# Seminar: Degenerate plaquette physics as key ingredient of high-temperature superconductivity in cuprates

- Date: 3 October 2023, 11:15–12:15
- Location: Ångström Laboratory, Å4005
- Type: Seminar
- Lecturer: Mikhail Katsnelson, Radboud University
- Organiser: Division of Materials Theory, Department of Physics and Astronomy
- Contact person: Jorge Cayao

**Abstract**:

Cluster dynamical mean-field theory (CDMFT) approach to t-t’ Hubbard model [1] is a popular way to study superconductivity and other phenomena in cuprates. During last twenty years, enormous amount of such calculations was performed, and the main problem is not how to do more and more advanced calculations but, rather, to analyze a huge number of the data available and to separate them into more and less essential. We noticed some time ago [2] that for ‘realistic’ values of model parameters the ground state of Hubbard plaquette is close to be degenerate which naturally explains appearance of ‘soft fermion mode’ and pseudogap physics. Further studies demonstrated crucial importance of this accidental degeneracy to various aspects of physics of cuprates [3-6]. I will review the main results of this approach focusing on the discussion on the role of t’ which turns out to be essentially different in the regimes of small, large, and (realistically) moderate U. Within the model the binding energy of two holes is an order of magnitude higher than Tc which assumes existence of incoherent Cooper pairs in ‘normal’ phase [6]. I will discuss also in more detail calculations of effective Josephson coupling parameters between plaquettes and superfluid density [3] by the method similar to what we suggested long ago for magnetic interactions [7].

The research is done in a close collaboration with the group of Sasha Lichtenstein in Hamburg.

**References**:

[1] A. I. Lichtenstein and M. I. Katsnelson, Phys. Rev. B 62, R9283 (2000).

[2] M. Harland, M. I. Katsnelson, and A. I. Lichtenstein, Phys. Rev. B 94, 125133 (2016).

[3] M. Harland et al, Phys. Rev. B 100, 024510 (2019).

[4] M. Harland et al, Phys. Rev. B 101, 045119 (2020).

[5] A. A. Bagrov et al, Sci. Rep. 10, 20470 (2020).

[6] M. Danilov et al, NPJ Quant. Mat. 7, 50 (2022).

[7] M. I. Katsnelson and A. I. Lichtenstein, Phys. Rev. B 61, 8906 (2000).