Symmetry analysis of magnetoelastic couplings in the Fe-based superconductors*

Antiferromagnetism (AFM) is a reoccurring theme in the Fe-based superconductors. The stripe-type AFM on the Fe lattice in these compounds is accompanied by a sympathetic orthorhombic distortion of the tetragonal structure. This is not the only kind of magnetoelastic coupling possible between magnetic and structural degrees of freedom. Based on a symmetry analysis of the crystal structures we obtain the symmetry-allowed coupling terms in the Landau free energy. We propose that many of the compounds in this family (including FeSe, BaFe₂As₂ and CaKFe₄As₄) will develop a periodic structural modification when a magnetic field is applied to the AFM ordered phase. This should produce additional Bragg peaks which could provide a sensitive indicator of the AFM order under extreme conditions, such as in high pressure cells.