Elsevier

Physica B: Condensed Matter

Volumes 359–361, 30 April 2005, Pages 326-332
Physica B: Condensed Matter

Twenty-five years of heavy-fermion superconductivity

https://doi.org/10.1016/j.physb.2005.01.054Get rights and content

Abstract

We discuss first the physical properties of the prototypical heavy-fermion superconductor CeCu2Si2 and, second, the relationship of its superconductivity with both a spin-density-wave-type quantum phase transition and a weak valence transition of Ce that occurs at high pressure. We, then, briefly address the isostructural heavy-fermion compound YbRh2Si2, displaying a local type of quantum critical behavior which appears to be detrimental to 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 Tc0.6K (

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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