Magnetic excitations in one-dimensional spin-orbital models

TL;DR

This paper investigates the dynamics of one-dimensional spin-orbital models, revealing conditions for stable collective excitations and analyzing their renormalization, spectral features, and thermodynamic properties through various theoretical approaches.

Contribution

It introduces a detailed analysis of spin-orbital excitations, comparing mean-field and perturbative methods, and provides insights into their spectral and thermodynamic behavior.

Findings

Existence of long-lived collective excitations in ferromagnetic ground states.

Significant linewidth broadening and structures in the dynamical spin structure factor.

Low-temperature specific heat behavior indicating a dynamical critical exponent z=1.

Abstract

We study the dynamics and thermodynamics of one-dimensional spin-orbital models relevant for transition metal oxides. We show that collective spin, orbital, and combined spin-orbital excitations with infinite lifetime can exist, if the ground state of both sectors is ferromagnetic. Our main focus is the case of effectively ferromagnetic (antiferromagnetic) exchange for the spin (orbital) sector, respectively, and we investigate the renormalization of spin excitations via spin-orbital fluctuations using a boson-fermion representation. We contrast a mean-field decoupling approach with results obtained by treating the spin-orbital coupling perturbatively. Within the latter self-consistent approach we find a significant increase of the linewidth and additional structures in the dynamical spin structure factor as well as Kohn anomalies in the spin-wave dispersion caused by the scattering of spin excitations from orbital fluctuations. Finally, we analyze the specific heat c(T) by comparing a numerical solution of the model obtained by the density-matrix renormalization group with perturbative results. At low temperatures T we find numerically c(T) T pointing to a low-energy effective theory with dynamical critical exponent z=1.