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TT: Fachverband Tiefe Temperaturen

TT 16: Symposium “50 Years BCS Theory”

TT 16.2: Vortrag

Dienstag, 27. März 2007, 18:30–19:00, H20

Unconventional BCS States in Heavy-Fermion Superconductors — •Frank Steglich — Max Planck Institute for Chemical Physics of Solids, Dresden, Germany

Heavy-fermion (HF) superconductivity (SC) discovered in 1979 often occurs in the vicinity of an antiferromagnetic (AF) quantum critical point (QCP). Here, we concentrate on the prototypical HF compound CeCu₂Si₂, whose phase diagram also contains ‘phase A’, an ordinary spin density wave (SDW) with very small ordered moment. Application of low pressure was found to smoothly suppress ‘phase A’ and to establish an antiferromagnetic (AF) QCP of (3D) SDW type. A narrow superconducting dome centered around this QCP exists in the T-p phase diagram of CeCu₂Si_1.8Ge_0.2 - similar to what was discovered earlier for CePd₂Si₂ [Mathur et al. (1998)]. Different from its Pd homologue, however, CeCu₂Si_1.8Ge_0.2 exhibits a second superconducting dome, coinciding with a weak valence transition near p = 5 GPa. Because of the low-lying critical end point of the first-order valence transition line, SC under this high-p dome is likely to be mediated by charge-density fluctuations [Miyake et al. (1999)]. Here, we discuss experimental evidence for HFSC in CeCu₂Si₂ under the low-p dome to be due to AF quantum critical paramagnons, as first proposed in the case of CePd₂Si₂. The lack of SC in the isostructural quantum critical material YbRh₂Si₂ will also be addressed.
Work done in collaboration with: P. Gegenwart, C. Geibel, F.M. Grosche, S. Jeevan, M. Loewenhaupt, O. Stockert, S. Wirth and H. Q. Yuan.