Unusual hole dependence of Tc in HgBa2CuO4+δ

The anion doping effect on the superconducting transition temperature (Tc) of HgBa2CuO4+δ has been determined with a wide Tc variation. We found a parabolic variation of Tc with δ over the entire range of superconductivity. Remarkably, the dependence of Tc on p (the number of holes per CuO2) has richer features: for δ≤0.28, Tc is parabolic in p, but with an unusual prefactor and p=0.72δ, giving significantly less than two holes per oxygen; and for δ>0.28, a drastic departure from the parabolic Tc(p) occurs. Implications for the occurrence of superconductivity in this material are discussed.