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Ac magnetic response of fulleride superconductors
PHYSICA
Physica B 194-196 (1994) 2063-2064 North-Holland
Ac magnetic response of fulleride superconductors M. Baenitz a, E. Straubea, M. Kraus a, H. Wernerb, R. Schl6glb, K. Lfidersa alnstitut ftir Experimentalphysik, Freie Universitat Berlin, Amimallee 14, D-W-1000 Berlin 33, Germany, blnstitut ffir Anorganische Chemie, Universitfit Frankfurt, Niederurseler Hang, D-W-6000 Frankfurt 50, Germany
The properties of several fulleride superconductors were investigated using ac susceptibility measurements. Lower and upper critical magnetic fields were determined. Low field analyses of the imaginary part of the ac susceptibility show dissipation peaks allowing the interpretation in terms of inter- and intragrain flux motion.
1. I N T R O D U C T I O N The discovery that doped fullerenes show superconductivity with surprisingly high transition temperatures has stimulated the research activities considerably within this new class of materials. Many of these investigations were performed by means of dc susceptibility measurements yielding many fundamental characteristics of the superconducting state [ 1]. In the case of granular superconductors, however, ac susceptibility studies are important. In this paper the application of such measurements to superconducting C60 compounds is reported.
_
•
_
] /
v "X
0
O
10
20
30
EO
30
2. E X P E R I M E N T A L Samples were prepared from pure C60 as starting material [2]. After preparing phase-pure Rb6C60 and K6C60, these compounds were ground with another equivalent of C60 and pelletized by applying a pressure of 20 MPa (200 bar). The crushed pellets were sealed in a quartz ampoule and annealed at 350 °C for 5 days to result in phase-pure Rb3C60 and K3C60, respectively. The average particle size of the material could be determined to be (10 + 5) ~tm by applying scanning electron microscopy.
3. RESULTS
AND
.-.s
2G
:
0,3G
. :.
~
i o.G 0
10
temperature (K)
DISCUSSION
Figs. 1 and 2 show ac susceptibility results for Rb3C60 and K3C60. In the case of Rb3C60 two
Figure 1. Ac susceptibility of Rb3C6o for different ac field amplitudes.
0921-4526/94/$07.00 © 1994 - Elsevier Science B.V. All rights reserved S S D I 0921-4526(93)1664-8
2064
significant peaks occur in the imaginary part. Varying ac amplitude does not affect the first peak at higher temperature whereas the second is shifted u) lower temperatures with increasing am-plitude.(ln the case of K3C60 only the second peak occurs.) This is typical for samples with a microscopic structure consisting of an array of superconducting grains (0.1 - 1 bun) embedded in a matrix with different superconducting properties. According to this picture the first peak is due to intragrain dissipation in the Shubnikov phase of the grains between the upper and the lower critical magnetic field. Bcl values can be estimated approximately from the minimum between the two peaks (Rb3C60:200 _+ 50 G, [3]). The second peak represents the dissipation m the Shubnikov phase of the matrix material where the diamagnetic signal is superimposed by that of the grains. Applying Bean's model leads to intergram critical current densities of jc(17 K) _= 104 A/cm 2 (Rb3C60) and jc(4 K) = 104 A/cm 2 (K3C60), respectively. An intragrain Jc value is estimated to be 105 - 106 A/cm 2 (28 K, Rb3C60). The influence of external dc fields is shown in Fig. 3. All curves exhibit a linear behaviour (for the Rb3C60 samples above approximately 1.5 - 2 T). The resulting values of B~2 at zero temperature and the Ginzburg-Landau coherence length ~GL are determined to be [3]: 44 + 3 T and 27_+ 1 (Rb3C60, sample 1), 17 _+ 4 T and 44 + l A (K 3C 6o), respectively.
>-.
L 0
5
10
15
:ZO
i i 20G ~ 8G
II ~
i
>. '3G 9(j
L c5
i LG :0.5(; ~ 0.1G O.OlO
i
5
10
t cmperal
15 urc
20
(gl
Figure 2. Ac susceptibility of K3C60 for different ac field amplitudes.
15
Acknowledgement. This work was supported by the Bundesministerium ftir Forschung und Technologie (1<+ E-Vorhaben 13 N 60 74 1).
References 1. M. Kraus, M. Baenitz, M. Kanowski, E. Straube, E.-W. Scheidt, S. G/irtner, H.M. Vieth, H. Werner, R. Schl6gl, W. Krfitschmer, K. Ltiders, Applied Superconductivity 1, 901 (1993). 2. M. Baenitz, E. Stranbe, S. Gartner, H. Werner, R. SchlOgl, K. Ltiders, Fullerenes Science and Technology 1, No. 2 (1993). 3. M. Baenitz, E. Sraube, M. Kraus, H. Werner, R. Schl6gl, K. Ltiders, to be published in Springer Series of Solid State Sciences.
RbaCso
,-,.j 10
k:¢/Go 10
s a l n p l e ~1
~D
C e-,
sample 0 20.0
22.5
~2 ~ • 95.0
~ 27.5
" 30.0
L e m p e r a t . l U ' e (K)
10.0
19.5
15.0
temperature
17.5
20.0
(K)
Figure 3. Upper critical magnetic fields (sample 2 prepared from RbT1 alloy [2]).
Physica B 194-196 (1994) 2063-2064 North-Holland
Ac magnetic response of fulleride superconductors M. Baenitz a, E. Straubea, M. Kraus a, H. Wernerb, R. Schl6glb, K. Lfidersa alnstitut ftir Experimentalphysik, Freie Universitat Berlin, Amimallee 14, D-W-1000 Berlin 33, Germany, blnstitut ffir Anorganische Chemie, Universitfit Frankfurt, Niederurseler Hang, D-W-6000 Frankfurt 50, Germany
The properties of several fulleride superconductors were investigated using ac susceptibility measurements. Lower and upper critical magnetic fields were determined. Low field analyses of the imaginary part of the ac susceptibility show dissipation peaks allowing the interpretation in terms of inter- and intragrain flux motion.
1. I N T R O D U C T I O N The discovery that doped fullerenes show superconductivity with surprisingly high transition temperatures has stimulated the research activities considerably within this new class of materials. Many of these investigations were performed by means of dc susceptibility measurements yielding many fundamental characteristics of the superconducting state [ 1]. In the case of granular superconductors, however, ac susceptibility studies are important. In this paper the application of such measurements to superconducting C60 compounds is reported.
_
•
_
] /
v "X
0
O
10
20
30
EO
30
2. E X P E R I M E N T A L Samples were prepared from pure C60 as starting material [2]. After preparing phase-pure Rb6C60 and K6C60, these compounds were ground with another equivalent of C60 and pelletized by applying a pressure of 20 MPa (200 bar). The crushed pellets were sealed in a quartz ampoule and annealed at 350 °C for 5 days to result in phase-pure Rb3C60 and K3C60, respectively. The average particle size of the material could be determined to be (10 + 5) ~tm by applying scanning electron microscopy.
3. RESULTS
AND
.-.s
2G
:
0,3G
. :.
~
i o.G 0
10
temperature (K)
DISCUSSION
Figs. 1 and 2 show ac susceptibility results for Rb3C60 and K3C60. In the case of Rb3C60 two
Figure 1. Ac susceptibility of Rb3C6o for different ac field amplitudes.
0921-4526/94/$07.00 © 1994 - Elsevier Science B.V. All rights reserved S S D I 0921-4526(93)1664-8
2064
significant peaks occur in the imaginary part. Varying ac amplitude does not affect the first peak at higher temperature whereas the second is shifted u) lower temperatures with increasing am-plitude.(ln the case of K3C60 only the second peak occurs.) This is typical for samples with a microscopic structure consisting of an array of superconducting grains (0.1 - 1 bun) embedded in a matrix with different superconducting properties. According to this picture the first peak is due to intragrain dissipation in the Shubnikov phase of the grains between the upper and the lower critical magnetic field. Bcl values can be estimated approximately from the minimum between the two peaks (Rb3C60:200 _+ 50 G, [3]). The second peak represents the dissipation m the Shubnikov phase of the matrix material where the diamagnetic signal is superimposed by that of the grains. Applying Bean's model leads to intergram critical current densities of jc(17 K) _= 104 A/cm 2 (Rb3C60) and jc(4 K) = 104 A/cm 2 (K3C60), respectively. An intragrain Jc value is estimated to be 105 - 106 A/cm 2 (28 K, Rb3C60). The influence of external dc fields is shown in Fig. 3. All curves exhibit a linear behaviour (for the Rb3C60 samples above approximately 1.5 - 2 T). The resulting values of B~2 at zero temperature and the Ginzburg-Landau coherence length ~GL are determined to be [3]: 44 + 3 T and 27_+ 1 (Rb3C60, sample 1), 17 _+ 4 T and 44 + l A (K 3C 6o), respectively.
>-.
L 0
5
10
15
:ZO
i i 20G ~ 8G
II ~
i
>. '3G 9(j
L c5
i LG :0.5(; ~ 0.1G O.OlO
i
5
10
t cmperal
15 urc
20
(gl
Figure 2. Ac susceptibility of K3C60 for different ac field amplitudes.
15
Acknowledgement. This work was supported by the Bundesministerium ftir Forschung und Technologie (1<+ E-Vorhaben 13 N 60 74 1).
References 1. M. Kraus, M. Baenitz, M. Kanowski, E. Straube, E.-W. Scheidt, S. G/irtner, H.M. Vieth, H. Werner, R. Schl6gl, W. Krfitschmer, K. Ltiders, Applied Superconductivity 1, 901 (1993). 2. M. Baenitz, E. Stranbe, S. Gartner, H. Werner, R. SchlOgl, K. Ltiders, Fullerenes Science and Technology 1, No. 2 (1993). 3. M. Baenitz, E. Sraube, M. Kraus, H. Werner, R. Schl6gl, K. Ltiders, to be published in Springer Series of Solid State Sciences.
RbaCso
,-,.j 10
k:¢/Go 10
s a l n p l e ~1
~D
C e-,
sample 0 20.0
22.5
~2 ~ • 95.0
~ 27.5
" 30.0
L e m p e r a t . l U ' e (K)
10.0
19.5
15.0
temperature
17.5
20.0
(K)
Figure 3. Upper critical magnetic fields (sample 2 prepared from RbT1 alloy [2]).