Oxygen isotope effect in the superconducting BiSrCaCuO system

Physica C 156 ( 1988 ) 481-484 North-Holland, Amsterdam

OXYGEN I S O T O P E EFFECT I N T H E S U P E R C O N D U C T I N G B i - S r - C a - C u - O

SYSTEM

H. KATAYAMA-YOSHIDA, T. HIROOKA, A. OYAMADA, Y. OKABE, T. TAKAHASHI, T. SASAKI, A. OCHIAI and T. SUZUKI Department of Physics, Tohoku University, Sendai 980, Japan

A.J. MASCARENHAS, J.I. PANKOVE, T.F. CISZEK and S.K. DEB Solar Energy Research Institute, Golden, CO 80401, USA

R.B. GOLDFARB and Yongkang LI National Bureau of Standards, Boulder, CO 80303, USA

Received 28 July 1988

An oxygen isotope effect is observed in mixecl-phaseBi-Sr-Ca-Cu-O superconductors when ~80 is substituted for ~60. The superconducting transition temperature To, measured by electrical resistivity and magnetic susceptibility, is lowered by about 0.32 K for the higher-To ( 110 K) phase and by about 0.34 K for the lower-To (75 K) phase. These results suggest a measurable contribution to the superconductivityfrom phonons.

Recently, bulk superconductivity at 110 K has been observed in the B i - S r - C a - C u - O system [ 1 ], and more recently at 125 K in the T 1 - B a - C a - C u - O system [2,3 ]. The mechanism giving rise to such high transition temperatures (To) has not been established [4-9]. Considerable theoretical and experimental efforts have been made to elucidate the mechanism of the high-To superconductivity. The isotope effect on Tc is an essential test to study the role of a phonon-related electron-pairing mechanism in the superconductor. A nonzero isotope shift in Tc was observed in the L a - S r - C u - O system [ 10,11 ]. A zero isotope shift was initially reported in the Y - B a C u - O system [ 12,13 ], but later precise measurements demonstrated a measurable isotope effect, much smaller than predicted from phonon-mediated coupling and BCS theory [ 14-17 ]. To clarify the high-To mechanism, we report the oxygen isotope effect for the B i - S r - C a - C u - O system. We calcined samples of composition Bi2Sr2Ca2Cu3Ox using a solid-phase reaction of powders of Bi203, SrCO3, CaCO3 and CuO, all at least 99.999% pure, at 860°C in air for 24 h. Each pellet, sintered at 870°C in air for 12 h, was broken into

two pieces. They were placed in a specially designed furnace with divided furnace tubes to conduct equivalent thermal and gas-exchange processes for 160 and 180 enrichment [ 15 ]. The gas-exchange process consisted of ( 1 ) removal of oxygen from the samples by holding them under vacuum for 5 h, (2) exposing one sample to 1602 and the other to ~802 at 850°C for 30 h, and (3) cooling to room temperature at the rate of 100°C/h. The 1802 gas was 98.11 at% 180, 0.57 at% 170, and 1.32 at% 160. Electrical resistivity and magnetic susceptibility measurements showed that the samples thus obtained had two phases with critical temperatures of about 75 and 110 K. We simultaneously made two pairs of samples and confirmed the isotope effect on each pair with independent measurements of resistivity and susceptibility at Tohoku University (pair ~1) and at NBS-Boulder (pair #2). The resistivity and susceptibility curves were compared in terms of normalized parameters. Figure 1 shows the resistivity of the 1SO-enriched sample from pair ~1 normalized to its value at 273 K. The sample resistance was measured with separate current and voltage contacts, with a current of

0921-4534/88/$03.50 © Elsevier Science Publishers B.V. ( North-Holland Physics Publishing Division )

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10 mA. The observed isotope shift (ATe) with substitution is 0.34_+0.03 K for the 110 K phase as shown in fig. 2. The shift was determined by extrapolating the linear portions of the curves. Because the linear fits are parallel, ATe is the same at the inflection of the curves. Separate resistivity measurements on samples from pair ~2, with an AC current of 4 I.tA at 177 Hz, gave a ATe of 0.30+0.05 K for the l l 0 K transition. 0.6

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The shift for the 75 K phase is less certain. An extrapolation procedure similar to that used above gives a ATe of about 0.42_0.05 K for the 75 K phase, shown in fig. 3 for pair ~l. The apparent isotope shift for this phase, however, could depend somewhat on the value of the normalization temperature, 90 K in this case. Separate resistivity measurement on samples from pair g2, with an AC current of 100 ~tA, gave a ATe of 0.40_+0.05 K for the 75 K phase. A measurement of AC volume susceptibility (Z=Z' +ix" ) of the ~80-enriched sample from pair ~2 in a measuring field of 80 A / m ( 1 . 0 0 e ) rms at 1000 Hz is shown in fig. 4. Temperature was increased at the rate of 15 K/h. No correction was made for the small demagnetization factor of the sample. The real part (Z') measures the bulk shielding of the sample. The imaginary part (Z") measures the losses. The transition in Z' for the two superconducting phases are seen at about 1 l0 and 75 K. The Z' transition centered at 60 K, accompanied by a broad peak in Z", corresponds to "weak-link" coupling [ 18 ]. In fig. 5, the Z' data for pair g2 are normalized to the value of - Z ' at 80 K. With reference to fig. 4, 80 K is well below Tc for the 110 K phase, but above Tc for the 75 K phase. The value of ATe is 0.30_+0.02 K measured at 108 K, near the inflections of the curves. Separate susceptibility measurements on pair ~1 gave a ATe of 0.34+0.04 K for the 110 K phase. t

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In fig. 6, the X' curves were first shifted vertically to intercept zero at 80 K a n d then n o r m a l i z e d to the values of - X ' at 9 K. This allowed the 75 K transitions to be compared. For these transitions, ATe is 0.23 _+0.05 K measured at 74 K, near the inflections of the curves. Several variations o f shifts a n d normalizations were tried. Shifting to intercept zero at 90 K, rather than 80 K, gave identical ATe. Changing the normalization temperature from 9 K to 40 K gave ATe equal to about 0.26 K. The key to valid normalization is to choose a shift p o i n t a n d a scaling

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484

H. Katayama- Yoshida et a[. /Oxygen isotope effect in the Bi-Sr-Ca-Cu-O system

ATe does not change appreciably with To. This indicates that an electron-phonon interaction could contribute to the electron pairing mechanism for the superconductivity in these compounds, but some additional mechanism, like spin fluctuation [5,6] or charge fluctuation [ 9 ], may raise Tc from that of BiPb-Ba-O ( T c = l l K) to that of B i - S r - C a - C u - O (T~= 110 K). In order to confirm the substitution of the oxygen isotope, we performed Raman spectroscopy with a resolution of 1.0 cm-1 on pair ~1. On comparing the 160 and 180 samples, we observed a shift from 469 c m - 1 to 449 c m - 1 of the best-resolved phonon frequency. Such an isotope shift is consistent with almost perfect substitution. A detailed analysis of the phonon frequency assignments using single-crystal and oxygen isotope-enriched samples will be published elsewhere [ 19 ]. In summary, we have observed a small but nonzero isotope shift in T~ when substituting 180 for 160 in superconducting Bi-Sr-Ca-Cu-O. This suggests that a phonon mechanism contributes to the electron pairing in this high-To system. Because AT~ does not change appreciably with Tc over all high-To compounds, strong electron-phonon coupling alone is insufficient to explain the high value of T~. Some additional mechanism must be involved.

Acknowledgements We thank Professors Y. Endoh, A. Kotani and Dr. S. Sugai for valuable discussions. We also thank R.L. Spomer and D.L. Rule for their assistance in processing some of the data. This work was partially supported by a Grant-in-Aid for Scientific Research on Priority Area from the Ministry of Education, Science, and Culture of Japan, and by the U.S. Department of Energy under Contract No. DE-AC0283CH10093.

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