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n - Doped superconducting compounds: Granular and pressure effects
Physica B 165&166 (1990) 1541-1542 North-Holland
n - DOPED SUPERCONDUCTING COMPOUNDS: GRANULAR AND PRESSURE EFFECTS. A.OERBER, J.BEILLE, T.ORENET, and M.CYROT. Laboratoire Louis Neel, CNRS, 25 avenue des Martyrs, 166X, 38042 Grenoble cedex, France. We studied the superconducting properties of the electron-doped compounds Ln2_xMxCU04_y (Ln = Pr, Nd, Sm, Eu; M = Ce and Th). The granular structure of the samples influences strongly theIr superconducting behavior, causing an unusual "double - peak" resistive transition in low magnetic fields. The intragrain transition temperature was found to vary weakly in the series of the studied compounds in contrast with the macroscopic resistive transition midpoint. The ceramics remain superconducting under quasihydrostatic pressure up to 100 kbar. We found that the chemical pressure alone can't be responsible for the absence of superconductivity in the Od - based compound. The recently discovered family of high temperature superconductors Ln2_xMxCU04_y (Ln = Pr, Nd, Sm, Eu; M = Ce, Th) presents a wide range of interesting, not yet resolved, physical properties. The charge carriers are believed to be electrons according to Hall [1,2] and Seebeck [3] coefficients measurements, as well as to X-ray absorption spectroscopy [4] results. However, no evidence for the formation of Cu1+ was found in electron energy loss spectroscopy [5] experiments. The study of intrinsic superconducting properties is additionally complicated by interaction between superconductivity and rare-earth magnetic order [6]. Moreover, the sintered compounds demonstrate a fascinating combination of intra- and intergrain superconducting properties, the last being governed by an interplay between Josephson coupling and quasiparticle tunneling [7]. Superconductivity has not yet been observed in the Od based compounds, although they form the same T' phase as the other LnMCuO systems. The reported [8] decrease of the macroscopic critical temperature from SmCeCuO to EuCeCuO, and the absence of superconductivity in OdCeCuO could be explained in terms of chemical pressure effects, according to the reduction of the lattice parameters. From this point of view, superconductivity should be suppressed in EuCeCuO by external pressures higher than a predicted value of 30 kbar. We found, however, [9] that this compound, as well as Sm and Eu based ones, remain superconducting till 100 kbar.
Intergrain effects. The Ln2_xMxCu04_ samples studied here were of the same origin as in Ref.[S] with x=0.15. We show in Fig.l the resistance of a SmCeCuO sample measured as a function of temperature at zero and under 0.5 kOe and 60 kOe applied magnetic fields. All the curves coincide at high temperatures and deviate significantly below T cg, which we define as a higher limit of intragrain superconducting critical temperature. When measured at zero or under weak fields, the resistance demonstrates a clear enhancement in a temperature interval below Tcg. We relate this behavior to the quasiparticle dominated tunneling between superconducting - superconducting, or superconducting 0921-4526/90/$03.50
© 1990 -
normal grains of the material. The suppression of the superconducting energy gap by a high magnetic field leads to the progressive reduction of the resistance to its normal state value (curve C in Fig. 1).
B,..-----------.., Sml.8SCeO.1SCu04_y .......
UJ
o
...... 0 E""=/'
~~
:.~Cg
::::
,,'S
..c. o
:.
'At"I'''to,~,
o
A
E
"""0 0:: 0 N
.
B O.9(kOQ) A:
C
49
i
I -----l 0+-----4----4-
o
10
T CK)
20
30
Fig.I. Electrical resistance of SmCeCuO sample as a function of temperature at zero (A) and under 0.9 kOe (B) and 60 kOe (C) magnetic fields. We found the intergrain coupling to be very weak in these compounds. When a low magnetic field (H < 1 kOe) is applied on the samples, an unusual "double peak" superconducting transition can be observed (curve B in Fig.l). We associate the first high temperature resistance reduction to not completely percolating chain of well touching superconducting grains. This is followed by a resistance increase due to thermally activated quasiparticle tunneling. At low enough temperatures Josephson coupling becomes efficient and the samples resistance reduces again. The detailed picture depends, of course, on the variations of intragrain critical temperatures, as well as on the distribution of intergrain resistances and Josephson coupling energies. Consistent results were obtained by measuring the I-V characteristics, demonstrating clearly separated and well defined values of inter- and intragranular critical currents.
Elsevier Science Publishers B.V. (North-Holland)
1542
A. Gerber, J. Beille, T. Grenet, M. Cyrot
Three characteristic temperatures of the superconducting transition can be discussed: the inrtagrain transition temperature Tcg,.the diamagnetic signal onset temperature
TcX;' and the macroscopic resistive transition midpoint Tcmid' Two of these temperatures: Tcg and Tcmid are shown in Fig.2 as a function of the host lanthanide element. Tcg and TcX; (not shown here) were found to change weakly with respect to the host element, in contrast with Tcmid' The large reduction of this last critical temperature observed [8] in EuCeCuO and SmThCuO compounds could be therefore related to a weaker intergrain coupling and not to the intrinsic crystal properties.
sample showed not a complete but, rather, a quasireentrant superconducting transition. Under quasihydrostatic pressure its superconducting transition temperature was identified by comparison between R(1) curve measured at zero and under 5T applied magnetic field Tcon data for the three studied samples is plotted in Fig.3 as a function of pressure. In contrast with the hole-doped superconductors, the quantity dLogTc/dLogV was found to be weakly positive and of the same order of magnitude than under hydrostatic pressure. The effect of the applied pressure on the superconducting critical temperature does not scale with the chemical one. ~-r----------,
.....0 ::S:::N
§
'"' C
a
0 N
.
UlJ1
f--
1J
°e
u
~
I-
(0)
40 80 P (kbar)
0
0
l'l
0
__
I
III
!
~N
~
Fig.3. The onset superconducting transition temperatures of Sm1.85Ceo.15CU04-y (A), Nd1.85CeO.15CU04-y (B) and EU1.85CeO.15CU04-y (C) samples as a function of applied quasihydrostatic pressure.
!:!
Cl 0
.-~-
(b) 1 - - _...... 1 --1 o ......-~ 1 _ - 11_ _1
Pr
Nd
Pm
8m
Eu
120
Gd
Fig.2. Characteristic temperatures of the superconducting transition of Ln1.85Mo.15Cu04-y for M=Ce (0) and Th (e) corresponding to: a) the resistive superconducting midpoint Tcmid (reproduced from Ref.8); b) the intragrain transition temperature Tcg.
Pressure effects. One of the possible explanations for the reported decrease of Tcmid [8] from SmCeCuO to EuCeCuO and the absence of superconductivity in GdCeCuO compounds could have been based on the increase of chemical pressure, leading to an external critical pressure for disappearence of superconductivity below 30 kbar for the two first materials. Using other definitions of the critical temperature like TcX; or Tcg, critical pressure values of 60 and 92 kbar can be respectively calculated for SmCeCuO. We have studied three LnCeCuO sintered samples with Ln = Nd, Sm, and Eu under quasihydrostatic pressure up to 100 kbar. All these samples remain superconducting in this range of applied pressures. Although no zero resistance state was observed, well defined transition onset temperatures Tcon were found in Nd and Sm based samples. At normal pressure, the studied tiny EuCeCuO
Absence of superconductivity in the GdCeCuO compound could possibly be explained by a depairing effect due to the interaction between the lanthanide and conduction electrons spins, which, according to the deGennes factor [10], reaches a maximum value in GdCeCuO. We acknowledge E.A.Early, J.T.Markert and M.B.Maple for providing the samples.
References.
1. Y.Tokura, H.Takagi and S.Uchida, Nature (London) 337, (1989) 345. 2. H.Takagi, S.Uchida and Y.Tokura, Phys.Rev.Lett., 62, (1989) 1197. 3. Y.Hikada and M.Suzuki, Nature (London) 338, (1989) 635. 4. J.M.Tranquada, S.M.Heald, A.R.Moodenbaugh, G.Liang and M.Croft, Nature (London) 337, (1989) 720. 5. N.Nucher, P.Adelmann, M.Alexander, H.Romberg, S.Nakai, J.Fink, H.Rietschel, G.Roth, H.Schmidt and H.Spille, Z.Phys.B 75, (1989) 421. 6. Y.Dalichaouch, B.W.Lee, C.L.Seaman, J.T.Markert and M.B.Maple, Phys.Rev.Lett. 64, (1990) 599. 7. A.Gerber, J.Beille, T.Grenet and M.Cyrot, to be published. 8. J.T.Markert, J.Beille, J.J.Neumeier, E.A.Early, C.L.Seaman, T.Moran and M.B.Maple, Phys.Rev.Lett. 64, (1990) 80. 9. J.Beille, A.Gerber, T.Grenet and M.Cyrot, to be published. 10. A.A.Abrikosov,and L.P.Gorkov, Sov.Phys.JETP 12, (1961) 1243.
n - DOPED SUPERCONDUCTING COMPOUNDS: GRANULAR AND PRESSURE EFFECTS. A.OERBER, J.BEILLE, T.ORENET, and M.CYROT. Laboratoire Louis Neel, CNRS, 25 avenue des Martyrs, 166X, 38042 Grenoble cedex, France. We studied the superconducting properties of the electron-doped compounds Ln2_xMxCU04_y (Ln = Pr, Nd, Sm, Eu; M = Ce and Th). The granular structure of the samples influences strongly theIr superconducting behavior, causing an unusual "double - peak" resistive transition in low magnetic fields. The intragrain transition temperature was found to vary weakly in the series of the studied compounds in contrast with the macroscopic resistive transition midpoint. The ceramics remain superconducting under quasihydrostatic pressure up to 100 kbar. We found that the chemical pressure alone can't be responsible for the absence of superconductivity in the Od - based compound. The recently discovered family of high temperature superconductors Ln2_xMxCU04_y (Ln = Pr, Nd, Sm, Eu; M = Ce, Th) presents a wide range of interesting, not yet resolved, physical properties. The charge carriers are believed to be electrons according to Hall [1,2] and Seebeck [3] coefficients measurements, as well as to X-ray absorption spectroscopy [4] results. However, no evidence for the formation of Cu1+ was found in electron energy loss spectroscopy [5] experiments. The study of intrinsic superconducting properties is additionally complicated by interaction between superconductivity and rare-earth magnetic order [6]. Moreover, the sintered compounds demonstrate a fascinating combination of intra- and intergrain superconducting properties, the last being governed by an interplay between Josephson coupling and quasiparticle tunneling [7]. Superconductivity has not yet been observed in the Od based compounds, although they form the same T' phase as the other LnMCuO systems. The reported [8] decrease of the macroscopic critical temperature from SmCeCuO to EuCeCuO, and the absence of superconductivity in OdCeCuO could be explained in terms of chemical pressure effects, according to the reduction of the lattice parameters. From this point of view, superconductivity should be suppressed in EuCeCuO by external pressures higher than a predicted value of 30 kbar. We found, however, [9] that this compound, as well as Sm and Eu based ones, remain superconducting till 100 kbar.
Intergrain effects. The Ln2_xMxCu04_ samples studied here were of the same origin as in Ref.[S] with x=0.15. We show in Fig.l the resistance of a SmCeCuO sample measured as a function of temperature at zero and under 0.5 kOe and 60 kOe applied magnetic fields. All the curves coincide at high temperatures and deviate significantly below T cg, which we define as a higher limit of intragrain superconducting critical temperature. When measured at zero or under weak fields, the resistance demonstrates a clear enhancement in a temperature interval below Tcg. We relate this behavior to the quasiparticle dominated tunneling between superconducting - superconducting, or superconducting 0921-4526/90/$03.50
© 1990 -
normal grains of the material. The suppression of the superconducting energy gap by a high magnetic field leads to the progressive reduction of the resistance to its normal state value (curve C in Fig. 1).
B,..-----------.., Sml.8SCeO.1SCu04_y .......
UJ
o
...... 0 E""=/'
~~
:.~Cg
::::
,,'S
..c. o
:.
'At"I'''to,~,
o
A
E
"""0 0:: 0 N
.
B O.9(kOQ) A:
C
49
i
I -----l 0+-----4----4-
o
10
T CK)
20
30
Fig.I. Electrical resistance of SmCeCuO sample as a function of temperature at zero (A) and under 0.9 kOe (B) and 60 kOe (C) magnetic fields. We found the intergrain coupling to be very weak in these compounds. When a low magnetic field (H < 1 kOe) is applied on the samples, an unusual "double peak" superconducting transition can be observed (curve B in Fig.l). We associate the first high temperature resistance reduction to not completely percolating chain of well touching superconducting grains. This is followed by a resistance increase due to thermally activated quasiparticle tunneling. At low enough temperatures Josephson coupling becomes efficient and the samples resistance reduces again. The detailed picture depends, of course, on the variations of intragrain critical temperatures, as well as on the distribution of intergrain resistances and Josephson coupling energies. Consistent results were obtained by measuring the I-V characteristics, demonstrating clearly separated and well defined values of inter- and intragranular critical currents.
Elsevier Science Publishers B.V. (North-Holland)
1542
A. Gerber, J. Beille, T. Grenet, M. Cyrot
Three characteristic temperatures of the superconducting transition can be discussed: the inrtagrain transition temperature Tcg,.the diamagnetic signal onset temperature
TcX;' and the macroscopic resistive transition midpoint Tcmid' Two of these temperatures: Tcg and Tcmid are shown in Fig.2 as a function of the host lanthanide element. Tcg and TcX; (not shown here) were found to change weakly with respect to the host element, in contrast with Tcmid' The large reduction of this last critical temperature observed [8] in EuCeCuO and SmThCuO compounds could be therefore related to a weaker intergrain coupling and not to the intrinsic crystal properties.
sample showed not a complete but, rather, a quasireentrant superconducting transition. Under quasihydrostatic pressure its superconducting transition temperature was identified by comparison between R(1) curve measured at zero and under 5T applied magnetic field Tcon data for the three studied samples is plotted in Fig.3 as a function of pressure. In contrast with the hole-doped superconductors, the quantity dLogTc/dLogV was found to be weakly positive and of the same order of magnitude than under hydrostatic pressure. The effect of the applied pressure on the superconducting critical temperature does not scale with the chemical one. ~-r----------,
.....0 ::S:::N
§
'"' C
a
0 N
.
UlJ1
f--
1J
°e
u
~
I-
(0)
40 80 P (kbar)
0
0
l'l
0
__
I
III
!
~N
~
Fig.3. The onset superconducting transition temperatures of Sm1.85Ceo.15CU04-y (A), Nd1.85CeO.15CU04-y (B) and EU1.85CeO.15CU04-y (C) samples as a function of applied quasihydrostatic pressure.
!:!
Cl 0
.-~-
(b) 1 - - _...... 1 --1 o ......-~ 1 _ - 11_ _1
Pr
Nd
Pm
8m
Eu
120
Gd
Fig.2. Characteristic temperatures of the superconducting transition of Ln1.85Mo.15Cu04-y for M=Ce (0) and Th (e) corresponding to: a) the resistive superconducting midpoint Tcmid (reproduced from Ref.8); b) the intragrain transition temperature Tcg.
Pressure effects. One of the possible explanations for the reported decrease of Tcmid [8] from SmCeCuO to EuCeCuO and the absence of superconductivity in GdCeCuO compounds could have been based on the increase of chemical pressure, leading to an external critical pressure for disappearence of superconductivity below 30 kbar for the two first materials. Using other definitions of the critical temperature like TcX; or Tcg, critical pressure values of 60 and 92 kbar can be respectively calculated for SmCeCuO. We have studied three LnCeCuO sintered samples with Ln = Nd, Sm, and Eu under quasihydrostatic pressure up to 100 kbar. All these samples remain superconducting in this range of applied pressures. Although no zero resistance state was observed, well defined transition onset temperatures Tcon were found in Nd and Sm based samples. At normal pressure, the studied tiny EuCeCuO
Absence of superconductivity in the GdCeCuO compound could possibly be explained by a depairing effect due to the interaction between the lanthanide and conduction electrons spins, which, according to the deGennes factor [10], reaches a maximum value in GdCeCuO. We acknowledge E.A.Early, J.T.Markert and M.B.Maple for providing the samples.
References.
1. Y.Tokura, H.Takagi and S.Uchida, Nature (London) 337, (1989) 345. 2. H.Takagi, S.Uchida and Y.Tokura, Phys.Rev.Lett., 62, (1989) 1197. 3. Y.Hikada and M.Suzuki, Nature (London) 338, (1989) 635. 4. J.M.Tranquada, S.M.Heald, A.R.Moodenbaugh, G.Liang and M.Croft, Nature (London) 337, (1989) 720. 5. N.Nucher, P.Adelmann, M.Alexander, H.Romberg, S.Nakai, J.Fink, H.Rietschel, G.Roth, H.Schmidt and H.Spille, Z.Phys.B 75, (1989) 421. 6. Y.Dalichaouch, B.W.Lee, C.L.Seaman, J.T.Markert and M.B.Maple, Phys.Rev.Lett. 64, (1990) 599. 7. A.Gerber, J.Beille, T.Grenet and M.Cyrot, to be published. 8. J.T.Markert, J.Beille, J.J.Neumeier, E.A.Early, C.L.Seaman, T.Moran and M.B.Maple, Phys.Rev.Lett. 64, (1990) 80. 9. J.Beille, A.Gerber, T.Grenet and M.Cyrot, to be published. 10. A.A.Abrikosov,and L.P.Gorkov, Sov.Phys.JETP 12, (1961) 1243.