U2PtC2and Systematics of Heavy Fermions

Our results for superconducting ${\mathrm{U}}_2$Pt${\mathrm{C}}_2$ place it intermediate to heavy-fermion (U${\mathrm{Be}}_{13}$, U${\mathrm{Pt}}_3$) and less anomalous (${\mathrm{U}}_6$Fe) superconductors. Heavy-fermion behavior represented by the electronic specific heat correlates well with spacing between $f$-electron atoms in U, Np, and Ce materials. Two regimes of spacing are evident and arise from direct and indirect interactions between $f$-electron atoms. We suggest that this correlation provides a reasonable criterion for heavy-fermion behavior and a test for theories.