Journal of Physics and Chemistry of Solids
CeCu2Si2 AND UBe13: NEW QUESTIONS—OLD ANSWERS?
Introduction
The tetragonal compound CeCu2Si2 was the first of a—by now—long list of `novel' (i.e. non-BCS) superconductors [1]. It belongs to the `heavy-fermion' (HF) metals, in which quasiparticles composed of both local, f degrees of freedom and itinerant conduction–electron degrees of freedom form well below the characteristic `Kondo temperature', TK (≈15 K). The f degrees of freedom determine the huge effective quasiparticle mass, m* (≈300 mel) as inferred from a giant Sommerfeld coefficient γ≈0.7 J K−2 mol−1 [1]. Both the discontinuity in the specific heat, ΔC [1], and the slope of the upper-critical-field curve, B′C2=−(dBC2/dT) [2], at TC≈0.65 K were found to scale with the large γ value and ascribed to massive Cooper pairs. Very similar observations were subsequently made [3]for the cubic HF metal UBe13 (TC≈0.9 K). Compared with their later-discovered counterparts, UPt3 [4], URu2Si2 (W. Schlabitz, J. Baumann, B. Pollit, U. Rauchschwalbe, H. M. Mayer, U. Alheim and C. D. Bredl, unpublished data,[5]), UNi2Al3 [6]and UPd2Al3 [7], CeCu2Si2 and UBe13 are phenomenologically more complex and less understood. On the one hand, in the first four compounds, HF superconductivity forms at TC (0.5–2 K) out of a heavy Landau Fermi-liquid (FL) state, which coexists with antiferromagnetic (afm) order at T<TN. Below TC (≪TN), HF superconductivity and afm order coexist on a microscopic scale. On the other hand, for CeCu2Si2 an as yet unidentified (presumably SDW) `phase A' is almost degenerate with HF superconductivity, whereas for UBe13 no clear-cut evidence for any kind of magnetic order exists so far. Further on, the low-temperature normal (n) states of these two `heaviest' superconductors are characterized by strong violations of Landau FL behavior. It is the main purpose of this paper to explore the `non-Fermi-liquid' (NFL) properties of n-state CeCu2Si2 and UBe13. In addition, we report the discovery of new anomalies in the superconducting (sc) state of pure UBe13 and its thoriated variant, for which a very complex sc phase diagram has been established 8, 9.
Section snippets
CeCu2Si2: breakdown of the heavy-fermion picture in the vicinity of the A-phase transition
A thorough investigation of CeCu2Si2 single crystals revealed the existence of distinctly different groundstate properties, depending on the composition of the starting melt and/or the subsequent heat treatment 10, 11. However, any differences in the lattice parameters of these different variants are much too small to be resolved by X-ray diffractometry. A subsequent systematic study of polycrystalline samples enabled Geibel et al. [12]to map the different physical groundstate behaviors (`S',
UBe13: non-Fermi-liquid normal state and new anomalies below Tc
Two variants of UBe13 with markedly different sc and n-state properties have recently been identified [20]: `H-type' (TC≈0.9 K) and `L-type' (TC≈0.75 K) UBe13. Since the former variant is mostly found among polycrystals while all `L-type' samples are single crystals, we believe that their phenomenological differences must be related to differences in the actual composition originating in different preparation procedures [20].
In the following, we discuss some low-T properties of a high-quality UBe
Conclusions
HF superconductivity in CeCu2Si2 as well as in UBe13 forms out of a NFL normal state. For CeCu2Si2, anomalous T-dependences at intermediate temperatures (T>0.6 K) and sufficiently low magnetic fields (B<6 T) are strongly suggesting an afm QCP on the brink of phase A (TA→0). When approaching this QCP by further cooling, however, Δρ(T) and γ(T) behave quite disparately. This highlights a decoupling of the itinerant and local (4f) parts of the heavy-fermion quasiparticles. But even when TA is
Acknowledgements
We are grateful to W. Assmus for supplying the CeCu2Si2 single crystal. We acknowledge a fruitful correspondence with P. Coleman. Work in Darmstadt was supported by the SFB 252, work in Gainesville by the US Department of Energy, Contract no. De-FG05-86ER45258.
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