Excitons and Dark Fermions as Origins of Mott Gap, Pseudogap and Superconductivity in Cuprate Superconductors --- General Idea and Basic Concept Based on Gap Physics

TL;DR

This paper proposes a novel theory where excitons and dark fermions, arising from electron fractionalization in doped Mott insulators, explain the Mott gap, pseudogap, and high-temperature superconductivity without spontaneous symmetry breaking.

Contribution

It introduces a new framework linking excitons and dark fermions to the origins of gaps and superconductivity in cuprates, avoiding symmetry breaking.

Findings

Identifies Mott-gap fermions and dark fermions as emergent from electron fractionalization.

Proposes exciton hybridization as the mechanism behind pseudogap formation.

Suggests high-Tc superconductivity arises from Wannier-type exciton attraction.

Abstract

Theory of doped Mott insulators is revisited in the light of recent understanding on the singular self-energy structure of the single-particle Green's function. The unique pole structure in the self-energy induces the high-temperature superconductivity in the anomalous part, while it generates Mott gap and pseudogap in the normal part. Here, we elucidate that fractionalization of electrons, which is exactly hold in the Mott insulator in the atomic limit, more generally produces the emergent Mott-gap fermion and dark (hidden) fermions. It does not require any spontaneous symmetry breaking. The two gaps are the consequences of the hybridization of these two fermions with quasiparticles. We further propose that the Mott-gap fermion and dark fermions are the fermionic component of Frenkel- and Wannier-type excitons, respectively, which coexist in the doped Mott insulator. The Bose-Einstein condensation of the Frenkel-type excitons allowed without spontaneous symmetry breaking holds a key for understanding the unique pole structure and the pseudogap through the instantaneous hybridization between the fractionalized quasiparticle and the dark fermion in analogy with the Mott gap. We argue that the high-$T_{\rm c}$ superconductivity is ascribed to the dipole attraction of the Wannier-type excitons. The gap formation mechanism is compared with that caused by conventional spontaneous symmetry breaking known over condensed matter and elementary particle physics including quantum chromodynamics. We propose a theoretical framework and discuss experimental tests to analyze this idea and concept.