Volution of upper critical field in fullerene superconductors near the Mott transition

We here report systematic investigations of the upper critical field $H_{c2}$ of alkali-metal-doped fullerene superconductors $A_3$C$_{60}$ ($A$: Alkali metal) including Rb$_x$Cs$_{3-x}$C$_{60}$ ($0 < x < 1$), which is a new series of expanded fullerene superconductors. Using Rb$_x$Cs$_{3-x}$C$_{60}$, we can access the novel regime from the $T_c$ maximum to the antiferomagnetic phase even at ambient pressure. We have successfully synthesized high-purity Rb$_x$Cs$_{3-x}$C$_{60}$ compounds with several Rb compositions of $x$. Determination of $H_{c2}$ has been demonstrated by rf-penetration depth measurements under pulsed magnetic field up to 62~T. With expanding lattice volume with decreasing $x$, the system approaches to the Mott insulator from the superconducting phase. We found that $H_{c2}$ continuously increases with decreasing $x$ and it reaches as large as 80~T in the lowest $x=0.35$, which is almost the verge of the Mott transition. Combining with specific heat measurements, underlying phenomena in the superconductor-insulator transition in the fullerene compounds will be discussed.