Physica B 165&166 (1990) 1541-1542North-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 theIrsuperconducting 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 incontrast with the macroscopic resistive transition midpoint. The ceramics remain superconducting underquasihydrostatic pressure up to 100 kbar. We found that the chemical pressure alone can't be responsible forthe absence of superconductivity in the Od - based compound.

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Fig.I. Electrical resistance of SmCeCuO sample as afunction of temperature at zero (A) and under 0.9 kOe (B)and 60 kOe (C) magnetic fields.

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We found the intergrain coupling to be very weak in thesecompounds. When a low magnetic field (H < 1 kOe) isapplied on the samples, an unusual "double peak"superconducting transition can be observed (curve B inFig.l). We associate the first high temperature resistancereduction to not completely percolating chain of welltouching superconducting grains. This is followed by aresistance increase due to thermally activated quasiparticletunneling. At low enough temperatures Josephson couplingbecomes efficient and the samples resistance reduces again.The detailed picture depends, of course, on the variations ofintragrain critical temperatures, as well as on the distributionof intergrain resistances and Josephson coupling energies.

Consistent results were obtained by measuring the I-Vcharacteristics, demonstrating clearly separated and welldefined values of inter- and intragranular critical currents.

normal grains of the material. The suppression of thesuperconducting energy gap by a high magnetic field leadsto the progressive reduction of the resistance to its normalstate value (curve C in Fig.1).

B,..-----------..,UJ Sml.8SCeO.1SCu04_y

The recently discovered family of high temperaturesuperconductors Ln2_xMxCU04_y (Ln = Pr, Nd, Sm, Eu;M = Ce, Th) presents a wide range of interesting, not yetresolved, physical properties. The charge carriers arebelieved to be electrons according to Hall [1,2] and Seebeck[3] coefficients measurements, as well as to X-rayabsorption spectroscopy [4] results. However, no evidencefor the formation of Cu1+ was found in electron energy ­loss spectroscopy [5] experiments. The study of intrinsicsuperconducting properties is additionally complicated byinteraction between superconductivity and rare-earthmagnetic order [6]. Moreover, the sintered compoundsdemonstrate a fascinating combination of intra- and inter­grain superconducting properties, the last being governedby an interplay between Josephson coupling andquasiparticle tunneling [7].

Superconductivity has not yet been observed in the Odbased compounds, although they form the same T' phase asthe other LnMCuO systems. The reported [8] decrease ofthe macroscopic critical temperature from SmCeCuO toEuCeCuO, and the absence of superconductivity inOdCeCuO could be explained in terms of chemical pressureeffects, according to the reduction of the lattice parameters.From this point of view, superconductivity should besuppressed in EuCeCuO by external pressures higher than apredicted value of 30 kbar. We found, however, [9] thatthis compound, as well as Sm and Eu based ones, remainsuperconducting till 100 kbar.

Intergrain effects.

The Ln2_xMxCu04_ samples studied here were of thesame origin as in Ref.[S] with x=0.15. We show in Fig.lthe resistance of a SmCeCuO sample measured as a functionof temperature at zero and under 0.5 kOe and 60 kOeapplied magnetic fields. All the curves coincide at hightemperatures and deviate significantly below Tcg, which wedefine as a higher limit of intragrain superconducting criticaltemperature. When measured at zero or under weak fields,the resistance demonstrates a clear enhancement in atemperature interval below Tcg. We relate this behavior tothe quasiparticle dominated tunneling betweensuperconducting - superconducting, or superconducting -

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1542 A. Gerber, J. Beille, T. Grenet, M. Cyrot

Three characteristic temperatures of the superconductingtransition can be discussed: the inrtagrain transitiontemperature Tcg,.the diamagnetic signal onset temperature

TcX;' and the macroscopic resistive transition midpointTcmid' Two of these temperatures: Tcg and Tcmid areshown in Fig.2 as a function of the host lanthanide element.Tcg and TcX; (not shown here) were found to changeweakly with respect to the host element, in contrast withTcmid' The large reduction of this last critical temperatureobserved [8] in EuCeCuO and SmThCuO compounds couldbe therefore related to a weaker intergrain coupling and notto the intrinsic crystal properties.

sample showed not a complete but, rather, a quasireentrantsuperconducting transition. Under quasihydrostatic pressureits superconducting transition temperature was identified bycomparison between R(1) curve measured at zero and under5T applied magnetic field Tcon data for the three studiedsamples is plotted in Fig.3 as a function of pressure. Incontrast with the hole-doped superconductors, the quantitydLogTc/dLogV was found to be weakly positive and of thesame order of magnitude than under hydrostatic pressure.The effect of the applied pressure on the superconductingcritical temperature does not scale with the chemical one.

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.UlJ1f--

40 80 120P (kbar)

Fig.3. The onset superconducting transition temperaturesof Sm1.85Ceo.15CU04-y (A), Nd1.85CeO.15CU04-y (B)and EU1.85CeO.15CU04-y (C) samples as a function ofapplied quasihydrostatic pressure.

Absence of superconductivity in the GdCeCuOcompound could possibly be explained by a depairing effectdue to the interaction between the lanthanide and conductionelectrons spins, which, according to the deGennes factor[10], reaches a maximum value in GdCeCuO.

We acknowledge E.A.Early, J.T.Markert andM.B.Maple for providing the samples.

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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 andH.Spille, Z.Phys.B 75, (1989) 421.6. Y.Dalichaouch, B.W.Lee, C.L.Seaman, J.T.Markertand M.B.Maple, Phys.Rev.Lett. 64, (1990) 599.7. A.Gerber, J.Beille, T.Grenet and M.Cyrot, to bepublished.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 bepublished.10. A.A.Abrikosov,and L.P.Gorkov, Sov.Phys.JETP 12,

(1961) 1243.

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One of the possible explanations for the reported decreaseof Tcmid [8] from SmCeCuO to EuCeCuO and the absenceof superconductivity in GdCeCuO compounds could havebeen based on the increase of chemical pressure, leading toan external critical pressure for disappearence ofsuperconductivity 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 berespectively calculated for SmCeCuO.

We have studied three LnCeCuO sintered samples withLn =Nd, Sm, and Eu under quasihydrostatic pressure up to100 kbar. All these samples remain superconducting in thisrange of applied pressures. Although no zero resistancestate was observed, well defined transition onsettemperatures Tcon were found in Nd and Sm basedsamples. At normal pressure, the studied tiny EuCeCuO

Fig.2. Characteristic temperatures of the superconductingtransition of Ln1.85Mo.15Cu04-y for M=Ce (0) and Th (e)corresponding to: a) the resistive superconducting midpointTcmid (reproduced from Ref.8); b) the intragrain transitiontemperature Tcg.

Pressure effects.