Centipede ladder at quarter filling

TL;DR

This paper investigates the ground state and excitation spectra of a quasi-one-dimensional 'centipede ladder' nanostructure at quarter filling, revealing different behaviors in charge and spin excitations depending on hopping parameters and exchange interactions.

Contribution

It provides a detailed analysis of the spin and charge excitation spectra of the centipede ladder, highlighting the effects of varying longitudinal hopping and exchange couplings on its quantum states.

Findings

At small $t_\|$, ferromagnetic on-rung exchange leads to dimerization instability.

Charge transfer excitons can propagate coherently in the small $t_\|$ limit.

At large $t_\|$, the system resembles a two-orbital Hubbard chain with a charge gap.

Abstract

We study the ground state and excitation spectrum of a quasi one-dimensional nanostructure consisting of a pole and rungs oriented in the opposite directions ("centipede ladder", CL) at quarter filling. The spin and charge excitation spectra are found in the limits of small and large longitudinal hopping $t_\|$ compared to the on-rung hopping rate $t_\perp$ and exchange coupling $I_\perp$. At small $t_\|$ the system with ferromagnetic on-rung exchange demonstrates instability against dimerization. Coherent propagation of charge transfer excitons is possible in this limit. At large $t_\|$ CL behaves like two-orbital Hubbard chain, but the gap opens in the charge excitation spectrum thus reducing the symmetry from SU(4) to SU(2). The spin excitations are always gapless and their dispersion changes from quadratic magnon-like for ferromagnetic on-rung exchange to linear spinon-like for antiferromagnetic on-rung exchange in weak longitudinal hopping limit.