Strong Electronic Correlations: From High Temperature Superconductors to the Chemical Bond

Speaker: Prof. Józef Spałek
Affiliation: Institute of Theoretical Physics, Jagiellonian University, Kraków, Poland

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One of the fundamental problems od condensed matter physics is to explain the nature of electron states and their microscopic pairing mechanism in the strongly correlated systems. Their specific characteristic is the circumstance that the pairing may occur in real space when the reference state is either the Mott (magnetic) insulator or the molecular hydrogen (diamagnetic) insulator. The canonical examples are the cuprates (copper-oxygen) and molecular-hydrogen condensed systems, in which a transformation to the metallic state appears either by doping (change of stoichiometry) or extreme external pressure, respectively. These systems at the start differ from the ordinary superconductors, where the starting state is that of a rather ordinary metal and the principal question is to what extent the former systems represent a different state of matter from that described by the Bardeen-Cooper-Schrieffer theory. A proper answer to this question would introduce a new type of quantum liquids in any condensed-matter solid with strong correlations; also in such diverse condensed matter systems as are the neutron stars and quark-gluon plasma. The role of the evolution from the Mott insulating (atomic) initial state of involved electrons (fermions) is stressed.

In our group we have constructed over the last 10 years a unified theoretical model of the solid-state cuprates that contains both the strong correlations and magnetic fluctuations. The model has been applied to the description of high-temperature superconducting cuprates [1], heavy fermions, and to the twisted graphene bilayer systems. The aim of my presentation is to summarize selected properties of those systems, that is: (i) to discuss evolution from the antiferromagnetic Mott insulator to high temperature superconductor and to provide universal features of the unconventional superconducting condensate arising from it; (ii) to describe the dynamic (paramagnons) and plasmon excitations across the phase diagram. One should underline that pairing is regarded as taking place in real space, what distinguishes them in a principal manner from the corresponding description in either the BCS or Eliashberg theories. Selected theoretical results are compared with experiment in a quantitative manner.

If time allows, I will discuss briefly our recent work on the insulating/chemical hydrogen systems.

The work involves the projects from National Science Centre (NCN) of Poland, Grants Nos.
UMO-2018/29/B/ST3/02646 and UMO-2021/41/B/ST3/04070.

[1] Review: J. Spałek, M. Fidrysiak, M. Zegrodnik, and A. Biborski, Phys. Rep. 959, 1-117 (2022).

 Chairman: Prof. Ireneusz Weymann