Identification of a superoxide in superconductingLa2CuO4+δby x-ray photoelectron spectroscopy

Abstract
X-ray photoelectron spectroscopic and annealing studies of ceramic La2 CuO4+δ prepared by high-temperature and high-pressure oxygen enrichment have identified a superoxide as the predominant oxygen species (δ≊0.13) correlated with a superconducting transition at Tc≊30 K.