High-resolution thermal expansion of superconducting fulleridesA3C60(A=K,Rb)

The thermal expansion of air-sensitive $A_3{\mathrm{C}}_{60}$ compounds ($A=\mathrm{K},\mathrm{R}\mathrm{b}$) was investigated using a high-resolution capacitance dilatometer operating in an inert atmosphere. The discontinuity in the linear thermal expansivity at the superconducting transition, $\Delta \alpha$, of each compound is presented, from which we determine the corresponding discontinuity in the specific heat $\frac{\Delta c_p}{T_c}$, via the Ehrenfest relation (75±14 mJ/mol ${\mathrm{K}}^2$ for ${\mathrm{Rb}}_3$ ${\mathrm{C}}_{60}$ and 64±14 mJ/mol ${\mathrm{K}}^2$ for ${\mathrm{K}}_3$ ${\mathrm{C}}_{60}$). On the basis of the calculated $\frac{\Delta c_p}{T_c}$ values, the density of states $N\left(E_F\right)$ and the electron-phonon coupling constant $\lambda$ are derived using the McMillan strong-coupling formalism and recent theoretical results. We find $N\left(E_F\right)=6.6\mathrm{}\pm 1.1$ states/eV spin ${\mathrm{C}}_{60}$ and 7.4±1.0 states/eV spin ${\mathrm{C}}_{60}$ for ${\mathrm{K}}_3$ ${\mathrm{C}}_{60}$ and ${\mathrm{Rb}}_3$ ${\mathrm{C}}_{60}$, respectively. The associated $\lambda$ is estimated to be ∼0.5 for both ${\mathrm{K}}_3$ ${\mathrm{C}}_{60}$ and ${\mathrm{Rb}}_3$ ${\mathrm{C}}_{60}$. The ratio $\frac{N{\left(E_F\right)}_{{\mathrm{Rb}}_3{\mathrm{C}}_{60}}}{N{\left(E_F\right)}_{{\mathrm{K}}_3{\mathrm{C}}_{60}}}$ is consistent with a superconducting mechanism involving high-frequency...