- Christoph Geibel: Recent surprises in the Heavy Fermion systems YbRh2Si2 and CeCu2Si2
- 10. 12. 2014, 14:30
- lecture room F2, first floor Ke Karlovu 5
- more information
Abstract:
CeCu2Si2 and YbRh2Si2 are archetypal systems in the field of strongly correlated electron systems: The discovery of superconductivity in CeCu2Si2 in 1979 started the field of unconventional superconductivity, while YbRh2Si2 was observed in 2001 to be extremely close to an antiferromagnetic quantum critical point. Despite both compounds have been investigated since many years, basic questions in understanding the unusual physics in these compounds are still open. In the past two years we obtained new data on both systems which resulted in surprising and unexpected conclusions.
In YbRh2Si2 recent ARPES results gave some direct insight into the Fermi surface and indicate the presence of a large Fermi surface (including the 4f degree of freedom) in a wide temperature region from about 1 K up to 100 K [1]. This results is in conflict with the so called T* line being a transition from a large Fermi surface at high fields to a small Fermi surface at low fields. Furthermore a thorough investigation of the alloy system YbCo2Si2-YbRh2Si2 proved that at intermediate Co-content magnetic order switches from an in-plane antiferromagnet to a c-axis ferromagnet [2]. While the occurrence of ferromagnetism is not surprising in view of the long known ferromagnetic correlations in the pure Rh compounds, the ordering along the c direction is an extreme surprise in view of the huge easy-plane crystal electric field anisotropy. I shall analyze both observations in view of the previous knowledge on these compounds and discuss implications for future models.
In CeCu2Si2, quite a number of experimental results led to the prevailing view that the superconducting order parameter is d-wave in this compound, although different experiments concluded in different types of d-waves. Surprisingly recent specific heat results on so-called S-type single crystals provide strong evidence that the heavy part of the Fermi surface is fully gapped, which is incompatible with previously favored superconducting order parameters [3]. I shall discuss the reliability of the recent observations, and discuss implications for the SC order parameter in the context of previous experimental results and renormalized band structure calculations.
[1] K. Kummer et al., submitted
[2] S. Lausberg et al., PRL 110 (2013) 256402
[3] S. Kittaka et al., PRL 112 (2014) 067002.