Field Re-entrant Superconductivity Induced by the Enhancement of Effective Mass in URhGe

High quality single crystals of a ferromagnetic superconductor URhGe were successfully grown. The electrical resistivity was measured for the field along b-axis in the orthorhombic crystal structure in order to study precisely its field re-entrant superconductivity which occurs in the vicinity of the field $H_{\rm R}$, where the easy magnetization switches from $c$ to b-axis. The field re-entrant superconductivity of URhGe is analyzed with special focus on its dependence with the value of residual resistivity $\rho_0$ and the coefficient $A$ of $T^2$ term of the resistivity. The experimental results are well explained by a crude model related with the field dependence of the effective mass $m^\ast$, where the corresponding critical temperature $T_{\mathrm{sc}}(m^\ast)$ and the upper critical field $H_{\rm c2}$ are strongly enhanced. Discussion is made on the interplay between magnetic and superconducting phase diagram as well as the link between $T_{\mathrm{sc}}$ and $A$ in other heavy fermion superconductors.