Copper isotope shifts in Pr and La substituted 123 and 124 cuprate superconductors; comparison with oxygen isotope shifts

Physica C 298 Ž1998. 203–208

Copper isotope shifts in Pr and La substituted 123 and 124 cuprate superconductors; comparison with oxygen isotope shifts D.E. Morris b

a,)

, A.P.B. Sinha a , V. Kirtikar a , A.V. Inyushkin

b

a Morris Research, Inc., 44 Marguerita Road, Berkeley, CA 96707, USA Institute of Molecular Physics, Russian Research Centre ‘KurchatoÕ Institute’, Moscow, Russia

Received 23 June 1997; revised 11 September 1997; accepted 12 September 1997

Abstract Copper isotope shifts of Tc have been measured in 123 and 124 cuprate superconductors with partial substitutions by Pr and La to reduce hole concentration. The 123 compositions were Pr xY1yx Ba 2 Cu 3 O 7 Ž x s 0.2, 0.3, 0.4. and YLa 0.3 Ba 1.7 Cu 3 O 7 . The 124 compounds were Pr xY1yx Ba 2 Cu 4 O 8 Ž x s 0.2, 0.3. and YLa 0.3 Ba 1.7 Cu 4 O 8 . Oxygen isotope experiments in materials with these compositions have shown substantial isotope shifts. We find that the Cu isotope shifts are also substantial, but somewhat smaller. Other findings are: Ž1. the Cu isotope exponent a Cu is positive, Ž2. a Cu increases rapidly as Tc is reduced by substitution, and rises to a substantial fraction of the BCS value, behavior which is similar to that of the oxygen isotope exponent a O , and Ž3. when Tc is reduced by non-isovalent substitution, both O and Cu isotope shifts increase greatly, but the ratio a Cura O remains 0.75 " 0.1 in both 123 and 124, regardless of the amount and site of the non-isovalent substitution. q 1998 Published by Elsevier Science B.V. Keywords: Superconductivity; Copper isotope effect; Pr substitution; La substitution

1. Introduction Isotope effects in cuprate superconductors have been widely investigated in order to clarify the role of electron–phonon interactions in the microscopic pairing mechanism. Published results on these measurements have been reviewed by Franck w1x. Nearly all the experiments relate to the effects of oxygen isotope substitution, and numerous experiments indicate that a O Žwhere Tc A MyA . is nearly zero in optimally doped Ži.e. the highest Tc . cuprate superconductors, but that a O increases considerably when )

Corresponding author. Tel.: q1 510 525 0122; Fax: q1 510 525 8131.

the hole concentration and Tc are reduced below the optimum by partial substitution of a non-isovalent cation or removal of oxygen. As Tc decreases with increasing substitution, a O rises rapidly into the range of the BCS value for phonon mediated superconductivity. It has been further established by siteselective isotope work, pioneered by Morris et al. w2x and followed up by Condor et al. w3x, and Zech et al. w4x, that the small isotope shift, which is observed at optimum hole concentration Žand Tc ., is caused by isotope substitution of the CuO 2 sheets and not in the apical or CuO chain sites. Pr doped 123 was used by Zhao et al. w5x in site selective oxygen isotope substitution experiments, since Pr greatly increases the oxygen isotope shift Žsee Ref. w1x.. Again the

0921-4534r98r$19.00 q 1998 Published by Elsevier Science B.V. All rights reserved. PII S 0 9 2 1 - 4 5 3 4 Ž 9 7 . 0 1 7 3 0 - 9

204

D.E. Morris et al.r Physica C 298 (1998) 203–208

shift, although larger, was associated with oxygen isotope substitution in the CuO 2 sheets. The above observations showed the significance of phonons involving vibrations of the O ions in the CuO 2 sheets in the pairing interaction, and it is natural to also investigate the role of the Žlower frequency. phonons which involve motion of the Cu ions. This can be explored by measurement of copper isotope effects. Unfortunately, such experiments are technically more demanding than the corresponding oxygen isotope experiments, because oxygen isotopes can be substituted by gas diffusion into identical pieces broken from the same superconducting pellet. Therefore, very few Cu isotope experiments have been done so far. A copper isotope shift was observed w6x in La 2y x Sr x CuO4 for x s 0.125 and x s 0.15. The copper isotope exponent values were found to be very close to those of the oxygen isotope exponent. The copper isotope effect was also investigated in fully oxygenated YBa 2 Cu 3 O 7 Žsee Refs. w7–12x. but no discernible shift was observed. In hindsight this is not surprising since the oxygen isotope DTc is very small Ž0.3 K. and the results of the present paper suggest that the Cu isotope shift should be somewhat smaller. Recently, Franck et al. w13x investigated copper isotope effect in oxygen deficient YBa 2 Cu 3 O y with transition temperature between 40 and 91 K. They reported negative isotope exponent. It was small in the 60 K plateau region and unobservable for Tc s 91 K. In regions away from these two special conditions the a was reported to reach large values. Zhao et al. w14x have carefully measured the Cu isotope effect in YBa 2 Cu 3 O y in the underdoped region with reduced oxygen content, and found a large positive copper isotope exponent Ž a Cu s 0.37.. The exponent was found to decrease to zero Žor slightly negative. in the optimally doped region Ž y s 6.94.. These results indicate that phonon modes involving Cu as well as O in the CuO 2 planes are important in the pairing mechanism of high Tc superconductivity. It is well known that Pr substitution at Y site or La substitution at Ba site decreases Tc drastically in both 123 and 124. Oxygen isotope experiments on these substituted materials showed substantial isotope shifts, as large as 1.5–2 K. It was therefore anticipated that the copper isotope shift DT and the

isotope exponent a Cu might also have relatively large values. Furthermore, these compounds are easy to prepare in pure single-phase form, so these results can be more definitive. We report here results of our measurements on seven 123 and 124 compounds: The 123 compositions were Pr0.2Y0.8 Ba 2 Cu 3 O 7 , Pr 0 .3Y 0 .7 Ba 2 Cu 3 O 7 , Pr 0 .4Y 0 .6 Ba 2 Cu 3 O 7 and YLa 0.3 Ba 1.7 Cu 3 O 7 . The 124 compositions were Pr 0 .2Y 0 .8 Ba 2 Cu 4 O 8 , Pr 0 .3Y 0 .7 Ba 2 Cu 4 O 8 and YLa 0.3 Ba 1.7 Cu 4 O 8 .

2. Experimental 2.1. Starting materials Oxides of isotopically pure copper with ultra-high chemical purity ŽTable 1. were prepared at the Institute of Molecular Physics, Kurchatov Institute, Moscow. The analyzed impurity levels are given in Table 1. It can be seen that the difference in the concentration levels of all impurities between 63 CuO and 65 CuO is 1 ppmw or less except in case of Sn, where it is 2 ppmw. These extremely small differences in impurity levels will not measurably shift Tc . For example, in the case of Zn, which is very effective in depressing the Tc of 123, the estimated decrease due to this amount Ž1 ppmw. would be only about 1.5 mK, which is two orders of magnitude smaller than the copper isotope shifts observed in our experiments.

Table 1 Analysis of isotopically pure CuO samples Žimpurities in ppmw. Element

63

Ni Zn Co Fe Mn Ca Ti Cr Pb Sn Mo K Na

7 3 3 4 1 5 1 20 20 4 2 -10 -10

CuO

66

CuO

6 4 3 3 1 4 1 20 20 2 2 -10 -10

D.E. Morris et al.r Physica C 298 (1998) 203–208

The other ingredients, Y2 O 3 , BaCO 3 , Pr4 O 11 , La 2 O 3 , ZnO and NiO, had purity of 99.99% or better; this is quite sufficient because the same batch of each oxide is used for both Cu isotope samples, so impurities are the same. La 2 O 3 was ignited at 9508C for 3 h before weighing. All other materials were dried at 2008C for 6 h. 2.2. Precursor preparation For each pair of samples Ž 63 Cu, 65 Cu., a precursor was prepared by mixing all the required ingredients Žexcept copper oxide. in the required proportions.

205

The subsequent steps involved in the sample preparation were: 1. adding the 63 CuO and the 65 CuO to aliquots of the precursor in the correct ratios. Each sample was mixed in an agate mortar and pestle for 15 min under cyclohexane, 2. pressed into a 6 mm diameter pellet, 3. fired at 9508Cr15 h in pure flowing oxygen, 4. dry ground and re-pressed, 5. re-fired at 9508Cr15 h in pure flowing oxygen and slow cooled between 600–4008C for 15 h, and

Fig. 1. Plot of magnetization as a function of temperature; Ža. for Pr0.4Y0.6 Ba 2 Cu 3 O 7 , Žb. for Pr0.3Y0.7 Ba 2 Cu 3 O 7 and Žc. for Pr0.3Y0.7 Ba 2 Cu 4 O 8 . The curves for 63 Cu samples and 65 Cu samples are identified in the plots. The values of DTc were read off from each pair, and the values of a calculated therefrom. These values are listed in Table 2. In the 123 compounds, Pr xY1yx Ba 2 Cu 3 O 7 , Tc goes down as x increases, but DTc , and a Cu rise. In the 124 compounds, Pr xY1yx BaCu 4 O 8 the DTc and a Cu rise as x increases from x s 0 to x s 0.2; but then they level off and remain nearly constant as x increases further to x s 0.3.

D.E. Morris et al.r Physica C 298 (1998) 203–208

206

6. finally, each pellet was re-ground to very fine powder, pressed into a 4 mm diameter pellet and annealed in pure O 2 at 5008C for 10 h. In all these experiments, the two tablets were placed together Žone on the other with a gold foil separator. in the uniform temperature zone of the furnace. 2.3. Sample preparation and measurement For the 124 samples, after stage Ž5. each pellet was crushed and mixed in mortar and pestle with the appropriate amount of additional CuO Žusing the same Cu isotope. and Ag 2 O was added. The dry mixing was carried out for 15 min in each case, after which the sample was pressed into a 4 mm diameter pellet and heated in a commercial high P ŽO 2 . furnace w15x at 100 bar, 9008C for 15 h. This step was repeated twice Ž4 times for the La substituted samples to obtain pure single phase 124.. Then each pellet was reground to very fine powder, pressed into a 4 mm diameter pellet and annealed in flowing, pure O 2 at 5008C for 10 h. The susceptibility was measured with a Quantum Design SQUID magnetometer. The field-cooled, measured-on-warning susceptibility was measured in a field of 20 Oe. The temperature measurements were performed with a platinum resistance thermometer ŽLakeshore PT-111. in direct contact with the sample and driven by the microprocessor con-

trolled AC bridge in the SQUID. The resolution was 2.5 mK and reproducibility 10 mK at 77 K after cycling to room temperature w16x. The compositions were so chosen, and conditions of preparations so controlled, that changes in Tc due to any factor other than the isotope effect were minimized. The precautions taken were: 1. The superconductors selected were all derivatives of YBa 2 Cu 3 O 7 and YBa 2 Cu 4 O 8 , which are both line compounds so there is no scope for composition variation in the parent superconducting phase; 2. any difference in concentrations of the substituent ŽPr or La. in the 66 Cu vs. 63 Cu samples was avoided by mixing in the substituent at the precursor stage; 3. during every thermal treatment, the isotopic pair was kept in very close proximity Žone on top of the other. in the constant temperature zone to ensure that both received identical thermal treatment. The 123 compounds were annealed in oxygen long enough to ensure complete oxidation. Since 124 has a fixed oxygen stoichiometry, problems associated with changes in oxygen content are obviated. Error bars on our measurements were experimentally determined by carrying out several blank runs using CuO Ž99.999%. with natural isotope abundance. Eight samples of Pr0.2Y0.8 Ba 2 Cu 3 O 7 were prepared in four runs Žtwo per run., following the procedure and precautions described above, and their

Table 2 Comparison of copper and oxygen isotope effects in doped cuprate superconductors Ratio a Cu ra O

Composition of HTSC compound

Oxygen isotope effect T c ŽK. DT c ŽK.

aO

Copper isotope effect TcŽ63. ŽK. TcŽ65. ŽK.

DT c ŽK.

a Cu

Pr2Y0.8 Ba 2 Cu 3 O 7

75.6

0.80

0.11 ŽRef. w17x.

73.2

73.0

0.20

0.09

0.82

1.16

0.23 ŽRef. w17x.

59.7

59.4

0.30

0.16

0.70

1.50

0.32 ŽRef. w17x.

45.3

45.0

0.30

0.22

0.69

–

–

70.2

70.0

0.20

0.09

–

–

63.5

63.3

0.20

0.10

57.45

57.1

0.35

0.20

52.6

52.0

0.60

0.36

Pr0.3Y0.7 Ba 2 Cu 3 O 7

60.4

Pr0.4Y0.6 Ba 2 Cu 3 O 7

46.2

Pr0.2Y0.8 Ba 2 Cu 4 O 8

–

a

Pr0.3Y0.7 Ba 2 Cu 4 O 8

–

YLa 0.3 Ba 1.7 Cu 3 O 7

60

1.93

0.27 ŽRef. w16x.

YLa 0.3 Ba 1.7 Cu 4 O 8

–

–

–

a

Not measured.

0.74

D.E. Morris et al.r Physica C 298 (1998) 203–208

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susceptibility vs. temperature for three representative samples in the Pr substituted series Žtwo of 123 and one of 124.. It may be seen that there is a substantial linear portion in the xm –T plot near Tc . Tc was obtained by extending this linear portion to xm s 0 and a calculated therefrom: a s wln TcŽ63. y ln TcŽ65. xrwlnŽ65. y lnŽ63.x. Our results are presented in Table 2. Fig. 2 presents results of magnetisation of La substituted 123 and 124 of two representative samples. Table 2 gives the values of Tc , DTc and a for all seven samples. Results are analyzed and discussed below. 3. Results

Fig. 2. Plot of magnetization as a function of temperature; Ža. for YLa 0.3 Ba 1.7 Cu 3 O and Žb. for YLa 0.3 Ba 1.7 Cu 4 O 8 . The curves for 63 Cu and 65 Cu samples are identified in the figure. The values of DTc measured from these figures are listed in Table 2, with the corresponding of a Cu . The DTc Žand a Cu . values are larger than unsubstituted Y-123 and Y-124, respectively. We note that a Cu of YLa 0.3 Ba 1.7 Cu 4 O 8 is larger than those of the other Pr and La substituted samples in Table 2. Note that, in all cases, the value of a Cu is about 3r4 of the value of a O .

Tc values were measured. They were then converted to four pairs of Pr0.2Y0.8 Ba 2 Cu 4 O 8 by addition of CuO and Ag 2 O, following the procedure described above, and their Tc values were measured once more. Although the difference of Tc from batch to batch was as large as 500 mK, the difference between the Tc values of the two samples of any pair in the same batch was well below 100 mK, and this is taken as our confidence limit for the isotope shift DTc . The substituted 123 and 124 compounds described in this paper showed Cu isotope shifts considerably larger than this. Fig. 1 presents molar

1. A significant copper isotope effect was found in several measurements on a variety of samples of several composition of the 123 and 124 superconductors. The higher mass Ž 65 Cu. isotope has a lower Tc , i.e. a Cu is positive, the same sign as the oxygen isotope effect Žsee Table 2.. 2. The copper isotope effect and oxygen isotope effect behave similarly in several other respects: Ža. For optimally doped materials Ži.e. those having the highest Tc ., both oxygen and copper isotope effects are near zero. Žb. When doping moves the hole concentration away from the optimum, causing Tc to fall, the copper isotope DTc and a Cu rise rapidly. When the doping has reduced Tc substantially, a Cu is quite large, a substantial fraction of the BCS value. In contrast, as we show in another paper Žto be published. when Tc is decreased by substitutions for Cu such as Ni and Zn, which will cause pair breaking, and do not shift the hole concentration away from optimal, the DTc ŽCu. and a Cu remain small even though Tc is suppressed. This behavior is also seen in the oxygen isotope shift in these materials Žsee Refs. w16,17x.. 3. The dependence of the oxygen and copper isotope effects on composition is very similar, giving a ratio of a Cu ra O which is nearly constant in these Pr and La doped 123 and 124 superconductors. Considering the systematic errors involved, we find that the ratio a Cu ra O s 0.75 " 0.1 well represents our results ŽTable 2, last column..

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4. Discussion Our results Žthe large a Cu . establish that the importance of the lower frequency phonon modes which involve copper motion and their effect on Tc is comparable to the high frequency modes involving oxygen motion. 1. Although these Cu isotope experiments cannot identify the specific Cu phonon modes which are involved, i.e. those originating in the plane or in the chain layers, it is very likely that they comprise motions of the Cu in CuO 2 layers. 2. We come to this conclusion, because of a close similarity in the behavior of the copper and oxygen isotope effects vs. Tc and dopant type and level. Attempts have been made to argue against the importance of phonons in the HTSC pairing mechanism by explaining the large oxygen isotope effect in off-optimally doped HTSC as due to a dependence of hole concentration on the mass of the apical oxygen, which was assumed to move nonadiabatically in a large asymmetric double-well potential. Even if such a model could provide a correct explanation for the observed oxygen isotope effect, the large copper isotope shifts which we report in the present paper cannot be explained by this model which is specific to the apical oxygen; there is no theoretical reason or experimental evidence for the existence of a double potential well for the motion of the Cu. A successful theory of HTSC should explain why the Cu and O isotope effects both decrease to near zero as doping is optimized to the maximum Tc in spite of such a strong involvement of phonons as demonstrated by the large Cu and O isotope effects in the non-optimally doped materials. Also to be explained is the observation that in all cases studied the minimum Žor zero. of the Cu and O isotope shifts has been found at the same hole concentration which optimizes Tc , even if this requires partial substitution Že.g., Ca in 124.. It is now clear that the isotope effect is pervasive in cuprate superconductors, and any explanation which attaches only a marginal role to phonons and

assumes that most of the attractive electron–boson interaction comes from some other mechanism is bound to face difficulty in the face of these isotope results. Any successful theory has to involve phonons, more particularly phonons involving motion of both copper and oxygen in the CuO 2 planes.

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