Finite temperature spin-dynamics and phase transitions in spin-orbital models

TL;DR

This paper investigates the finite temperature behavior of a spin-orbital model relevant to transition metal compounds, revealing phase transitions and spectral changes that resemble phenomena observed in Fe-pnictide materials.

Contribution

It introduces a comprehensive analysis of a coupled spin-orbital model, highlighting the phase transition and spectral dynamics relevant to real materials.

Findings

The model exhibits a 2D Ising-like phase transition to orbital order.

Orbital excitations cause a redistribution of spin spectral weight.

Magnetic susceptibility sharply increases below the transition.

Abstract

We study finite temperature properties of a generic spin-orbital model relevant to transition metal compounds, having coupled quantum Heisenberg-spin and Ising-orbital degrees of freedom. The model system undergoes a phase transition, consistent with that of a 2D Ising model, to an orbitally ordered state at a temperature set by short-range magnetic order. At low temperatures the orbital degrees of freedom freeze-out and the model maps on to a quantum Heisenberg model. The onset of orbital excitations causes a rapid scrambling of the spin spectral weight away from coherent spin-waves, which leads to a sharp increase in uniform magnetic susceptibility just below the phase transition, reminiscent of the observed behavior in the Fe-pnictide materials.