| The discovery of iron-based superconductors opened a new direction in the research of quantum materials. Besides an unconventional superconducting state, it has been proposed that these systems also display an anisotropic electronic state, since experiments in the orthorhombic phase revealed strong anisotropies that cannot be attributed solely to the small distortion of the lattice. Starting from a microscopic electronic model, I will show that Ising-nematic degrees of freedom naturally emerge from the itinerant magnetism of the iron pnictides, being a strong candidate for such anisotropic electronic order. In the nematic state, the tetragonal symmetry is spontaneously broken, but the spin-rotational symmetry is preserved. While nematic order induces a small orthorhombic distortion and a finite orbital polarization, nematic fluctuations soften the shear modulus in the tetragonal phase. Moreover, nematic order enhances magnetic fluctuations, leading to the opening of a pseudogap in the electronic spectral function due to magnetic precursors. I will also discuss the magneto-structural phase diagram that follows from this model, as well as the anisotropic transport properties of the nematic state, comparing the results with experimental observations. This model provides a simple framework to understand the interplay between the several degrees of freedom present in the iron pnictides, unveiling the primary role of magnetism. |
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