Strong Correlations and the Normal State of the High Temperature Superconductors

International Workshop & Seminar
17 May - 27 May 2016

High temperature superconductivity has proved both a source of great excitement and deep mystery since the first discovery of the copper oxide superconductors in the 1980s and the recent discovery of the iron pnictide superconductors. The wast majority of researchers agree that, to understand superconductivity, one needs to understand first the normal state of these materials. The latter, is, however, highly non-trivial and contains, e.g., the pseudogap phase in the cuprates and the nematic phase in iron pnictides and chalcogenides. Arriving at a theoretical understanding of the normal state in both these materials is particularly challenging given the likely involvement of multiple of these contributing factors. Within the last few years, however, an infusion of new experimental results has finally made a resolution of normal state physics in the family of high temperature superconductors a tangible possibility. The workshop will focus on in-depth discussion of the origin of the pseudogap in the cuprates (e,g, whether it is a precursor to some other state or a state with broken symmetry) and of unconventional normal state properties of Fe pnictides and chalcogenides. Similarities and differences between these two classes of high-temperature superconductors will be discussed.

• Is the pseudogap in the cuprates a phase with a broken symmetry (if yes, what symmetry or symmetries are broken), or does it represent a crossover to Mott physics as suggested by some DMFT-based theoretical studies?
• If the pseudogap involves a broken symmetry (most likely related to charge order), can the symmetry-breaking order be better described in terms of charge order (or strong fluctuations), or pair-density order (a pairing instability with a non-zero total momentum of a pair)? Which instability is stronger for a realistic fermionic dispersion?
• Is there a single phase transition at T= T*, or a series of transitions at different temperatures involving for example the breaking of time-reversal symmetry and U(1) translational symmetry?
• What is the interplay between the description of the pseudogap in the metallic scenario and in the strong coupling scenario describing a doped Mott insulator? Are the two approaches qualitatively different or do they represent the two different descriptions of qualitatively the same physics?
• How can we reconcile the breakdown of the Fermi liquid paradigm when superconductivity is suppressed by elevated temperatures, with Fermi liquid behaviour observed at low temperatures when superconductivity is suppressed by a magnetic field?
• What is the role of the quantum critical points associated with density wave order that underlie the maxima of the two-dome superconducting structure? How are these related to the pseudogap regime and especially the strange metal regime where the Fermi liquid breaks down?
• How can the angle resolved photoemission data away from antinodal regions be explained? Does the Fermi arc represent a nodal quasiparticle density of states at the Fermi energy, or does it just reflect a strong incoherence of excitations at the antinodal region?
• What is the origin of the nematic phase in iron pnictides? Is it due to orbital order or is the result of magnetic fluctuations?
• If orbital order is the primary one, what gives rise to an attraction in the orbital space and is orbital transition continuous or discrete? (both types of transitions have been detected in Fe-pnictides)
• If nematic phase is the result of a composite spin order (a four-fermion condensate), what is the effect of such order on single electron properties?
• Can nematic fluctuations mediate an attractive pairing interaction? If yes, in what channel?
• Is it possible to have a non-superconducting phase with time-reversal symmetry breaking in Fe-pnictides?
• Is Mott physics and the concept of “orbital selective Mott transition” relevant for at least some Fe-pnictides?