Effect of pressure and of oxygen content on superconductivity in the cuprate HgBa2CuO4+δ. A quantitative analysis based on indirect-exchange pairing

A quantitative analysis is presented of experimental pressure and oxygen-doping effects on the critical temperature of HgBa₂CuO_4+δ over a wide range of δ, covering both under- and overdoped compositions, on the basis of indirect-exchange pairing between conduction electrons via oxygen anions. The results clearly show that for δ 0.28, the oxygen doping process is not the same as cation doping in e.g. La_2−xSr_xCuO₄: part of the doped oxygen fulfills a double role as hole dopant and as Cooper pair mediator. Furthermore, adding or extracting oxygen under experimental conditions is accompanied by a redistribution of oxygen anions in the unit cell, to or from the neighborhood of the CuO₂ layer. This effect explains the surprisingly wide range of superconducting compositions (0.03 δ 0.40) of the system. The observed, anomalous, upward shift in thermoelectric power for δ />/ 0.28 is attributed to different sites of doped oxygen beyond that limit, affecting only hole conductivity.