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Unconventional Pairing in Heavy Fermion Metals

The Fermi-liquid theory of superconductivity is applicable to a broad range of systems that are candidates for unconventional pairing. Fundamental differences between unconventional and conventional anisotropic superconductors are illustrated by the unique effects that impurities have on the low-temperature transport properties of unconventional superconductors. For special classes of unconventional superconductors the low-temperature transport coefficients are {\it universal}, i.e. independent of the impurity concentration and scattering phase shift. The existence of a universal limit depends on the symmetry of the order parameter and is achieved at low temperatures $k_B T \ll γ\ll Δ_0$, where $γ$ is the bandwidth of the impurity induced Andreev bound states. In the case of UPt$_3$ thermal conductivity measurements favor an $E_{1g}$ or $E_{2u}$ ground state. Measurements at ultra-low temperatures should distinguish different pairing states.