Pressure Dependence of the Upper Critical Field of the Heavy Fermion SuperconductorUBe13

Abstract

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....

Superconducting phases off-electron compounds

“…In the zero temperature limit H c2 (T → 0) = 14 T. H c2 exhibits strong Pauli limiting and as an additional feature a change of curvature at T /T s ∼ 0.5 K. The unusual temperature dependence of H c2 has been attributed to a combination of very strong coupling superconductivity and the tendency to form a FFLO state (see also section V.B.1). While the coupling constant λ = 15 in these calculations is suspiciously large and exceeds coupling constants in comparable systems by an order of magnitude, this scenario finds further support in the pressure dependence of λ, which tracks the mass enhancement inferred from dH c2 /dT | Ts and the specific heat (Glémot et al, 1999).…”

“…It is now broadly accepted that these features do not yield microscopic characteristics related to a FFLO state, but instead may be due to subtle forms of defect related pinning. Further candidates for an FFLO state are URu 2 Si 2 and UBe 13 , which display additional contributions in H c2 Buzdin and Brison, 1996a,b;Glémot et al, 1999). For URu 2 Si 2 this contribution is seen for the c-axis and rather small.…”

Scaling properties of the magnetic-field-induced specific heat of superconductingUBe13

“…It is well established 4,24 and has recently been confirmed 25 that UBe 13 is a strong coupling superconductor, which implies that the energy-gap amplitude is substantially larger than the BCS weak-coupling value of ⌬Ϸ1.76k B T c . The Fermi-energy of UBe 13 , given by E F /k B , is of the order of 10 K. 4 Using these values and the predictions for s-wave superconductors, we find the lowest level of the low-energy excitations in the vortex core to be at significantly higher energies than the typical thermal energies k B T of this experiment.…”

Temperature-Dependent Ginzburg–Landau Parameter

“…This case demands further investigation because it seems that this material having extremely high upper critical field 12 and T 3 behavior of specific heat at low temperatures 13 belongs to triplet superconductors with point nodes in the quasiparticle spectrum.…”

“…It is temperature independent and given by the numerator of Eq. (12). While in a superconductor with strong paramagnetic effect that is at large enough Maki parameters the value of |dH c2 /dT | rapidly decreases with decreasing temperature, which leads in its turn to the fast decrease of the Ginzburg-Landau parameter (4).…”