Ž .Physica C 294 1998 249–256

Suppression of superconductivity in Sm and Co substitutedBi Sr Ca Cu O system2 2 1 2 8qd

Sandeep SinghSurface Physics Group, National Physical Laboratory, Dr. K.S. Krishnan Road, New Delhi 110012, India

Received 12 June 1997; revised 25 September 1997; accepted 8 October 1997

Abstract

The present work investigates the effect of Sm and Co substitution in Bi Sr Ca Cu O system. The substitution of2 2 1 2 8qd

Sm leads to an initial increase of transition temperature followed by a decrease. No such increase of transition temperature isobserved in Sm and Co co-substituted samples, thus reflecting localization of excess holes in these samples. The suppressionof superconductivity in both kind of samples occurs due to hole filling and disorder effects. The insulating state of thesamples is described by 3D variable range hopping of conduction mechanism with energy dependent density of states.q 1998 Elsevier Science B.V.

Keywords: X-ray diffraction; Transition temperature; Doping ; Resistivity ; Magnetoresistance; Metal insulator transition

1. Introduction

Doping in high temperature superconductors hasbeen found to be of considerable importance due toproximity of superconducting and insulating phases.The parent compounds of oxide superconductors areall antiferromagnetic insulators consisting of CuO2

planes. As the antiferromagnetic order is destroyedby doping of holes or electrons, they become metal-lic resulting in superconductivity at low temperature.The mechanism of superconductivity is not yet clearand is intimately related to symmetry of the orderparameter. A mechanism which is as puzzling as theoccurrence of superconductivity is suppression of Tc

due to introduction of impurities. The studies relatedto suppression of T are of fundamental importancec

as they may provide a clue to the symmetry of theorder parameter.

The substitution of both magnetic and nonmag-netic impurities in oxide superconductors displaysdetrimental effects on T . This is in contrast withc

isotropic s-wave superconductors where nonmag-netic impurities have virtually no effect on T pro-c

w xvided density of states remains invariant 1 . Theproposed mechanisms for suppression of supercon-ductivity are largely system dependent and no con-sensus on the issue has emerged yet. The in-planeŽ .CuO plane and out of plane behavior of defects2

diverge as far as T suppression is concerned. Forc

example, both magnetic and nonmagnetic impuritiesat Cu sites in the La Sr CuO system general1.85 0.15 4

local magnetic moments in CuO plane which in-2w xduce pair breaking effect 2 . The suppression of

superconductivity due to nonmagnetic impurities inY Ba Cu Zn O and Bi Sr Ca Y Cu O1 2 3yx x 7yd 2 2 1yx x 2 8qd

has been interpreted as being due to pair breaking

0921-4534r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved.Ž .PII S0921-4534 97 01783-8

( )S. SinghrPhysica C 294 1998 249–256250

w xcaused by nonmagnetic disorder 3 . A strong hy-bridization between Pr 4f states and conduction bandis found responsible for T suppression inc

w xY Pr Ba Cu O 4 . Thus, the prominent can-1yx x 2 3 7yd

didates for the pair breaking scattering are a non-magnetic scattering process for other than s-typesymmetry of order parameter or an indirect magneticAbrikosov–Gor’kov process by the local magneticmoments of Cu induced by dopants.

In the present paper we investigate the conse-quence of Sm and Co substitution in Bi Sr -2 2

Ca Cu O system through X-ray and electrical1 2 8qd

resistivity measurements. In Sm and Co co-sub-stituted samples it is anticipated that the presence ofan interplanar exchange correlation would lead to a

strong suppression of superconductivity. Our resultsshow localization of excess holes in Sm and Coco-substituted samples. The suppression of supercon-ductivity occurs due to hole filling and disordereffects.

2. Experiment

All the samples were prepared by standard solidstate reaction method. Stoichiometric amounts ofBi O , SrCO , CaCO , CuO and Sm O were thor-2 3 3 3 2 3

oughly ground. For Co doped samples, high puritycobalt powder was weighed in the required amountand was dissolved in HNO . The solution was slowly3

Fig. 1. XRD patterns of the Bi Sr Ca Sm Cu O system.2 2 1yx x 2 8qd

( )S. SinghrPhysica C 294 1998 249–256 251

heater dried up till tan color and was mixed with therest of the powders. The calcination profile consistedof heating in air at 7008C for 2 h and at 8008C for 12h. The product was reground, pressed into pelletsand sintered for 40 h with intermediate grindings at860–8908C depending upon the value of x. Powderdiffraction measurements of all samples were doneon a Siefert X-ray diffractometer using Cu K a radia-tion. The DC resistivity of rectangular bar shapedsamples was measured in an APD closed cycle he-lium refrigerator using four probe technique. Lowresistive electrical contacts were made by applyingsilver paint on cleaned surface of the samples andfiring at 4008C for 30 min. Thin copper wires wereindium soldered on these pads. AC susceptibilitymeasurements were performed at 0.1 Oe and at 73Hz using phase sensitive detection technique.

3. Results and discussion

3.1. X-ray study

Ž .The X-ray diffraction XRD patterns of some ofthe samples are shown in Figs. 1 and 2. All thesamples show a single phase behavior consisting of

Ž .Bi Sr Ca Cu O 2:2:1:2 phase. Complete solu-2 2 1 2 8qd

Žbility of Sm in Bi Sr Ca Sm Cu O Sm sys-2 2 1yx x 2 8qd

.tem was obtained upto xs1.0 with no traces ofimpurity phase in the system. However, in

Ž .Bi Sr Ca Sm Cu Co O Sm, Co system2 2 1yx x 1.95 0.05 8qd

traces of impurity phases were detected for x)0.6.The background intensity remains nearly unchangedin Sm samples whereas it shows slight increase inSm, Co samples. We notice that for low Sm concen-

Ž .tration x-0.3 the most intense line is observed

Fig. 2. XRD patterns of the Bi Sr Ca Sm Cu Co O system.2 2 1yx x 1.95 0.05 8qd

( )S. SinghrPhysica C 294 1998 249–256252

Fig. 3. Variation of lattice parameters a and c with Sm concentra-tion x in the Bi Sr Ca Sm Cu O and Bi Sr -2 2 1y x x 2 8q d 2 2

Ca Sm Cu Co O systems.1y x x 1.95 0.05 8qd

Ž .from the 115 plane in both types of samples.However, with increase of Sm concentration theŽ .117 plane gives rise to the peak of highest inten-sity. The variation of lattice parameters obtainedassuming a pseudo tetragonal cell is shown in Fig. 3.The solid line is a linear best fit to the data points.The present samples exhibit an increase in a-latticeparameter and a decrease in c-lattice parameter withincreasing x. The slope dcrd xsy0.60 when com-

Žpared to dcrd xsy1.17 of Gd ionic radii rs0.93˚ ˚. Ž . w xA doped for Sr rs1.10 A in 2212 in Ref. 5 and

˚Ž .dcrd xsy0.78 of Gd doped for Ca rs0.99 A inw xRef. 6 is in agreement with smaller ionic radii

3q ˚ 2qŽ .difference of Sm rs0.96 A and Ca ions.This indirectly suggests incorporation of Sm in theunit cell of the present samples. The decrease inc-lattice parameter in Gd and Sm doped samples,however, is not proportional to the ionic radii differ-ence of the ions. This is due to the fact that a largercontraction would be opposed by the repulsive forcesbetween cationic layers.

3.2. Transition temperature and resistiÕity data

Ž .Fig. 4 a,b show the results of AC susceptibilitymeasurements of Sm and SmCo samples respec-tively. The susceptibility signal measured per mg ofthe samples decreases with increasing value of x. In

SmCo samples susceptibility signal per mg is severaltimes smaller than the same in Sm samples. The factshows that a small amount of Co ions in the CuO2

plane has decreased the superconducting volumefraction considerably.

ŽThe variation of transition temperature defined as.the onset temperature of Meissner signal with x is

shown in Fig. 5. In the Sm samples T increasesc

from 81 K for xs0.0 to 90 K for xs0.2 and thendecreases with increasing x. On the other hand, inthe SmCo samples, T decreases from 70 K forc

xs0.0 to 64 K at xs0.01 and then remains con-stant upto xs0.2. T starts decreasing with furtherc

increase of x in this system. The increase of T isc

Ž .Fig. 4. AC susceptibility vs. T of a Bi Sr Ca Sm Cu O2 2 1yx x 2 8qd

Ž .and b Bi Sr Ca Sm Cu Co O systems.2 2 1yx x 1.95 0.05 8qd

( )S. SinghrPhysica C 294 1998 249–256 253

Fig. 5. Variation of transition temperature with Sm concentrationŽ .in the Bi Sr Ca Sm Cu O shown by triangles and2 2 1yx x 2 8qd

Ž .Bi Sr Ca Sm Cu Co O shown by circles systems.2 2 1yx x 1.95 0.05 8qd

attributed to filling of excess holes by extra electronsprovided by Sm3q ions substituted for divalent Ca2q

ion. Surprisingly, SmCo samples do not show anincrease of T though the samples were prepared inc

the same way as the Sm samples. This can occur ifthe excess holes in this system get localized in thevicinity of impurity Co2q ions in the CuO plane.2

It is evident that superconductivity vanishes for0.5-x-0.6 in the Sm samples, whereas in theSmCo samples it vanishes for 0.3-x-0.4. Thevanishing of superconductivity for values of x be-tween 0.5 and 0.6 in the Sm samples is identical to

w xthe Gd doped 2:2:1:2 system 6 . According to thew xAbrikosov–Gor’kov theory 7 the decrease of transi-

Ž .tion temperature is given by the relation ln T rTc c0Ž . Ž .sC 1r2 yC 1r2qr where T is the transi-c0

tion temperature of the pure material and C is thedigamma function. For a magnetic impurity r is

w Ž . 2Ž .2 Ž .xgiven as rs n N E j g y1 J Jq1 r2kTi F J cŽ .where n is the number density of impurity, N Ei F

is the density of states at Fermi level, j is theŽ .2 Ž . Žexchange parameter and g y1 J Jq1 smJ J

.say is the de Gennes factor. The calculated values ofm for Sm3q and Gd3q ions are 4.5 and 15.8m2

J B

respectively. Despite the large difference of deGennes factors of Sm3q and Gd3q ions, the super-conductivity disappears in the same domain of x for

both rare earth impurities, thus indicating that mag-netic pair breaking does not play a dominant role insuppression of superconductivity.

A noteworthy observation of T vs. x curve ofc

SmCo samples is that with increasing Sm concentra-tion no rapid decrease in T is observed. Also thec

trend of decrease of T beyond xs0.20 in SmCoc

sample is identical to that of Sm samples. Thisshows a negligible interplanar magnetic interactionin SmCo samples and suppression of T in both thec

systems is due to identical reasons. We must mentionthat in the Fe substituted Gd Ba Cu O system a1 2 3 7yd

decrease of T is observed upon quenching whereasc

Fig. 6. Temperature dependence of electrical resistivity of super-Ž . Ž .conducting a Bi Sr Ca Sm Cu O , and b Bi Sr -2 2 1yx x 2 8qd 2 2

Ca Sm Cu Co O systems.1y x x 1.95 0.05 8qd

( )S. SinghrPhysica C 294 1998 249–256254

a similar Fe substituted Y Ba Cu O system1 2 3 7yd

w xshows an increase of T 8 . The decrease of T in Fec c

substituted Gd Ba Cu O has been attributed to1 2 3 7yd

w xstrong interplanar magnetic interaction 9 . The ab-sence of such an interaction in SmCo samples is inconformity with the higher anisotropic character ofthe Bi Sr Ca Cu O than the Y Ba Cu O2 2 1 2 8qd 1 2 3 7yd

system.The temperature dependence of resistivities of Sm

and Sm, Co samples which show superconductivityŽ .is shown in Fig. 6 a,b . It is clear that Sm concentra-

tion affects transition width, increases normal stateresistivity and residual resistivity.

Ž .The normal state resistivity r as a function ofnŽ .Sm concentration x is plotted in Fig. 7. Here r isn

defined as the resistivity value measured at 300 K. Itis evident that r values of the SmCo samples aren

higher than in the Sm samples. As the normal stateresistivity is regarded as the measure of disorderw x10,11 , it shows that the SmCo samples have ahigher amount of disorder than the Sm samples.Though Sm substitution is beyond the CuO plane, it2

presumably generates random potential in the CuO2

plane. This is evident from Fig. 8, which exhibits aŽ .variation of residual resistivity r with x. r is0 0

obtained by fitting a straight line in the linear portionof normal state resistivity of the superconducting

Fig. 7. Room temperature resistivity vs. Sm concentration x in theŽ .Bi Sr Ca Sm Cu O shown by triangles and Bi Sr -2 2 1yx x 2 8qd 2 2Ž .Ca Sm Cu Co O shown by circles systems.1y x x 1.95 0.05 8qd

Fig. 8. Dependence of residual resistivity on Sm concentration xŽ .in the Bi Sr Ca Sm Cu O shown by triangles and2 2 1yx x 2 8qd

Ž .Bi Sr Ca Sm Cu Co O shown by circles systems.2 2 1yx x 1.95 0.05 8qd

samples. Residual resistivity arises due to impurityscattering in the CuO plane. The increase of r2 0

with x implies that impurity scattering in the CuO2

plane also increases with Sm concentration. More-over, higher values of r in the SmCo samples are0

in line with the presence of a higher amount ofimpurity content in the SmCo samples.

The temperature dependence of resistivity of theSm and SmCo samples as shown in Fig. 6 follows asimilar behavior upto xs0.40 in the Sm system andxs0.20 in the SmCo system. The normal state r–Tcurves of all these samples are nearly parallel andcan be derived from each other by a displacementalong the resistivity axis. The r–T curves of xs0.50in the Sm system and xs0.30 in the SmCo systemare interesting as they show the same transitiontemperature and display identical shallow minima inresistivity before the onset of superconducting transi-

Ž .tion. However, the high temperature )200 K r–Tcurves of these samples differ qualitatively fromeach other. Whereas, in xs0.50 it is closely parallelto the rest of the samples in the Sm system, the r–Tcurve at high temperature of xs0.30 in the SmCosystem shows a reduction of slope as compared torest of the samples. As the normal state resistivity,which determines disorder, is higher in the Sm,xs0.50, sample than in the SmCo, xs0.30, sam-

( )S. SinghrPhysica C 294 1998 249–256 255

ple, the reduction of slope in SmCo, xs0.30, couldnot be due to localization effect related to disorder.The described behavior can be understood in terms

) Ž 2 .of Drude’s model of resistivity rsm r ne t ,where m) is the effective mass of the electron, n iscarrier concentration and ty1 is the scattering rate.

y1 y1 y1 y1Ž .t can be expressed as t st qt T , where0 iny1 y1Ž . Ž .t and t T are the elastic impurity scattering0 in

rate and the inelastic scattering rate, respectively.y1Ž .Again t T is related to the electron boson cou-in

Ž . y1Ž .pling constant l and is given as t T ;2plT.in

Within this model, the change of resistivity deriva-tive in the SmCo, xs0.30, sample is because ofvanishing inelastic scattering rate, presumably stimu-lated by Co ions in the SmCo system. The shallowupturn in resistivity in these samples is interpreteddue to weak localization effect. Thus, the hole fillingby an extra electron of Sm3q, sudden increase innormal state resistivity, increase in residual resistiv-ity and the appearance of resistivity minima showthe hole filling and disorder effects as the dominantmechanism for the suppression of superconductivityin both the systems.

The resistivity curves of the sample in the insulat-ing state are shown in Fig. 9. In the insulating stateFermi energy lies in the region of localized stateswhich results in the phonon assisted variable range

Fig. 9. r –T curves of the Sm, xs0.6, 1.0, and SmCo, xs0.4,0.5, samples. The current scale is for the Sm, xs0.6, and SmCo,xs0.4, samples. For the SmCo, xs0.5, and Sm, xs1.0, sam-ples one small division equals 0.005 and 0.50 V cm, respectively.

Ž . Ž . y1 r4Fig. 10. a ln s vs. T plot for the Sm, xs0.6, and SmCo,Ž . Ž . Ž .ymxs0.6 samples. b ln s vs. T plot of the Sm, xs1.0,

sample for various values of exponent m.

Ž .of hopping VRH of conduction mechanism. Thew xVRH conductivity is given as 12

Ž .1r nq1w xsss exp yT rT .0 0

The characteristic temperature T is related to den-0Ž .sity of states N E at E asF F

16a 3

T s ,0 kN EŽ .F

where ay1 is the localization length. The leastŽ . y1r nsquare fitting of ln s vs. T for ns2, 3 and 4

in the present samples showed the best fit for ns4.Ž . Ž . y1r4In Fig. 10 a we have shown ln s vs. T curves

( )S. SinghrPhysica C 294 1998 249–256256

for two samples: Sm, xs0.6, and SmCo, xs0.6.Ž . Ž . Ž .ymFig. 10 b shows typical ln s vs. T curves for

Sm, xs1.0, sample for values of ms1r2, 1r3and 1r4. It is evident that ms1r4 yields a betterfitting. Except in the case of Sm, xs1.0, a kinkappeared at around 30–35 K in all the samples. Sucha kink is shown by deviation from straight line in

Ž .Fig. 10 a . The occurrence of kink can be accountedw xfor within the Ortuno and Pollak 13 model. The

model has the special feature that the density ofstates near E is concave in nature giving rise toF

Ž . y1r4discontinuity in ln s vs. T curve at low tem-perature. In the Sm, xs1.0, sample probably thediscontinuity appears at lower temperature and hencewas not observed.

4. Conclusion

We studied Bi Sr Ca Sm Cu O and2 2 1yx x 2 8qd

Bi Sr Ca Sm Cu Co O systems through2 2 1yx x 1.95 0.05 8qd

X-ray and electrical resistivity measurements. Thesubstitution of Sm in the former leads to an increaseof transition temperature before it starts decreasing.The absence of similar behavior in Sm and Coco-substituted samples reflects localization of excessholes in these samples. The suppression of supercon-ductivity due to Sm substitution occurs due to holefilling and disorder effects. The samples which are inthe insulating state follow 3D variable range hopping

of conduction mechanism with energy dependentdensity of states.

Acknowledgements

The author is thankful to Drs. S.M. Shivprasadand C. Anandan for helpful discussions.

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