Flat bands and the physics of strongly correlated Fermi systems

TL;DR

This paper explores how flat bands in electronic spectra lead to unusual properties in strongly correlated Fermi systems, challenging traditional theories and explaining recent experimental anomalies in materials like heavy-fermion compounds, graphene, and cuprates.

Contribution

It introduces the fermion condensate approach to explain phenomena that violate Landau Fermi liquid theory, such as asymmetric tunneling and universal scaling in correlated materials.

Findings

Explains asymmetric tunneling conductivity in heavy-fermion compounds and graphene.

Accounts for violation of Leggett theorem in overdoped cuprates.

Describes universal scaling between resistivity and superfluid density.

Abstract

Some materials can have the dispersionless parts in their electronic spectra. These parts are usually called flat bands and generate the corps of unusual physical properties of such materials. These flat bands are induced by the condensation of fermionic quasiparticles, being very similar to the Bose condensation. The difference is that fermions to condense, the Fermi surface should change its topology, leading to violation of time-reversal (T) and particle-hole (C) symmetries. Thus, the famous Landau theory of Fermi liquids does not work for the systems with fermion condensate (FC) so that several experimentally observable anomalies have not been explained so far. Here we use FC approach to explain recent observations of the asymmetric tunneling conductivity in heavy-fermion compounds and graphene and its restoration in magnetic fields, as well as the violation of Leggett theorem, recently observed experimentally in overdoped cuprates, and recent observation of the challenging universal scaling connecting linear-$T$-dependent resistivity to the superconducting superfluid density.