Partial substitution of 18 O in YBa 2 Cu 3 O 7 : Investigations of inhomogeneities and their effect on T c
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Dimitry Krillov and David T. Hodul Varian Research Center, 611 Hansen Way, Palo Alto, California 94303
Ronald H. Fleming Charles Evans and Associates, 301 Chesapeake Drive, Redwood City, California 94063 (Received 7 September 1988; accepted 27 December 1988) The possibility of site-selective substitution of 18O into YBa2Cu307 was explored. Samples containing various quantities of 18O were prepared by processing in 18O2 both at 950 °C and at 400 °C. The samples were characterized by secondary ion mass spectroscopy (SIMS), temperature programmed desorption (TPD) and reduction (TPR), Raman spectroscopy, and magnetization measurements. Measurements of the shifts in the Raman active modes with 18O substitution and of the ratios of 18O to 16O by TPD, TPR, and SIMS show that even for temperatures as low as 400 °C and times as short as 2 h, 18O is not substituted exclusively into the chain site (01) in YBa2Cu307. In addition, there is no consistent variation in the shifts in Tc with the degree of substitution; therefore, the isotope effect for a sample with 100% 18O cannot be predicted by a linear extrapolation of data obtained for samples with partial 18O substitution. The mechanism of oxygen substitution, the difficulties of measuring the true magnitude of the oxygen isotope shift, and the meaning of the small isotope shift are discussed.
I. INTRODUCTION The observation that the superconducting transition temperature Tc varied with isotopic substitution in elemental mercury1'2 gave an important clue that lattice vibrations and electron-phonon interactions are important in superconductivity. This observation led to the phonon-mediated Bardeen-Cooper-Schrieffer (BCS) theory,3 which is widely accepted as the best theoretical description of nearly all known superconductors. According to BCS theory, electrons are coupled into pairs in the superconducting ground state through electron-phonon interactions. One of the most debated issues concerning the recently-discovered high temperature oxide superconductors4"6 is whether a novel pairing mechanism is involved.7 To determine whether phonons play a role, either alone or in concert with another mechanism, oxygen isotope effects have been studied. Since the report by Leary et al.,s six independent groups have confirmed a shift in the transition temperature of YBa2Cu3O7 with oxygen isotope substitution.9"14 This observation of an oxygen isotope effect provides supporting evidence that phonons play a role in the electron pairing mechanism. Unfortunately, none of the samples that have been studied to date has been exchanged completely with 18O. The full magnitude of the isotope effect has been estimated by assuming that Tc scales linearly with the degree of iso1
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J. Mater. Res., Vol. 4, No. 3, May/Jun 1989
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tope substitution. However, there is no fundamental reason to expect such a simple relationship between Tc and isotopic mass, especially in view of the crystallographic complexi
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