Dual model for magnetic excitations and superconductivity in UPd2Al3

The Heavy Fermion state in UPd2Al3 may be approximately described by a dual model where two of the three U-5f electrons are in a localized state split, by the crystalline electric field into two low lying singlets with a splitting energy Delta similar or equal to 6 meV. The third 5f electron has itinerant character and forms the Heavy Electron bands. Inelastic neutron scattering and tunneling experiments suggest that magnetic excitons, the collective propagating crystal field excitations of the localized 5f electrons, mediate superconducting (sc) pairing in UPd2Al3. A theory for this novel mechanism is developed within a nonretarded approach. A model for the magnetic exciton bands is analyzed and compared with experiment. The se pair potential which they mediate is derived and the gap equations are solved. It is shown that this mechanism favors an odd parity state which is nondegenerate due to the combined symmetry breaking by the crystalline electric field and the AF order parameter. A hybrid model including the spin fluctuation contribution to the pairing is also discussed