Excitonic density-waves, bi-excitons and orbital selective pairing in two-orbital correlated chains

TL;DR

This paper explores various unconventional phases in a one-dimensional two-orbital model, including excitonic density waves, bi-excitons, and orbital selective pairing, supported by analytical and numerical methods.

Contribution

It introduces a comprehensive analysis of excitonic and pairing phenomena in a two-orbital chain, revealing new phases and mechanisms not previously detailed.

Findings

Discovery of excitonic density wave with finite momentum

Identification of bi-excitons near charge density-wave instability

Observation of orbital selective pairing with one metallic and one gapped orbital

Abstract

We present a comprehensive study of a one-dimensional two-orbital model at and below quarter-filling that realizes a number of unconventional phases. In particular, we find an excitonic density wave in which excitons quasi-condense with finite center of mass momentum and an order parameter that changes phase with wave-vector $Q$. In this phase, excitons behave as hard-core bosons without charge order. In addition, excitons can pair to form bi-excitons in a state that is close to a charge density-wave instability. When pairing dominates over the inter-orbital repulsion, we encounter a regime in which one orbital is metallic, while the other forms a spin gapped superconductor, a genuine orbital selective paired state. All these results are supported by both, analytical and numerical calculations. By assuming a quasi-classical approximation, we solve the three-body hole-electron-spinon problem and show that excitons are held together by forming a bound state with spinons. In order to preserve the antiferromagnetic background, excitons acquire a dispersion that has a minimum away from $k=0$. The full characterization of the different phases is obtained by means of extensive density matrix renormalization group calculations.