CORRELATION BETWEEN HOLE DENSITY AND OXYGEN EXCESS IN THE BI2SR2CACU2O8+DELTA SUPERCONDUCTOR

Charge-carrier density in Bi-2212 oxide superconductor samples with we ll-defined cation molecularity has been measured by means of Cu-L-III x-ray absorption spectroscopy, as a function of the oxygen content, wh ich was fixed to known values by annealing at predetermined T and P(O- 2), and subsequent quenching. The main results of this work can be sum marized as follows: (i) at low oxygen content the hole density is grea ter than that one determined by the oxygen excess only, (ii) at interm ediary oxygen content values the hole density is almost entirely deter mined by the oxygen excess only; each extra oxygen atom contributes tw o holes to the overall charge-carrier density; (iii) for the highest v alues of the oxygen content, each extra oxygen atom contributes only o ne hole to the overall charge-carrier density. From these results a ba nd model is drawn in which the extra oxygen levels Lie in energy near the Fermi level.