Phase transitions in spin-orbital models with spin-space anisotropies for iron-pnictides: A study through Monte Carlo simulations

Abstract

The common phase diagrams of superconducting iron pnictides show interesting material specificities in the structural and magnetic phase transitions. In some cases the two transitions are separate and second order, while in others they appear to happen concomitantly as a single first order transition. We explore these differences using Monte Carlo simulations of a two-dimensional Hamiltonian with coupled Heisenberg-spin and Ising-orbital degrees of freedom. In this spin-orbital model, the finite-temperature orbital-ordering transition results in a tetragonal-to-orthorhombic symmetry reduction and is associated with the structural transition in the iron-pnictide materials. With a zero or very small spin space anisotropy, the magnetic transition separates from the orbital one in temperature, and the orbital transition is found to be in the Ising universality class. With increasing anisotropy, the two transitions rapidly merge together and tend to become weakly first order. We also study the case of a single-ion anisotropy and propose that the preferred spin-orientation along the antiferromagnetic direction in these materials is driven by orbital order.

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• Spin reorientation driven by the interplay between spin-orbit coupling and Hund's rule coupling in iron pnictides
• Elastic coupling and spin-driven nematicity in iron-based superconductors
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• Lifted electron pocket and reversed orbital occupancy imbalance in FeSe
• How many quantum phase transitions exist inside the superconducting dome of the iron pnictides?
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• Double stage nematic bond-ordering above double stripe magnetism: application to BaTi${}_2$Sb${}_2$O
• Tunneling spectroscopy for probing orbital anisotropy in iron pnictides
• Spin excitation anisotropy in optimal-isovalent-doped superconductor BaFe2(As0.7P0.3)2
• Elementary Excitations due to Orbital Degrees of Freedom in Iron Based Superconductors
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• Itinerant scenario for Fe-pnictides: comparison with quantum Monte Carlo
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