Challenging Questions from IRONIC14

Questions and Grand Challenges for discussion in the final two weeks:

A. Overarching questions: questions that cut between the families of unconventional superconductivity.

  1. What is the nature of the strange metal that exists at optimal doping in the cuprates, but which is also seen
in CeCoIn5 and most probably, in P-doped BaFe2As2? (Piers Coleman)
  • How do we understand the observation of T-linear resistivity, and (in at least the first two cases) T^2 Hall Angle?
  • Is this a consequence of quantum criticality, or a signature of a new class of metal?
  • What new experiments can be carried out to shed new light on the strange metal at optimal doping?

2. Do we/can we understand the systematics of Tc even qualitatively in any class of unconventional superconductors? (Peter Hirschfeld)

3. Bad Metals: What, if any, are the limits or bounds on scattering as measured in transport (for example: k_F l ~ 1, E_F \tau ~ 1, \Gamma = kB T)? (Andy Schofield)

B. Focused questions: questions that pertain to specific families or instances of unconventional superconductivity.

  1. What is the pairing mechanism in UBe(13), and how is the pair condensate formed from the local degrees of freedom? (Piers Coleman)
  2. What is the origin of the zero bias anomaly seen in tunneling into Fe(Se,Te): could this be a signature of fully gaped p-wave pairing? (Piers Coleman)
  3. I would like to see a discussion (which may have occurred at some point while I was away) of to what extent the existence of members of the Fe SC family that do not have hole pockets at the gamma point calls into question the consensus understanding of the mechanism of SC in this material. Specifically, I would love a review of to what extent it is really established that there is S\pm pairing in any
    (or even the majority) of these materials, and to what extent some form of spin-fluctuation exchange mechanism
    is called into question by this observation. (Steven Kivelson)
  4. How come UPt3 is an f-wave superconductor when the proximate magnetism is antiferromagnetic? (Qimiao Si)
    • One could consider additional factors (spin-orbit etc.), but why is UPt3 special?
    • Are there other systems in any class of unconventional superconductors where p- or f-wave pairing occurs near antiferromagnetism?
5. Is there a shooting gun experiment which can tell us whether superconductivity in iron superconductors has weak or strong coupling origin? (Leni Bascones)
6. Nematic order/fluctuations (Andriy Nevidomskyy): Can nematic order exist far from a magnetic instability (e.g. FeSe), or is observed nematicity (ARPES) due to spin/orbital fluctuations? What is the role of built-in strain? Does orbital nematicity play an active role in superconductivity?
7. Why is the T-linear resistivity so ubiquitous in the iron pnictides and chalcogenides? (Qimiao Si)

Possible discussion topics compiled at the organizational mtgs in wks 3,6,7,8:

(added in week 9):

Iron-based systems:
  • m* vs. x (discussed in wk 9)
  • FeSe intercalted: (XYZ) FeSeS; Double dome; KFeSe
  • Pairing state, pairing amplitude, etc.. (discussed in wk 10)
  • Tuning by pressure or strain
  • 110K SC(?) and single-layer FeSe (discussed in wk 6)
  • Various FeSe's: universal among them? how do they compare w/ pnictides?
  • Orbital selective physics
  • Correlation strength (discussed in wk 1)
  • Electronic anisotropies (discussed in wk 2)
  • pairing symmetry
  • multi-orbital
  • Quantum criticality (discussed in wk 3)
  • disorder
  • Nature of magnetism (discussed in wk 6)
  • Pnictides vs cuprates
  • Multiple tuning parameters (isoelectronic vs el or hole dopings, etc.)
  • Materials basis
  • Variations of Fermi surfaces across different families (hole & el pockets)
  • Effects of S.O.C., Glide reflection symm, etc on pairing
  • Higher Tc

Other systems:
  • m* vs x (discussed in wk 9)
  • Magnetism & SC in (Cs,Rb,K)3C60 (part of discussion wk 8)
  • SC near magnetism (discussion wk 7)
  • Strange metals
  • Pseudo-gap: cuprates and beyond
  • Intertwined orders
  • Composite pairing
  • Heavy fermions
  • Ruthenates
  • Quantum criticality (discussed in wk 3)
  • Organics

Theoretical aspects:
  • Weak vs. strong coupling approaches

LB: I would like to propose a question to more "strongly-correlated" people:

- to what level are spin-fermion models a la Wei-Ku compatible with the results of DMFT calculations?
- Can the parameters of these models be extracted from first-principles calculations? What would be the smoking-gun in an electronic structure
calculation that speaks in favour of these models?

I would be grateful to anybody who has an answer ;)

IM: If you mean arXiv:1205.3509, I can only say that a rose by any other name smells just as sweet. Yes, magnetic moments formed by itinerant electrons are soft. Yes, if there is a Fermi surface, there will be double exchange (RKKY by other name) interaction, largely ferromagnetic. Why does this need a new name, I am not sure.

Possible discussion topics for Wednesdays 4pm discussion (from 1st organizational mtg.)
In no particular order:

  1. Bad metal behavior and importance for superconductivity
  2. Importance of spin-orbit coupling for pairing problem
  3. Pnictides vs. chalcogenides and more generally FeSC vs. cuprates (Wed. 9/3 discussion)
  4. eta pairing and other consequences of glide plane symmetry
  5. Time reversal symmetry breaking
  6. Nematicity: spin vs. orbit (9/10 discussion)
  7. Consequences of multiorbitals bands for low-energy effective physics
  8. Sr2RuO4
  9. Nature in magnetism in FeSe
  10. Higher Tc?
  11. Differences among Fe-based materials
  12. Hole vs. electron pockets
  13. Quantum criticality: heavy fermions, ruthenates, pnictides?
  14. Disorder: role in unconverntional superconductors
  15. Theoretical methods for unconv..SC