Mott and Wigner-Mott transitions in doped correlated electron systems: effects of superlattice potential and inter-site correlation

TL;DR

This paper introduces the concept of superstructure Mottness to describe Mott and Wigner-Mott transitions in doped correlated electron systems, highlighting the effects of superlattice potential and inter-site interactions.

Contribution

It proposes a unified framework for Mott and Wigner physics based on superstructure Mottness, supported by a minimal ionic Hubbard model analysis.

Findings

Superstructure Mottness can exist without translation symmetry breaking.

The superlattice potential enhances correlation effects and local moment formation.

A phase diagram illustrating the interplay of superlattice potential and Coulomb interactions.

Abstract

We introduce the notion of superstructure Mottness to describe the Mott and Wigner-Mott transition in doped strongly correlated electron systems at commensurate filling fractions away from one electron per site. We show that superstructure Mottness emerges in an inhomogeneous electron system when the superstructure contains an odd number of electrons per supercell. We argue that superstructure Mottness exists even in the absence of translation symmetry breaking by a superlattice, provided that the extended or intersite Coulomb interaction is strong. In the latter case, superstructure Mottness offers a unifying framework for the Mott and Wigner physics and a nonperturbative, strong coupling description of the Wigner-Mott transition. We support our proposal by studying a minimal single-band ionic Hubbard $t$-$U$-$V$-$\Delta$ model with nearest neighbor Coulomb repulsion $V$ and a two-sublattice ionic potential $\Delta$. The model is mapped onto a Hubbard model with two effective ``orbitals'' representing the two sites within the supercell, the intra and interorbital Coulomb repulsion $U$ and $U^\prime \sim V$, and a crystal field splitting $\Delta$. Charge order on the original lattice corresponds to orbital order. Developing a cluster Gutzwiller approximation, we study the effects and the interplay between $V$ and $\Delta$ on the Mott and Wigner-Mott transitions at quarter-filling. We provide the mechanism by which the superlattice potential enhances the correlation effects and the tendency towards local moment formation, construct and elucidate the phase diagram in the unifying framework of superstructure Mottness.