The physics of coupled spin-orbital degrees of freedom and Fe pnictides

Motivated by neutron scattering experiments on Fe pnictides, we study the thermodynamic properties and finite-temperature spin dynamics of a model Hamiltonian with coupled quantum Heisenberg-spin and Ising-orbital degrees of freedom. The model system undergoes a phase transition to an orbitally ordered state at a temperature set by short-range magnetic order. The behavior of the specific heat and the order-parameter suggests that the transition is continuous and of second order, belonging to the 2D Ising universality class. The onset of orbital excitations and fluctuations causes a rapid scrambling of the spin spectral weight away from coherent spin-waves, leading to a sharp increase in uniform magnetic susceptibility just below the phase transition. The experimental consequences of this model are in qualitatively good agreement with the observed behavior in the Fe-pnictide materials.