Effect of Hund coupling in the one-dimensional SU(4) Hubbard model

TL;DR

This paper investigates how Hund coupling influences the spectral gap, symmetry, and possible phases like superconductivity or charge density waves in the one-dimensional SU(4) Hubbard model away from half-filling.

Contribution

It reveals the presence of a spectral gap in the spin-orbital sector regardless of Coulomb strength and distinguishes two regimes based on Hund coupling strength, showing symmetry enhancement and phase transitions.

Findings

Spectral gap exists in the spin-orbital sector at all Hund couplings.

Low Hund coupling leads to SU(4) symmetry and coherent excitations.

High Hund coupling suppresses superconductivity, favoring charge density waves.

Abstract

The one-dimensional SU(4) Hubbard model perturbed by Hund coupling is studied, away from half-filling, by means of renormalization group and bosonization methods. A spectral gap is always present in the spin-orbital sector irrespective of the magnitude of the Coulomb repulsion. We further distinguish between two qualitatively different regimes. At small Hund coupling, we find that the symmetry of the system is dynamically enlarged to SU(4) at low energy with the result of {\it coherent} spin-orbital excitations. When the charge sector is not gapped, a superconducting instability is shown to exist. At large Hund coupling, the symmetry is no longer enlarged to SU(4) and the excitations in the spin sector become {\it incoherent}. Furthermore, the superconductivity can be suppressed in favor of the conventional charge density wave state.