The electronic structure of heavy-fermion compounds arises from the interaction of nearly localized 4f- or 5f-shell electrons (with atomic magnetic moments) with the free-electron-like itinerant conduction-band electrons. In actinide or rare-earth heavy-fermion materials, this interaction yields itinerant electrons having an effective mass about 100 times (or more) the bare electron mass. Moreover, the itinerant electrons in UPd2Al3 are found to be superconducting well below the magnetic ordering temperature of this compound, whereas magnetism generally suppresses superconductivity in conventional metals. Here we report the detection of a dispersive excitation of the ordered f-electron moments, which shows a strong interaction with the heavy superconducting electrons. This 'magnetic exciton' is a localized excitation which moves through the lattice as a result of exchange forces between the magnetic moments. By combining this observation with previous tunnelling measurements on this material, we argue that these magnetic excitons may produce effective interactions between the itinerant electrons, and so be responsible for superconductivity in a manner analogous to the role played by phonons in conventional superconductors.

CeCoIn5 is an anomalous superconductor which exhibits a high-magnetic-field phase that consists of a modulated magnetic coupling together with persistent superconducting order. Here we use a generic microscopic model to argue that this state is a pattern of coexisting condensates: a d-wave singlet superconducting (SC) state, a staggered π-triplet SC state, and a spin density wave (SDW). Our microscopic picture allows a calculation of the phase diagram, and physical consequences including NMR. We interpret the appearance of the SDW order in the Q phase as being induced by odd-triplet pairing.

We have studied the competition and coexistence of staggered triplet SC with d-wave singlet SC and SDW in the mean-field approximation. Detailed numerical studies demonstrate that particle-hole asymmetry mixes these states and therefore they are simultaneously present. Even more interesting were the results of our study of the influence of a uniform magnetic field. We observe novel transitions that show the characteristics of Fulde-Ferrel phases, yet they concern transitions to different combinations of the above orders. For example, above a given field, in a particle-hole symmetric system we observe a transition from d-wave singlet SC to a state in which d-wave singlet SC coexists with staggered triplet SC and SDW. We believe our results may provide, among others, a direct explanation to recent puzzles about the Fulde Ferrel like states that are apparently observed in CeCoIn5.

Momentum decoupling arises when small momentum transfer processes dominate the electronphonon scattering and implies that anisotropies in superconductivity are driven by the anisotropies of the density of states. Considering an isotropic s-wave interaction in the momentum decoupling regime we give a natural simultaneous explanation to various aspects of ARPES and tunnel experiments on Bi2Sr2CaCu2O8, including the correlation of gap magnitude and visibility of the dip above the gap, the enhancement of anisotropy with temperature, the presence of gap minima away from the Γ − X direction and a gap maximum in the Γ − X direction, the similarity of tunnel and ARPES spectra in the Γ −M direction and the asymmetry in the SIN tunnel spectra where the dip structure is present only at negative sample bias.