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Vacancies as dopants in high-temperature superconductors

The shift in the Fermi energy EF and the modification of the density of states rho (E) have been calculated for oxygen vacancies in La1.85Sr0.15CuO4-y and YBa2Cu3O7-y and for lanthanum vacancies in La2-xCuO4. Oxygen vacancies are found to act as donors in every case, and La vacancies as acceptors. These conclusions are in accord with the well known observations that oxygen vacancies decrease Tc, and with the suggestion that the superconductivity of nominally pure La2CuO4 could be due to La vacancies.