This rather unexpected result shows that the explanation of the anomalous Tc-behaviour of CeRu, is not simply a superposition of a rather small pairbreaking effect and an alloying effect, at least if one assumes that alloying is well described by changing mean values as for instance the conduction electron concentration (rigid band picture). ear
420
LETTERS
23 October
1972
References
III AA. Abrikosov
and L.P. Gorkov, Sov. Phys. JETP 12 (1961) 1243. 121 B.T. Matthias, H. SUN and E. Corenzwit, Phys. Rev. Lett. 1 (1958) 92. J. Phys. Chem. Sol. 31 I31 M. Wilhelm and B. HiJlenbrand, (1970) 559; M. Wilhelm and B. Hillenbrand, Physica 55 (1971) 608. K. Schuster and M. Wilhelm, Z. Naturf. I41 B, Hillenbrand, 26a (1971) 1684. and M. Wilhelm, Phys. Lett. 40A (1972) I51 B. Hillenbrand 387. Z. Naturf. 26a (1971) I61 M. Wilhelm and B. Hillenbrand, 141.
ON THE ANOMALOUS TEMPERATURE
PHYSICS LETTERS
23 October 1972
DEPENDENCE OF THE SUPERCONDUCTING
TRANSITION
Tc OF CeRu2 ON MAGNETIC ION CONCENTRATION B. HILLENBRAND
and M. WILHELM
Siemens Research Laboratories Erlangen, Germany
Received 23 August 1972 It is shown that the anomalous concentration dependence of the superconducting transition temperature of CeRu2 on gadolinium concentration is not a simple superposition of the pair-breaking effect of magnetic ions and an alloying effect.
Generally is found that the superconducting transition temperature T, decreases linearly with the concentration c of magnetic ions at least in the low concentration range. This decrease is often very large, dTJdc being of the order 10 to 100 K/mole-%, and is attributed to a pair-breaking mechanism resulting from exchange interaction between local spins and conduction electron spins [l] . Strictly speaking there is a linear dependence of T, on a pair-breaking parameter p, p being proportional to cN(0)Jzfl, where N(0) is the density of states and Jeff the effective exchange interaction constant. Apparently CeRu2 and its mixed crystals with magnetic rare earth elements are an exception from this rule [2,3]. In this case dT,/dc is zero or even positive for c + 0. Cesium in CeRu, is tetravalent, the other rare-earth elements of interest here are trivalent. Therefore one can make the assumption that the observed T, versus c-curve is produced by superposition of an alloying effect because of different valency of cesium and its substitutes and of the usual pair-breaking effect. If we assume that the real Tco (transition temperature without local spins) of a Gd,Cel_,Ru2mixed crystal is obtained when replacing gadolinium by yttrium, then we can take into account the alloying effect in T,, but not in the pair-breaking parameter. Such a procedure, however, did not result in a linear concentration dependence of T, [4]. On the other hand, mixed crystals of the very similar intermetallic compound LaRu2 with GdRu, gave the linear concentration dependence [S] . Therefore we prepared mixed crystals of the composition Ce,,Y0.16_,GdXRu2 with a constant part of trivalent ions. Thus we intended to eliminate the alloy-
0
0
I
I
2
4
I
/
/I
I
I
8 12 6 10 COrlCerltr2tlUrlOf Gd RUZ ---*
I 14mole-X16
Fig. 1. The dependence of transition temperature T, and Curie temperature Bc for the isoelectronic mixed crystals Ceo&4(Yo_l,j_x~dx)Ru2 on GdRu2-concentration.
ing effect and expected the normal linear concentration dependence of T,. Preparation and measurements were done as described in [6] . The result is to be seen in fig. 1. There are plotted transition temperatures T, and Curie temperatures Bc against the concentration of GdRu,. Bc depends linearly on the concentration the straight line being the same as for alloys without yttrium. T, also shows no remarkable difference to the yttrium-free alloy [3]. Because of the yttrium content, T, amounts to 4.9 K in accordance with already published results [6], but the general feature has not changed, that is a nonlin-
419
Volume
41A, number
5
PHYSICS
dependence of T,_ on concentration, starting horizontally at the origin. This rather unexpected result shows that the explanation of the anomalous Tc-behaviour of CeRu, is not simply a superposition of a rather small pairbreaking effect and an alloying effect, at least if one assumes that alloying is well described by changing mean values as for instance the conduction electron concentration (rigid band picture). ear
420
LETTERS
23 October
1972
References
III AA. Abrikosov
and L.P. Gorkov, Sov. Phys. JETP 12 (1961) 1243. 121 B.T. Matthias, H. SUN and E. Corenzwit, Phys. Rev. Lett. 1 (1958) 92. J. Phys. Chem. Sol. 31 I31 M. Wilhelm and B. HiJlenbrand, (1970) 559; M. Wilhelm and B. Hillenbrand, Physica 55 (1971) 608. K. Schuster and M. Wilhelm, Z. Naturf. I41 B, Hillenbrand, 26a (1971) 1684. and M. Wilhelm, Phys. Lett. 40A (1972) I51 B. Hillenbrand 387. Z. Naturf. 26a (1971) I61 M. Wilhelm and B. Hillenbrand, 141.