Phase Diagram of a Model of Correlated Hopping of Electrons in a Lattice of Berry Molecules

TL;DR

This paper maps the one-dimensional phase diagram of a correlated electron hopping model with orbital degrees of freedom, revealing superconducting, metallic, and insulating phases depending on electron density and interaction strength.

Contribution

It introduces a 1D model incorporating orbital degeneracies via a spin-1 variable, analyzing its phase diagram and identifying conditions for superconductivity and charge order.

Findings

Superconducting correlations dominate at low densities and low U.

A charge gap and insulating phase emerge at half-filling for U > U_c.

A persistent spin gap exists across all phases.

Abstract

The $1D$ phase diagram of a model for correlated hopping of electrons in a lattice of Berry phase molecules is presented. Electrons hop in presence of an extra orbital degree of freedom at each site. This is mimicked as a spin-1 variable whose allowed states depend on the electron occupancy so as to take into account the orbital degeneracies of different molecular occupancies. In the $1D$ case we find that at low electron densities $n<<1$ there is a region with dominant superconducting correlations surviving an additional repulsive on-site interaction $U$ as strong as the bandwidth, $W=4t$. The critical value $U_c$ of $U$ below which superconductivity is found to be dominant decreases with increasing density $n$. For $n=1/2$ we find $U_c/t\approx 1$, whereas at $n=1$ (half-filling) our (less accurate) results are compatible with $U_c/t\approx 0$. For $U>U_c(n)$ and away from half-filling ($n\neq 1$) the system is metallic with dominant $2k_F$ charge density wave (CDW) correlations. At half-filling a charge gap opens for $U>U_c$ and the system becomes an insulator. A spin-gap characterizes the phase-diagram for all densities and for all values of $U$, even in the metallic regime $U>U_c$.