Determination ofC13NMR isotropic Knight shift and deviation from BCS relation inA3C60superconductors

We determined the isotropic ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$ NMR hyperfine coupling constant as $\frac{a_{\mathrm{iso}}}{2\pi }=0.69\mathrm{}\pm 0.06$ MHz for the ${\mathrm{C}}_{60}^{3-}$ ion near room temperature (RT). On this basis, the spin susceptibility at RT, ${\chi }_s\left(\mathrm{RT}\right)$, was estimated in various alkali-metal doped ${\mathrm{C}}_{60}$ compounds $A_3{\mathrm{C}}_{60}$, which is approximately proportional to the lattice constant $a_0$; we found $\frac{d{\chi }_s}{d{a}_0}=(0.61\mathrm{}\pm 0.06)\times {10}^{-3\mathrm{}}$ (emu/mole ${\mathrm{C}}_{60}$)/Å. The correlation between the superconducting transition temperature $T_c$ and ${\chi }_s\left(\mathrm{RT}\right)$ was determined. It was found that $T_c$ of ${\mathrm{Na}}_2$Rb${\mathrm{C}}_{60}$, ${\mathrm{Na}}_2$K${\mathrm{C}}_{60}$, ${\mathrm{Li}}_2$Cs${\mathrm{C}}_{60}$, and ammoniated $A_3{\mathrm{C}}_{60}$ is significantly lower than that expected from the BCS relation based on ${\chi }_s\left(\mathrm{RT}\right)$.