Cigdem Capan: Superconductivity and Quantum Criticality in CeIrIn_5

Cigdem Capan began with a reprise of the key physics of Kondo lattice symmetries, describing how the physics of heavy electron physics is driven by a competition between the screening of the local moments and the RKKY interaction between them.





She raised three general key questions:

• How does the single ion Kondo effect relate to the Kondo lattice effect?
• Is the phase diagram organized by the singular Quantum Critical Points?
• Does the presence of a QCP favor superconductivity?

Capan introduced the 115 heavy electron materials, already discussed in blogs last week. She introduced the Pagliuso phase diagram that links the Co, Rh and Iridium versions of these tetragonal systems. She mentioned CeCoIn5, for which a variety of measurements indicate dx^2-y^2 pairing. Her main focus is on CeIrIn5. This is a HF superconductor, thought to have a line of gap nodes, but the symmetry is currently unknown.

This was followed by a brief summary of the physics of CeRhIn5 - for which I will refer you to the previous talk by Joe Thompson.

The talk then proceeded to discuss CeCoIn5, where there is a field tuned QCP at the upper critical field, yet to be identified. Here, as one lowers the field towards Hc2=5T,

• The linear specific heat grows and appears to diverge at Hc2 (Bianchi et al)
  
• The A coefficient of the resistivity (rho(T) = rho(0) + AT^2), A diverges as one approaches Hc2 (Paglione et al)
  

It turns out that the QCP is not pinned to the top of the sc phase diagram. First, the position of the QCP is suppressed much faster than Hc2 under pressure. One can carry out an analysis of the Hall coefficient RH(T) which has an interesting field dependence. Cigdem claimed that the Hall data can be collapsed onto a single scaling curve, by scaling the field with respect to the field at which the Hall constant is a minimum. This procedure suggests that the QCP appears to be located at Hc =4.1T.

There is also recent work with tin (Sn) and Cadmium(Cd) doping. Tin doping fails to separate Recent Cd doping by Pham et al that indicates a link with antiferromagnetism. At the "top" of the Superconducting phase diagram, there is an additional phase - which may have links with antiferromagnetism and the FFLO incommensurate superconductor.

Returning to CeIrIn5 - here there is indication that some characteristic temperature scale in the specific heat drops with magnetic field, suggesting a QCP at 27T. The specific heat at high temperatures shows a transition, which appears to extrapolate to this same point. The talk focusses on this putative phase diagram. Here, more careful measurements suggest that at
the system narrowly misses a QCP at 27T, but makes a close "flyby", leading to a rapid evolution in the magnetization, or "meta-magnetic transition".

Sachdev suggested that this transition might be a very weak antiferromagnetic phase transition.

Capan showed dHvA data and argued that there is no significant change in the alpha orbits in the passage past the "metamagnetic transition". There is no significant change in the effective mass m* through the transition.

Varma pointed out that the observed masses were far too small to account for the huge linear specific heat (1000mJ/mol/K2) in this system. The amplitudes are however, anomalously depressed near the MMT - Capan would like to understand the origin of this anomally.

Key questions raised by Cigdem Capan:

• Is there a simple phenomenology to understand the scaling of the Hall effect in CeCoIn5?
• How do we undertand the electron hole assymetry in CeConIn5 (sn vs cd doping)?
• What is the damping mechanism for the dHva oscilations at the MMT transition in CeIrIn5?
• What is the origin of the upturn in the resitivity near the MMT transition in CeIrIn5?
• Why is there no QCP in CeIrIn5?

There is an interesting dichotomy between the larger Fermi surface of CeIrIn5, and the much higher mass of the system. Are the f-electrons more localized - as suggested by the large mass - or are they more delocalized - as suggested by the large Fermi surface. Is the Co or the Ir closer to the antiferromagnetic instability?