ELSEVIER Physica C 270(1996)317-326 PHYSICA The isotope effect coefficient dependence on nonstoichiometry for single CuO layer superconductors Cristina Buzea *,a, Tsutomu Yamashita a, Kensuke Nakajima a, Calin Gh. Buzea b, Maricel Agop c a Research Institute of Electrical Communication, Tohoku University, 2-1.1 Katahira, Aoba-ku, Sendai 980-77, Japan b Superconductioity Research Laboratory, Institute of Technical Physics, B-dul Mangeron 47, lasi, Romania c Department of Physics, Technical "'Gh. Asachi'" University, lasi, Romania Received 18 June 1996; revised manuscript received 5 August 1996 Abstract Considering that the variation of the critical temperature can be explained if the impurity potential acts only by the non-spin-flip part, in the presence of either a pure d-wave or anisotropic s-wave gap symmetry, and inserting the pair-breaking time dependence on temperature, we obtain a quantitative expression for the isotope effect coefficient t~ as a function of measured critical temperature T c maximal critical temperature T and degree of anisotropy of the energy gap. Our result predicts the achievement of a large range of values in ot for different ratios TIT c. Introducing the dependence of the critical temperature and pair-breaking time on dopant content for a single Cu-O layer superconductors, we can describe the variation of the isotope effect coefficient with nonstoichiometry for these materials. The result for the case of anisotropic s-wave gap symmetry is in good agreement with the experimental data. Keywords: Isotope effect; Single Cu-O layer superconductors; Pair-breaking time; Critical temperature; Dopant content I. Introduction The isotope effect coefficient was predicted by FrShlich [1], and experimentally observed by Maxwell [2] and Reynolds et al. [3]. The BCS theory [4] computes a value of 0.5 for the isotope effect coefficient a = - 8 In T~/~ In M for all weak-cou- pled superconductors. But even for elemental super- conductors a variety of experimental results have been found [5]: * Corresponding author. E-mail: cristi@riec.tohoku.ac.jp. • only the s-p metals exhibit ct--0.5 (i.e. the metals whose valence band is composed principally of s and p electrons. • for many transition-metal superconductors, a- values are in the 0.0 to 0.3 range. In multielement systems, such as high tempera- ture superconductors, the effect of isotope mass change AM i on the transition temperature ATc can be written as ATc/T~=Y'.iaIAMi/M i, where the summation index i runs over all the lattice sites in the structure. For copper-based superconductors the isotope effect coefficient values lie much below the BCS value or varies with nonstoichiometry [5]. Most 0921-4534/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved Pll S0921-4534(96)00509-6