Interplay of structural, magnetic and electronic properties of the iron-based superconductors FeSe and FeTe

Felix Lochner, Max-Planck-Institut für Eisenforschung, Düsseldorf, Germany
Ilya Eremin, Theoretische Physik III, Ruhr-Universität Bochum, Bochum, Germany
Tilmann Hickel, Max-Planck-Institut für Eisenforschung, Düsseldorf, Germany
Jörg Neugebauer, Max-Planck-Institut für Eisenforschung, Düsseldorf, Germany

Iron-based superconductors (FeSC) are layer structured compounds with a typical superconducting transition temperature Tc up to 35K for bulk compounds. The structural simplest and most interesting materials of this group iron-chalcogenides, where FeSe and FeTe are the most popular ones. For monolayers of FeSe Tc goes up to 100K. Moreover, the pressure depended phase diagram shows a lot of different magnetic and structural transitions, which are up to now not fully understood and differ from other FeSC. One of the most interesting phases is the so called nematic phase, where the magnetic moments are not randomly distributed, but elongated along a specific direction, without showing any long-range magnetic order like e.g. anti ferromagnetism (AFM). This behavior is combined with a strong dipole-dipole interaction in those materials. Therefore, we investigate the dependence of structural properties of FeSe and FeTe on the magnetic order with density functional theory (DFT). We have developed an efficient scheme to predict the correct ground state in the multi-dimensional space of structural parameters by employing similarities between the AFM and the paramagnetic (PM) state. Moreover, the inter-layer interaction in FeSe and FeTe turns out to weak and almost vanishes beyond a certain volume, if dipole-dipole corrections are not considered. Therefore, we include van der Waals (vdW) coupling to these systems. Our results show that a combination of magnetism and dipole interactions is necessary to determine the ground state structure and properties for FeSe. Since the coupling of these effects is specific for the materials investigated here, they might also explain the complexity of their phase diagrams.

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