Deformation of SU(4) singlet spin-orbital state due to Hund's rule coupling

TL;DR

This paper studies how Hund's rule coupling affects the ground state and excitations of a one-dimensional spin-orbital model, revealing a transition from SU(4) singlet to orbital-dimer states with distinct correlation behaviors.

Contribution

It provides a detailed analysis of the impact of Hund's rule coupling on spin and orbital correlations using DMRG, highlighting the transition from SU(4) symmetry to orbital-dimer states.

Findings

Orbital correlations shift to q=π with increasing J.

Orbital gap becomes larger than spin gap.

Orbital dimerization is enhanced by Hund's rule coupling.

Abstract

We investigate the ground-state property and the excitation gap of a one-dimensional spin-orbital model with isotropic spin and anisotropic orbital exchange interactions, which represents the strong-coupling limit of a two-orbital Hubbard model including the Hund's rule coupling ($J$) at quarter filling, by using a density-matrix renormalization group method. At J=0, spin and orbital correlations coincide with each other with a peak at $q=\pi/2$, corresponding to the SU(4) singlet state. On the other hand, spin and orbital states change in a different way due to the Hund's rule coupling. With increasing $J$, the peak position of orbital correlation changes to $q=\pi$, while that of spin correlation remains at $q=\pi/2$. In addition, orbital dimer correlation becomes robust in comparison with spin dimer correlation, suggesting that quantum orbital fluctuation is enhanced by the Hund's rule coupling. Accordingly, a relatively large orbital gap opens in comparison with a spin gap, and the system is described by an effective spin system on the background of the orbital dimer state.