Stefan Wirth (MPICpFS): Magnetotransport and tunneling investigations on heavy-fermion systems

Stefan Wirth introduced heavy fermion materials, describing them as a "Kondo lattice" of local moments embedded in a sea of conduction electrons. There are two types of interactions, he said - the local "Kondo" or s-d interaction betwen the local moments and conduction electrons

H = J S. s

that is responsible for the famous "resistance minimum", and a non-local "RKKY" interaction

H= J(R_ij) S_i.S_j

where the sign of the interaction oscillates due to Friedel oscillations. The competition between these two interactions in the Kondo lattice gives rise to a quantum critical point between the antiferromagnetic and heavy fermi liquid phases, according to the Doniach Scenario. The point that separates the antiferromagnet from the heavy fermi liquid is a quantum critical point - and it is in the vicinity of this point that new types of phases form - such as anisotropic superconductivity.

Steffan introduced the 115 materials, and discussed the rich interplay of antiferromagnetism and superconductivity in these systems. Of particular interest is Cd doped CeCo(In1-xCdx)5. Cd drives this system antiferromagnetic and leads to a two-stage transition (1) antiferromagnetism and (2) superconductivity. The H-T phase diagram of the SC and AFM are similar, with the sc nested inside the afm phase. The magnetic moment rises below the Neel temperature (4K?) but becomes T-independent in the superconducting phase. This suggests that f-electrons that would normally condense into the AFM order are instead, condensing into the sc condensate. Surely this is an argument for homogenious magnetism and sc?

There was a lot of discussion about whether the sc and magnetism exist homogeniously. Wirth argued that the comparable size of the SC and antiferromagnetic coherence length suggested they were homogenious - but surely NMR data by Curro et al refutes this?

Steffan also showed the global phase diagram including the Fermi liquid behavior. The FL extends out along the high field, low T part of the phase diagram, apparently extending inside the AFM dome.

Next Steffan turned to a discussion of STM data on heavy fermion compounds. He showed two sets of new data -

STM data on CeCoIn5 and STM data on YbRh2Si2.
In the CeCoIn5, which was cleaved at room temperature, the MPICpFS/Los Alamos group has observed a superconducting gap - but remarkably, the gap develops at 3K, while the s.c develops only at lower temperature - at 2.3K.  This raises the question as to whether there is a kind of pseudogap region above the sc transition - a point that was infered from bulk measurements some years ago.

In the YRS, which was cleaved at low temperature, the surface is much cleaner, and single-atom resolution is possible in the STM data.  The group does not know which layer they are cleaving on, but it is either Si or Yb layers.

Steffan discussed the STM curves, which show a kind of
V-shaped feature, with steps at negative bias that he is able to identify with the Crystal fields at 43, 25 and 17meV.  The overall shape of the dI/dV curve is however quite mysterious.  Wirth expressed the concern that they were mainly tunneling into the Si layers, with little direct coupling to the f-electrons


Bloggers aside: Interestingly enough - there is a remarkable similarity between the form of the dI/dV curve seen in this Non-Fermi liquid material, and the calculated density of states presented on tuesday by Massimo Capone, at a three-channel Kondo impurity quantum critical point.