Elementary excitations of the symmetric spin-orbital model: The XY limit

TL;DR

This paper analyzes elementary excitations in a 1D symmetric spin-orbital model, revealing phase separation and excitation gaps in XY and dimerized cases, highlighting the breakdown of mean-field theory.

Contribution

It provides a detailed analytical and numerical study of low-lying excitations in anisotropic spin-orbital models, emphasizing the nature of excitations and phase behavior.

Findings

Phase separation with gapless excitations in XY case

Gapped excitations in dimerized XXZ case

Elementary excitations involve simultaneous spin and orbital flips

Abstract

The elementary excitations of the 1D, symmetric, spin-orbital model are investigated by studying two anisotropic versions of the model, the pure XY and the dimerized XXZ case, with analytical and numerical methods. While they preserve the symmetry between spin and orbital degrees of freedom, these models allow for a simple and transparent picture of the low--lying excitations: In the pure XY case, a phase separation takes place between two phases with free--fermion like, gapless excitations, while in the dimerized case, the low-energy effective Hamiltonian reduces to the 1D Ising model with gapped excitations. In both cases, all the elementary excitations involve simultaneous flips of the spin and orbital degrees of freedom, a clear indication of the breakdown of the traditional mean-field theory.