Vortex-lattice melting in superconducting fullerene ${\mathrm{Rb}}_3$${\mathrm{C}}_{60}$

The temperature and field dependences of the dc magnetization have been characterized on a sample of ${\mathrm{Rb}}_3$${\mathrm{C}}_{60}$ fullerene with ${\mathit{T}}_{\mathit{c}}$=30.5 K. The sample exhibits a rather large critical current density of about 2×${10}^6$ A/${\mathrm{cm}}^2$ at T=5 K and H=0 T. An irreversibility line ${\mathit{H}}_{\mathrm{irr}}$(T) is defined by merging points of two sets of zero-field-cooling and field-cooling magnetization curves. The temperature dependence of the irreversibility field is in fairly good agreement with a power-law relation: ${\mathit{H}}_{\mathrm{irr}}$(T)=${\mathit{H}}_{\mathrm{irr}}$(0)[1 -T(H)/${\mathit{T}}_0$${]}^{\gamma }$. The best fit of experimental data to this equation provides the following values: γ=2.04±0.16, ${\mathit{H}}_{\mathrm{irr}}$(0)=470±52 kOe, and ${\mathit{T}}_0$=30.4±0.2 K. According to the nonlocal elasticity theory based on the Lindemann criterion, γ=2 strongly suggests the occurrence of thermally activated vortex-lattice melting in our ${\mathrm{Rb}}_3$${\mathrm{C}}_{60}$ sample. The Lindemann number of c≤0.074 is comparable to those of conventional type-II superconductors, but is much smaller than those for high-${\mathit{T}}_{\mathit{c}}$ oxides. Our data provide evidence of vortex-lattice melting in the superconducting fullerenes.