Strong Correlations and the Normal State of the High Temperature Superconductors

The workshop and seminar on strong correlations and the normal state of high temperature superconductors was held at MPIPKS in Dresden from May 17 to May 27, 2016.

The first week (May 17-20) was the workshop week, with around 70 participants and around 8 talks per day. The talks were 35 min. long. There was discussion at the end of each talk, and we allocated additional time for discussions at the end of every session. The session chairs directed the discussion and some presented summary slides.

We had two evening poster sessions on Tuesday and Wednesday, and there was a lively discussion around posters on both days.

During the second, seminar week, we had 4-5 talks per day, mostly by junior participants. We also had three talks by the organizers (Chubukov, Keimer, Sebastian), and one informal blackboard talk by Subir Sachdev, who also gave a colloquium at MPIPKS on Monday, May 23.

The goal of the workshop was to bring together leading experimentalists and theorists working in the field of strongly correlated systems, mostly cuprates and Fe-pnictides/chalogenides, with the goal to advance our understanding of the role of correlations in these materials, particularly of the origin of the pseudogap and charge-density-wave state in the cuprates, and nematic state in Fe-based systems. 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 discussion at the workshop and the seminar chiefly focused on two main issues:

• The origin on the pseudogap in the cuprates and its interplay with the charge order

• The origin of the nematic state in Fe-based systems.

The first issue included the discussion on: (i) Is the pseudogap in the cuprates a phase with a broken symmetry, or does it represent a crossover to Mott physics as suggested by some DMFT- based theoretical studies? (ii) 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)? (iii) 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? (iv) 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? (v)How can we reconcile the breakdown of the Fermi liquid paradigm when superconductivity is suppressed by elevated temperatures, with Fermi liquid behavior observed at low temperatures when superconductivity is suppressed by a magnetic field? (vi)What is the role of the quantum critical points associated with density wave order that underlie the maxima of the two-dome superconducting structure? (vii) 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?

The discussion on the nematicity was focused on the following topics: (i) What is the origin of the nematic phase in iron pnictides? Is it due to orbital order or is the result of magnetic fluctuations? (ii)If orbital order is the primary one, what gives rise to an attraction in the orbital space and is orbital transition continuous or discrete? (iii) 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? (iv) Can nematic fluctuations mediate an attractive pairing interaction? If yes, in what channel? (v) Is it possible to have a non-superconducting phase with time-reversal symmetry breaking in Fe-pnictides? (vi) Is Mott physics and the concept of “orbital selective Mott transition” relevant for at least some Fe-pnictides?

For the workshop, 30 speakers gave talks, and for the seminar, 15 speakers gave talks. The senior speakers at the workshop were L. Taillefer, A. Kapitulnik, D-H Lee, R. Greene, M. Rice, Y. Matsuda, H. Kontani, B. Buechner, C. Varma, W. Metzner, A. Mackenzie, J. Schmalian, and M. Vojta. S. Sachdev gave a colloquium and informal talk during the seminar week. A number of junior scientists gave talks during the workshop and the seminar week. All these talks went very well and generated a lot of questions. There wasn’t a single talk without at least 10 min discussion.

The key scientific results of the workshop and seminar is (i) the broad understanding that the emergence of the pseudogap in the cuprates gives rise to strong reconstruction of electronic states, consistent with the idea that the density of carriers sharply changes from 1+x to x, where x is the doping, and (ii) the understanding that there exists two mechanisms for nematic order in Fe-based systems --- one is a spontaneous orbital ordering, and the other is an Ising-nematic order due to magnetic fluctuations. Most, but not all researchers believe that for Fe-pnictides magnetic scenario is realized, but for FeSe a spontaneous orbital order is a clear possibility.