Common energy scale for magnetism and superconductivity in cuprates

Many compounds based on ${\mathrm{CuO}}_2$ planes (cuprates) superconduct below a critical temperature $T_c.$ Some of them show a second phase where a spontaneous static magnetic field appears below a critical temperature $T_g,$ which is lower than $T_c.$ By comparing $T_c$ and $T_g$ in numerous superconducting families, each with its own maximum $T_c,$ we find that the same energy scale determines both critical temperatures. This clearly indicates that the origin of superconductivity in the cuprates is magnetic.