Regensburg 2004 – wissenschaftliches Programm
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PV: Plenarvorträge
PV VIII
PV VIII: Plenarvortrag
Mittwoch, 10. März 2004, 14:30–15:15, H1
Heavy-Fermion Superconductivity: 25 years after — •F. Steglich — Max-Planck-Institut für Chemische Physik fester Stoffe, 01187 Dresden — Träger der Stern-Gerlach-Medaille 2004
25 years after the discovery of heavy-fermion superconductivity in CeCu2Si2 it has been widely accepted by now that this state may develop either out of a Landau-Fermi-liquid (LFL) state or out of a so-called non-Fermi-liquid (NFL) state. The hexagonal compound UPd2Al3 belonging to the former class is the first and yet only superconductor for which quasiparticle tunneling and inelastic neutron scattering strongly point towards a non-phononic, i.e., magnetic-exciton mediated, Cooper-pair state.
For the tetragonal NFL superconductor CeCu2Si2 pronounced deviations from LFL behavior are observed in its low-temperature normal-state properties. These can be related to the vicinity of an antiferromagnetic (AF) quantum critical point (QCP) at which a low-moment incommensurate spin-density-wave (SDW) phase disappears in a continuous way (TN → 0). For this compound, two distinct superconducting phases can be well separated on the pressure (p) axis by deliberately reducing the quasiparticle mean free path, i.e., by weak doping with Ge: The low-p phase is centered around the AF QCP, while the high-p phase occurs in the neighborhood of a weak first-order valence transition [Ce3+ → Ce(3+δ)+]. This observation highlights the (co-) existence of two different types of pairing mechanisms mediated by soft fluctuations of the spin density (at low p) and of the charge density (at high p).
While the NFL effects in CeCu2Si2 are consistent with an itinerant (SDW) scenario for the QCP, in YbRh2Si2 a new variant of quantum criticality has been found which seems to be phenomenologically related to that in the prototypical material CeCu5.9Au0.1. Here, the most critical spin fluctuations are apparently of a local rather than an itinerant nature. YbRh2Si2, isostructural to CeCu2Si2, is not a superconductor suggesting that a local-moment QCP might be unfavorable for heavy-fermion superconductivity.