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, e.g. heavy fermion, organic and cuprate superconductors. Ginzburg-Landau theory provides a link between the thermodynamic properties of these superconductors and Fermi-liquid theory. The multiple superconducting phases of UPt₃ illustrate the role that is played by the Ginzburg-Landau theory in interpreting these novel superconductors. 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 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_BT « γ « Δ₀, where γ is the bandwidth of the impurity induced Andreev bound states. In the case of UPt₃ thermal conductivity measurements favor an E_1g or E_2u ground state. Measurements at ultra-low temperatures should distinguish different pairing states.