“Flat/steep band model” for superconductors containing Bi square nets

Abstract

The crystal structures of a new family of superconductors containing a Bi square net and their electronic structures around the Fermi level have been reviewed. The structures of these compounds can be viewed as stacked layers denoted by [Bi][(RE)(M2Bi2)(RE)] RE = rare earth or alkaline earth metal, M = transition metal. Flat/steep band features are shown to exist in all these new superconductors, though the pairing mechanisms may be very different. The Dirac Fermion behavior is reviewed and its implications are discussed.