Inherent Spin-Density-Wave Instability in Heavy-Fermion Superconductivity

Both anisotropic superconducting (SC) states of heavy electrons with odd-parity $p$ pairing and with even-parity $d$ pairing are shown to have an inherent instability towards the spin-density wave which generally coexists with the SC state. A model is presented which contains an on-site repulsion characteristic of heavy-fermion materials, in addition to an intersite attraction. The second transition below the SC transition observed in $\left({\mathrm{U}}_{1-x}{\mathrm{Th}}_x\right){\mathrm{Be}}_{13}$ is interpreted in terms of the spin-density-wave instability.