Pressure and field dependence of superconductivity in ${\mathrm{Rb}}_3$${\mathrm{C}}_{60}$

Direct measurements of the pressure dependence of the superconducting transition temperature ${\mathit{T}}_{\mathit{c}}$ for single-phase ${\mathrm{Rb}}_3$${\mathrm{C}}_{60}$ provide for the first time substantial evidence that ${\mathit{T}}_{\mathit{c}}$ in ${\mathit{A}}_3$${\mathrm{C}}_{60}$ compounds may be a universal function of the lattice constant (inter-${\mathrm{C}}_{60}$ spacing). Determination of the lower (${\mathit{H}}_{\mathit{c}1}$) and upper (${\mathit{H}}_{\mathit{c}2}$) critical fields, and consequently the superconducting penetration depth and coherence length, is also reported. Compared to ${\mathrm{K}}_3$${\mathrm{C}}_{60}$, the measured quantities are consistent with a 15% larger electronic-state density in ${\mathrm{Rb}}_3$${\mathrm{C}}_{60}$ arising from a greater inter-${\mathrm{C}}_{60}$ spacing in this compound.