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Oxygen isotope effect in Nd1.85Ce0.15CuO4
Physica C 1 8 5 - 1 8 9 (1991) 1 3 8 5 - 1 3 8 6 North-Holland
OXYGEN ISOTOPE EFFECT IN Ndl.85Ce0.15CuO4 B. BATLOGO, S-W. CHEONG, G. A. THOMAS, $. L. COOPER,* L. W. RUPP, Jr., D. H. RAPKINE and A. S. COOPER AT&T Bell Laboratories, Murray Hill, NJ 07974 *University of Illinois, Champaign-Urbana, Illinois. Starting from high quali~ Ndl.ssCeo.tsCuO 4 polycrystalline samples with a Te of 24K, we have exchanged - 8 5 % of 160 by 180. The magnetically measured T c was found to change by less than 0.1K, equivalent to an oxygen-isotope exponent °ct_<0.05. Thus, the isotope effect is small for all classes of euprate superconductors when T¢ is optimized, irrespective of Cu-O coordination and conduction type. Ever since the original studies on YBa2Cu3OT, 1'2 the oxygen isotope effect in cuprate superconductors remains a much discussed, yet still enigmatic property. (For an up-to-date summary of results and discussions see Ref. 3). Here we report on isotope studies on "eleclron-doped" Nd t.85 Ce o. 15CuO 4. The polyerystalline samples for this study have been prepared using a multi-step grinding and annealing procedure according to the following schedule. The starling powders were mixed, heated at 950°C for 1 hour, reground, heated for 10 hours at 1000°C, reground again and fina!!"ykept at 1080°C for 20 and 50 hours, respectively. The last step involves annealing with the proper amount of Zr in order to adjust the oxygen content. This results in high quality material with a reproducible T e at 24.5K. (Fig. 1)
For the oxygen isotope exchange step, the samples were ground again and packed loosely (930°C, 1 hour) to facilitate diffusion of oxygen. Then the two halves of a sample were placed in the same furnace, but with regular r60 and with 180 enriched atmosphere. The various sets of samples were kept for 70, 100 and 120 hours at 870°C or 900°C. From differential weight changes and changes of the vibrational frequend:.~ (see Fig. 2 ), the concentration of 18O was deduced to be >_85% (e.g. samples $7: 84.3+2.5%, S10: 88.5+2.5%).
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Fig. 1 Meissner signal of loosely sintered smallgrained Nd l.S5 Ce0.!5 CuO4, cooled in a field of 5 0 e . Elsevier Science Publishers B.V.
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Fig, 2 Shift of phonon frequencies due to oxygen isotope exchange. The concentratior~ of 180 is ~85% (See text).
R Battogg et at / Oxygen isotope effect in Ndl.ssCe~tlsCu04
1386
The magnetically measured onset of superconductivity is shown on an extended temperature scale in Fig. 3 for three (of the five) sets of samples. The data are separated vertically for clarity, and the broken lines (for 180) allow a close comparison with the 160 results (solid circles). The shift in onset temperature is very small or zero, the largest one being -0.1K. This corresponds to an oxygen-isotope effect parameter °ct of _<0.05.
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REFERENCES
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[1]
B. Batlogg, G. Kourouklis, W. Weber, R. J. Cava, A. Jayaraman, A. E. White, K. T. Short, L. W. Rupp, Jr. and E. A. Rietman, Phys. Rev. Lett. 912 0987)°
[2]
T.A. Faltens, W. K. Ham, S. W. Keller, K. J. Leafy, J. N. Michaels, A. M. Staey, H-C. zur Loye, D. E. Morris, T. W. Barbee, HI, L. C. Bourne, M. L. Cohen, S. Hoen and A. Zettl, Phys. Rev. Lett. 59, 915 (1987).
[3]
R.B. Sehwarz, P. J. Yvon and D. Coffey, Vol. 8 of "Studies of High Temperature Superconductors," ed. A. Narlikar, Nova Science Publishers, New York (1991), (A complete list of references can be found in this summary.)
[4]
K.A. Miiller, Z.f. Physik (1990).
[5]
M. K. Crawford, W. E. Fameth, E. M. McCarron, III, R. L. Harlow and A. H. Moudden, Science 250, 1390 (1990).
[6]
J. P. Franck, J. Jung, M. A-K. Mohamed, S. Gygax and G. I. Sproule, Prec. of the LT-19 Conf., Physica B169, (1991).
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When cuprate superconductors are chemically modified to reduce T c from the optimal value, e.g. by changes of the oxygen content or the La/Sr ratio, the isotope effect becomes sizeable. 5,6 It is wortla pointing out in this context, that the reduction of T e is generally accompanied by a broadening of the transition and a reduction of the Meissner signal. Therefore it is somewhat difficult to discuss these "isotope" effects simply in terms of a microscopic model without considering materials-related issues.
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related to the fact that structural transitions occur for Sr compositions not far from the optimal one.) (2) The isotope effect is small in both "hole" a,d "electron"doped cuprates. (3) The isotope effect appears not to be influenced by the presence or absence of "apical oxygen", i.e. oxygen atoms outside the Cu02 layers. (For a discussion of the role of apical oxygen, see e.g. Ref. 4).
24
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26
TEMPERATURE (K)
Fig. 3 Magnetically determined onset temperatures for three sets of samples. The reproducibility of Tc and the very small (or zero) isotope effect are evident on this expanded temperature scale. The oxygen isotope exponent °c~ is <_0.05.
This result leads to three conclusions: (1) The oxygen isotope effect is very small in all cuprate superconductors, i.e. I~l<0.1, provided Tc is optimized within a given compound family. (La z.85Sr o.z5CuO 4 is an exception with a somewhat larger (~ of 0.15-0.2, which, we speculate, might be
OXYGEN ISOTOPE EFFECT IN Ndl.85Ce0.15CuO4 B. BATLOGO, S-W. CHEONG, G. A. THOMAS, $. L. COOPER,* L. W. RUPP, Jr., D. H. RAPKINE and A. S. COOPER AT&T Bell Laboratories, Murray Hill, NJ 07974 *University of Illinois, Champaign-Urbana, Illinois. Starting from high quali~ Ndl.ssCeo.tsCuO 4 polycrystalline samples with a Te of 24K, we have exchanged - 8 5 % of 160 by 180. The magnetically measured T c was found to change by less than 0.1K, equivalent to an oxygen-isotope exponent °ct_<0.05. Thus, the isotope effect is small for all classes of euprate superconductors when T¢ is optimized, irrespective of Cu-O coordination and conduction type. Ever since the original studies on YBa2Cu3OT, 1'2 the oxygen isotope effect in cuprate superconductors remains a much discussed, yet still enigmatic property. (For an up-to-date summary of results and discussions see Ref. 3). Here we report on isotope studies on "eleclron-doped" Nd t.85 Ce o. 15CuO 4. The polyerystalline samples for this study have been prepared using a multi-step grinding and annealing procedure according to the following schedule. The starling powders were mixed, heated at 950°C for 1 hour, reground, heated for 10 hours at 1000°C, reground again and fina!!"ykept at 1080°C for 20 and 50 hours, respectively. The last step involves annealing with the proper amount of Zr in order to adjust the oxygen content. This results in high quality material with a reproducible T e at 24.5K. (Fig. 1)
For the oxygen isotope exchange step, the samples were ground again and packed loosely (930°C, 1 hour) to facilitate diffusion of oxygen. Then the two halves of a sample were placed in the same furnace, but with regular r60 and with 180 enriched atmosphere. The various sets of samples were kept for 70, 100 and 120 hours at 870°C or 900°C. From differential weight changes and changes of the vibrational frequend:.~ (see Fig. 2 ), the concentration of 18O was deduced to be >_85% (e.g. samples $7: 84.3+2.5%, S10: 88.5+2.5%).
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10 15 20 TEMPERATURE (K)
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25
Fig. 1 Meissner signal of loosely sintered smallgrained Nd l.S5 Ce0.!5 CuO4, cooled in a field of 5 0 e . Elsevier Science Publishers B.V.
oL o
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q
i
I
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Fig, 2 Shift of phonon frequencies due to oxygen isotope exchange. The concentratior~ of 180 is ~85% (See text).
R Battogg et at / Oxygen isotope effect in Ndl.ssCe~tlsCu04
1386
The magnetically measured onset of superconductivity is shown on an extended temperature scale in Fig. 3 for three (of the five) sets of samples. The data are separated vertically for clarity, and the broken lines (for 180) allow a close comparison with the 160 results (solid circles). The shift in onset temperature is very small or zero, the largest one being -0.1K. This corresponds to an oxygen-isotope effect parameter °ct of _<0.05.
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REFERENCES
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[1]
B. Batlogg, G. Kourouklis, W. Weber, R. J. Cava, A. Jayaraman, A. E. White, K. T. Short, L. W. Rupp, Jr. and E. A. Rietman, Phys. Rev. Lett. 912 0987)°
[2]
T.A. Faltens, W. K. Ham, S. W. Keller, K. J. Leafy, J. N. Michaels, A. M. Staey, H-C. zur Loye, D. E. Morris, T. W. Barbee, HI, L. C. Bourne, M. L. Cohen, S. Hoen and A. Zettl, Phys. Rev. Lett. 59, 915 (1987).
[3]
R.B. Sehwarz, P. J. Yvon and D. Coffey, Vol. 8 of "Studies of High Temperature Superconductors," ed. A. Narlikar, Nova Science Publishers, New York (1991), (A complete list of references can be found in this summary.)
[4]
K.A. Miiller, Z.f. Physik (1990).
[5]
M. K. Crawford, W. E. Fameth, E. M. McCarron, III, R. L. Harlow and A. H. Moudden, Science 250, 1390 (1990).
[6]
J. P. Franck, J. Jung, M. A-K. Mohamed, S. Gygax and G. I. Sproule, Prec. of the LT-19 Conf., Physica B169, (1991).
/ .,4o,e o ® e ~ e ~ ~,,~4'-S10B
]ff
=!
o°'t Y 23
When cuprate superconductors are chemically modified to reduce T c from the optimal value, e.g. by changes of the oxygen content or the La/Sr ratio, the isotope effect becomes sizeable. 5,6 It is wortla pointing out in this context, that the reduction of T e is generally accompanied by a broadening of the transition and a reduction of the Meissner signal. Therefore it is somewhat difficult to discuss these "isotope" effects simply in terms of a microscopic model without considering materials-related issues.
o4
o° A
S6B
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related to the fact that structural transitions occur for Sr compositions not far from the optimal one.) (2) The isotope effect is small in both "hole" a,d "electron"doped cuprates. (3) The isotope effect appears not to be influenced by the presence or absence of "apical oxygen", i.e. oxygen atoms outside the Cu02 layers. (For a discussion of the role of apical oxygen, see e.g. Ref. 4).
24
25
26
TEMPERATURE (K)
Fig. 3 Magnetically determined onset temperatures for three sets of samples. The reproducibility of Tc and the very small (or zero) isotope effect are evident on this expanded temperature scale. The oxygen isotope exponent °c~ is <_0.05.
This result leads to three conclusions: (1) The oxygen isotope effect is very small in all cuprate superconductors, i.e. I~l<0.1, provided Tc is optimized within a given compound family. (La z.85Sr o.z5CuO 4 is an exception with a somewhat larger (~ of 0.15-0.2, which, we speculate, might be