Twenty-five years of heavy-fermion superconductivity
Section snippets
Heavy-Fermion superconductivity: a short history
Until the late 1970s superconductivity and magnetism were considered antagonistic phenomena. Since the local magnetic moments break up the spin-singlet state of the Cooper pairs [1], already very low concentrations of paramagnetic impurities suppress superconductivity in a classical (BCS) superconductor. In view of this antagonistic nature of superconductivity and magnetism, the discovery of the stoichiometric tetragonal compound CeCu2Si2 adopting a bulk superconducting state below (
Spin-density-wave quantum phase transition, valence change and heavy-fermion superconductivity in CeCu2Si2
Most of the HF superconductors, including the U-based compound UBe13 [13], show pronounced deviations from the properties of a LFL in their n-state. Such “non-Fermi-liquid” (NFL) phenomena are ascribed, in these clean stoichiometric compounds, to the existence of a “nearby” continuous AF quantum phase transition (QPT) [14]. In contrast to its classical counterpart at finite temperature, the QPT between an antiferromagnetically ordered state and a magnetically disordered one which comes about at
Unconventional quantum criticality and lack of superconductivity in YbRh2Si2
The compound YbRh2Si2, like CeNi2Ge2 isostructural to CeCu2Si2, undergoes a weak AF phase transition at a Néel temperature as low as 70 mK, cf. inset of Fig. 5 (Ref. [34]). By applying a small critical magnetic field Bc one can suppress the magnetic order and tune the material to a (“field-induced”) QPT [35]. Upon further increasing the field one enters a heavy LFL phase at sufficiently low temperatures. NFL phenomena are observed [36] in the close vicinity of Bc down to the lowest accessible
Outlook
Most remarkably, YbRh2Si2 is not a superconductor, although high-purity single crystals are now available. One may, therefore, speculate that, while a conventional (SDW) QPT favors an unconventional superconducting state to form in Ce-based HF metals, an unconventional QPT is unfavorable for superconductivity. Understanding the intimate relationship between superconductivity and quantum criticality more profoundly will remain a challenge for condensed matter physicists.
Acknowledgments
I am grateful for valuable discussions with J. Custers, P. Gegenwart, C. Geibel, F.M. Grosche, R. Küchler, S. Paschen, G. Sparn, O. Stockert, S. Wirth, H.Q. Yuan, P. Coleman, C. Pépin, Q. Si and G. Zwicknagl.
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