Spin and charge excitations in the anisotropic Hubbard ladder at quarter filling with charge-ordering instability

TL;DR

This study investigates the spin and charge excitations in an anisotropic Hubbard ladder at quarter filling, revealing a temperature-dependent spin-charge separation and coupling that may explain anomalous spin dynamics in charge-ordered materials.

Contribution

It provides a detailed analysis of spin-charge separation and coupling in a 1D Hubbard ladder model using quantum Monte Carlo and DMRG methods, highlighting temperature and parameter effects.

Findings

Spin degrees of freedom can behave like a 1D antiferromagnetic Heisenberg model.

Above a crossover temperature, spin excitations are influenced by charge fluctuations.

The results suggest a possible explanation for anomalous spin dynamics in charge-ordered materials.

Abstract

Quantum Monte Carlo and density-matrix renormalization group methods are used to study the coupled spin-pseudospin Hamiltonian in one-dimension (1D) that models the charge-ordering instability of the anisotropic Hubbard ladder at quarter filling. We calculate the temperature dependence of the uniform spin susceptibility and specific heat as well as the spin and charge excitation spectra of the system. We thereby show that there is a parameter and temperature region where the spin degrees of freedom are separated from the charge degrees of freedom and behave like a 1D antiferromagnetic Heisenberg model, and that, outside this parameter region and above a crossover temperature, the spin excitations are largely affected by the charge fluctuations. We argue that observed anomalous spin dynamics in the disorder phase of a typical charge-ordered material $\alpha'$-NaV$_2$O$_5$ may possibly be a consequence of this type of spin-charge coupling.