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Study of superconductivity in kondo alloys using Josephson tunneling
STUDY OF S U P E R C O N D U C T I V I T Y IN K O N D O ALLOYS USING J O S E P H S O N T U N N E L I N G C. VAN H A E S E N D O N C K , L. VAN DEN DRIES*, Y. BRUYNSERAEDE, Laboratorium voor Vaste Stof-Fysika en Magnetisme, KU Leuven, B-3030 Leuven, Belgium
and A. GILABERT Laboratoire de Physique de la Mati6re Condens~e**, Universite de Nice, F-06034 Nice, France
Information on the resonant spin flip scattering in dilute magnetic alloys can be obtained from superconducting experiments. We report the first measurements of the dc Josephson current in N b - N b x O y - C u C r / P b tunnel junctions where CuCr is a thin film Kondo alloy in proximity with a superconducting Pb film.
Since magnetic interactions strongly influence the superconducting pairing, superconductivity is a very sensitive tool for the study of dilute magnetic impurities in various matrices [1]. Although most Kondo alloys are normal metals, they can be given superconducting properties through the proximity effect, if they are in close contact with a superconductor S. Since this superconductivity is induced over a characteristic length of the order of 1000 A,, one is restricted to thin film systems (the C u C r / P b proximity sandwich can serve as an archetype). Josephson tunneling (transfer of Cooper pairs between two superconducting electrodes through an insulating barrier) in junctions with the proximity structure as one of the electrodes, enables to obtain information on the electron pairing in the Kondo alloy and on the scattering of the s electrons by magnetic impurities. In certain cases, we expect an anomalous behaviour of the maximum dc Josephson current I M versus temperature T for such superconductorinsulator-Kondo alloy/superconductor junctions. For a thin S layer, calculations of Kaiser [2] predict that below the transition temperature T c of the sandwich, there may exist a second transition back into the normal state. Indeed, when the Kondo temperature T K is small compared to To, the Kondo resonant scattering can destroy the superconducting pairing at T ~ T K. Theoretical calculations show that for weak superconducting order, a Josephson tunneling experiment is extremely sensitive to the magnetic depairing; therefore this *Research fellow of the Belgian Interuniversitair Instituut voor Kernwetenschappen. **Laboratoire associ6 au CNRS n ° 190.
method is more straightforward than the usual magnetization measurements [3] for studying the second transition. We present here the first preliminary dc Josephson measurements on Nb-NbxOy-C___u_uCr/Pb tunnel junctions. Fig. 1 shows the current-voltage characteristic I(V) and the dynamic conductance o(V) = d I / d V ( V ) for a typical junction. Since there is only a small difference between this o(V) curve and the c o r r e s p o n d i n g o ( V ) for a N b - N b x O - C u C r junction (dashed line in fig. 1), we may conclude that the density of states of the C u C r / P b sandwich approaches that of the normal Kondo alloy CuCr (which is nearly constant in the considered voltage region). Even though gapless, Y
_ _
0
o'-N
limA) -6
z.
2
0
~
I
J 2
0
V (meVl
Fig. 1. I(V) characteristic and dynamic conductance o(V) (normalized to the normal state conductance oN) for a typical Nb-NbxOy-C_.~uCr/Pb tunnel junction with Tc(CuCr/Pb)__ 3.5 K. The dashed line represents the conductance of a Nb-NbxOy-C._uuCr test junction.
Journal of Magnetism and Magnetic Materials 15-18 (1980) 85-86 ©North Holland
85
86
C. Van Haesendonck et al./Josephson effect in superconducting Kondo alloys
the C_uuCr/Pb is still superconducting since there is a finite Josephson current I M appearing at zero voltage (fig. 2a). T w o specific properties indicate that the observed zero voltage current I M is really due to the Josephson effect and not to microshorts: (i) Fiske steps are observed in the I ( V ) characteristic for a weak magnetic field (fig. 2b); (ii) the magnetic field dependence of I M shows a F r a u n hofer diffraction pattern (fig. 2c) Fig. 3 shows the temperature dependence of R I M ( R is the tunneling resistance) for a set of seven Nb-NbxOyC__uuCr/Pb tunnel j u n c t i o n s , fabricated in a single v a c u u m run. The thickness of the K o n d o alloy (with a concentration of ~ 2500 p p m of Cr) was varied from 240 to 650 ,~, while the thickness of the Pb film was kept constant (dpb 700 ,~). The C u C r test layer, evaporated simultaneously with the junctions, showed a clear K o n d o effect as indicated by the resistance m i n i m u m (insert fig. 3). The R I M ( T ) curves show no evidence for a second transition back into the normal state, since they are monotonically increasing with decreasing temperature (as is the casc for a constant pair breaking parameter). This could be explained by the fact that our Pb film was too thick: the criterion imposed by Kaiser would require dpb ~ 250 ,~ for our junctions. Such thin lead films were not considered for fabrication because they are below the electrical continuity limit. Recently, however, 1oo
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i
~
T
2 6 0 1 ~ V" "°°"°o
100
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•
50
10
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x
", ~_ "4 lO 30
T(K) ---P"
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,
f ~"; 25filP(~Qcml" %
• • . .
XXx
,
I
Xx× X XxeI
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15
20
25
30 T(K)
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Fig. 3. T e m p e r a t u r e d e p e n d e n c e of R I M in Nb-NbxOy-CuCr/Pb junctions with different CuCr thicknesses (dc_~cr--O 240 A;° × 300 A; O 325 A; +350 A; A 405 A; [] 520 A; ~' 650 A) and constant Pb thickness (dpb 700 A). Insert: temperature dependence of the resistivity for a CuCr layer.
we have evaporated C u / P b proximity sandwiches with a discontinuous Pb layer (dpb ~ 200 ,~) [4], which still showed a finite critical temperature. Therefore, in the near future, we will perform Josephson experiments on C__uuCr/Pb sandwiches satisfying the Kaiser criterion. We thank Prof. E. Mi.iller-Hartmann for helpful discussions and the Belgian Interuniversitair Instituut voor K e r n w e t e n s c h a p p e n for financial support.
02 o
References
Fig. 2. Josephson effects in a Nb-NbxOy-CuCr/Pb tunnel junction at T = 1.3 K: (a) maximum dc Josephson current IM; (b) Fiske current steps at external magnetic field H e = 0.16 Oe; (c) magnetic field dependence IM(He).
[1] E. Miiller-Hartmann and J. Zittartz, Phys. Rev. Lett. 26 (1971) 428. [2] A. B. Kaiser, J. Phys. F 7 (1977) L339. [3] J. G. Huber, W. A. Fertig and M. B. Maple, Solid State Commun. 15 (1974) 453. [4] Z. Ovadyahu, Ph.D. Thesis, Tel-Aviv University, Israel (1978); We are much indebted to Prof. G. Deutscher for bringing this work to our attention.
and A. GILABERT Laboratoire de Physique de la Mati6re Condens~e**, Universite de Nice, F-06034 Nice, France
Information on the resonant spin flip scattering in dilute magnetic alloys can be obtained from superconducting experiments. We report the first measurements of the dc Josephson current in N b - N b x O y - C u C r / P b tunnel junctions where CuCr is a thin film Kondo alloy in proximity with a superconducting Pb film.
Since magnetic interactions strongly influence the superconducting pairing, superconductivity is a very sensitive tool for the study of dilute magnetic impurities in various matrices [1]. Although most Kondo alloys are normal metals, they can be given superconducting properties through the proximity effect, if they are in close contact with a superconductor S. Since this superconductivity is induced over a characteristic length of the order of 1000 A,, one is restricted to thin film systems (the C u C r / P b proximity sandwich can serve as an archetype). Josephson tunneling (transfer of Cooper pairs between two superconducting electrodes through an insulating barrier) in junctions with the proximity structure as one of the electrodes, enables to obtain information on the electron pairing in the Kondo alloy and on the scattering of the s electrons by magnetic impurities. In certain cases, we expect an anomalous behaviour of the maximum dc Josephson current I M versus temperature T for such superconductorinsulator-Kondo alloy/superconductor junctions. For a thin S layer, calculations of Kaiser [2] predict that below the transition temperature T c of the sandwich, there may exist a second transition back into the normal state. Indeed, when the Kondo temperature T K is small compared to To, the Kondo resonant scattering can destroy the superconducting pairing at T ~ T K. Theoretical calculations show that for weak superconducting order, a Josephson tunneling experiment is extremely sensitive to the magnetic depairing; therefore this *Research fellow of the Belgian Interuniversitair Instituut voor Kernwetenschappen. **Laboratoire associ6 au CNRS n ° 190.
method is more straightforward than the usual magnetization measurements [3] for studying the second transition. We present here the first preliminary dc Josephson measurements on Nb-NbxOy-C___u_uCr/Pb tunnel junctions. Fig. 1 shows the current-voltage characteristic I(V) and the dynamic conductance o(V) = d I / d V ( V ) for a typical junction. Since there is only a small difference between this o(V) curve and the c o r r e s p o n d i n g o ( V ) for a N b - N b x O - C u C r junction (dashed line in fig. 1), we may conclude that the density of states of the C u C r / P b sandwich approaches that of the normal Kondo alloy CuCr (which is nearly constant in the considered voltage region). Even though gapless, Y
_ _
0
o'-N
limA) -6
z.
2
0
~
I
J 2
0
V (meVl
Fig. 1. I(V) characteristic and dynamic conductance o(V) (normalized to the normal state conductance oN) for a typical Nb-NbxOy-C_.~uCr/Pb tunnel junction with Tc(CuCr/Pb)__ 3.5 K. The dashed line represents the conductance of a Nb-NbxOy-C._uuCr test junction.
Journal of Magnetism and Magnetic Materials 15-18 (1980) 85-86 ©North Holland
85
86
C. Van Haesendonck et al./Josephson effect in superconducting Kondo alloys
the C_uuCr/Pb is still superconducting since there is a finite Josephson current I M appearing at zero voltage (fig. 2a). T w o specific properties indicate that the observed zero voltage current I M is really due to the Josephson effect and not to microshorts: (i) Fiske steps are observed in the I ( V ) characteristic for a weak magnetic field (fig. 2b); (ii) the magnetic field dependence of I M shows a F r a u n hofer diffraction pattern (fig. 2c) Fig. 3 shows the temperature dependence of R I M ( R is the tunneling resistance) for a set of seven Nb-NbxOyC__uuCr/Pb tunnel j u n c t i o n s , fabricated in a single v a c u u m run. The thickness of the K o n d o alloy (with a concentration of ~ 2500 p p m of Cr) was varied from 240 to 650 ,~, while the thickness of the Pb film was kept constant (dpb 700 ,~). The C u C r test layer, evaporated simultaneously with the junctions, showed a clear K o n d o effect as indicated by the resistance m i n i m u m (insert fig. 3). The R I M ( T ) curves show no evidence for a second transition back into the normal state, since they are monotonically increasing with decreasing temperature (as is the casc for a constant pair breaking parameter). This could be explained by the fact that our Pb film was too thick: the criterion imposed by Kaiser would require dpb ~ 250 ,~ for our junctions. Such thin lead films were not considered for fabrication because they are below the electrical continuity limit. Recently, however, 1oo
RIM(~eV} ,
- -
i
~
T
2 6 0 1 ~ V" "°°"°o
100
××
•
50
10
~,2~2~-
½
x
", ~_ "4 lO 30
T(K) ---P"
• XX
,
f ~"; 25filP(~Qcml" %
• • . .
XXx
,
I
Xx× X XxeI
000000 + + + + ++ O0000X
15
20
25
30 T(K)
3.5
Fig. 3. T e m p e r a t u r e d e p e n d e n c e of R I M in Nb-NbxOy-CuCr/Pb junctions with different CuCr thicknesses (dc_~cr--O 240 A;° × 300 A; O 325 A; +350 A; A 405 A; [] 520 A; ~' 650 A) and constant Pb thickness (dpb 700 A). Insert: temperature dependence of the resistivity for a CuCr layer.
we have evaporated C u / P b proximity sandwiches with a discontinuous Pb layer (dpb ~ 200 ,~) [4], which still showed a finite critical temperature. Therefore, in the near future, we will perform Josephson experiments on C__uuCr/Pb sandwiches satisfying the Kaiser criterion. We thank Prof. E. Mi.iller-Hartmann for helpful discussions and the Belgian Interuniversitair Instituut voor K e r n w e t e n s c h a p p e n for financial support.
02 o
References
Fig. 2. Josephson effects in a Nb-NbxOy-CuCr/Pb tunnel junction at T = 1.3 K: (a) maximum dc Josephson current IM; (b) Fiske current steps at external magnetic field H e = 0.16 Oe; (c) magnetic field dependence IM(He).
[1] E. Miiller-Hartmann and J. Zittartz, Phys. Rev. Lett. 26 (1971) 428. [2] A. B. Kaiser, J. Phys. F 7 (1977) L339. [3] J. G. Huber, W. A. Fertig and M. B. Maple, Solid State Commun. 15 (1974) 453. [4] Z. Ovadyahu, Ph.D. Thesis, Tel-Aviv University, Israel (1978); We are much indebted to Prof. G. Deutscher for bringing this work to our attention.