Correlation of $T_c$ with hole concentration in ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x\mathrm{Cu}{\mathrm{O}}_{4-\delta }$ superconductors

The electron deficiency (hole concentration) in the ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x\mathrm{Cu}{\mathrm{O}}_{4-\delta }$ system is determined by a chemical method. Our results show a direct correlation between $T_c$ and hole concentration. The hole concentration equals the Sr concentration to about $x=0.15\mathrm{}$. For $x>\mathrm{}0.15\mathrm{}$, the hole concentration decreases and oxygen vacancies are formed. These results strongly support an all-electronic mechanism for superconductivity in this system.