Ordering Phenomena in Transition Metal Oxides

Heavy electrons: Superconductivity and quantum critical points   

Strong electronic correlations on partially filled 4f or 5f shells, weakly hybridized with the itinerant ligand states, cause the formation of extremely heavy quasiparticles composed of a dominating local spin part and delocalized charge carrier contributions. As discovered in 1979 for CeCu₂Si₂ [1], these "heavy fermions" (HF) may form Cooper pairs, the superconducting glue presumably being of magnetic origin, at least in most cases [2]. This could be convincingly demonstrated with the aid of tunneling and neutron-scattering experiments on UPd₂Al₃ [3].

For a number of Ce-based HF metals superconductivity was found to be intimately related to the existence of an antiferromagnetic instability, most likely of the (3D) spin-density-wave type [4]. On the other hand, a quantum critical point of the local-moment variety at which the "composite fermions" appear to break up, as recently established in YbRh₂(Si,Ge)₂ [5], appears to be unfavorable for superconductivity.

Work done in collaboration with J. Custers, P. Gegenwart, C. Geibel, F.M. Grosche, R. Küchler, K. Neumaier, N.K. Sato, J. Sichelschmidt, G. Sparn, O. Stockert, Y. Tokiwa, H. Wilhelm, H.Q. Yuan, P. Coleman, C. Pepin, Q. Si, P. Thalmeier and G. Zwicknagl.

[1] F. Steglich et al., Phys. Rev. Lett 43, 1892 (1979).
[2] H.Q. Yuan et al., Science 302, 2104 (2003).
[3] N.K. Sato et al., Nature 410, 340 (2001).
[4] O. Stockert et al., Phys. Rev. Lett. 92, 136402 (2004).
[5] J. Custers et al., Nature 424, 524 (2003). R. Küchler et al., Phys. Rev. Lett. 91, 066405 (2003). J. Sichelschmidt et al., Phys. Rev. Lett. 91, 156401 (2003).