No abstract

Refined measurements are reported on the pressure dependence of the superconducting transition temperature in the heavy-fermion (HF) superconductor CeCu 2 Si 2 . Two characteristic pressures, p 1 ≈ 3 GPa and p 2 ≈ 7.6 GPa, delimit three ranges of the superconducting state. The present study confirms the unique behaviour of CeCu 2 Si 2 in the series of HF superconductors. This particular behaviour is interpreted as resulting from two contributions: a smooth one due to the pressure-increased Kondo temperature and sharper additional features reflecting topological changes in the renormalized heavy bands.

International audienceno abstrac

We report measurements under pressure of the upper critical field of the heavy fermion superconductor UBe 13 . An interpretation in the framework of a simple strong coupling model is achieved consistently with only one arbitrary parameter: the strong coupling constant l. We find that UBe 13 is in an extreme strong coupling regime and that the variation of l with pressure explains the pressure dependence of the thermodynamic properties of both the normal and the superconducting phases. It reveals a strong interplay between the mass renormalization and the pairing mechanisms, yielding the first quantitative indication of a nonphonon mediated pairing in a superconductor.[S0031-9007(98)08084-3] PACS numbers: 74.20. Mn, 74.60.Ec, 74.62.Fj, 74.70.Tx A characteristic feature of heavy fermion (HF) intermetallic compounds is the occurrence at low temperature (ഠ10 K) of a very large renormalization of the mass of the charge carriers: up to several hundred times the mass of a free electron. There is no complete understanding of this huge mass renormalization, but the consensus is that it arises from strong electronic correlations, which also produce different kinds of magnetic excitations. It has also long been suggested that the pairing mechanism responsible for superconductivity in HF may differ from the usual electron-phonon interaction, and is believed to involve the magnetic properties of the normal phase. But as well as for high-T c cuprates, real quantitative results on this point are still lacking. We propose here a new approach to this challenging question, taking advantage of the extreme strong coupling regime met in the HF superconductor UBe 13 : our measurements and analysis of the upper critical field H c2 of UBe 13 under pressure reveal a direct link between the mechanisms of mass renormalization and superconductivity in this system, yielding the first quantitative indication of a nonphonon mediated mechanism in a superconductor.The normal phase of UBe 13 already presents striking features. The quasiparticles have a record effective mass (renormalization by a factor of 1000 has been suggested [1,2]), and coherence in the lattice occurs only at very low temperature: the resistivity presents a maximum at 2.5 K [1]. Therefore, and as opposed to all other HF superconductors, UBe 13 has the very unusual feature to be in an ill defined Fermi liquid regime when superconductivity appears at T c ഠ 1 K [1,3]. With regard to the superconducting state, it is clearly in a strong coupling regime, as indicated by the relative jump of the specific heat at T c : of the order of 3, much larger than the BCS weak coupling value of 1.43 [4]. Quite curiously, the specific heat has long been the only superconducting property analyzed in a strong coupling scheme [4,5]. It is only very recently that strong coupling effects were quantitatively discussed on H c2 [6], providing a new but straightforward interpretation of its peculiar behavior.The temperature dependence of H c2 in UBe 13 at zero pressure (see the curve P 0 kbar of Fig....

No abstract