Electronic Structure and Magnetism of Fe-based Superconductors

Ab initio calculations of the electronic structures are carried out for the novel FeSe1–xTex superconductors to explain the experimentally found anomalous magnetic properties in the normal state. The calculations have shown that FeSe1–xTex systems are close to a magnetic instability with dominating enhanced spin paramagnetism. The magnetic susceptibility is found to increase gradually with the Te content. The temperature dependences of the magnetic susceptibility χ and its anisotropy ∆ χ = χ║ – χ┴ are investigated for FeSe, and a growth of the susceptibility with the temperature is revealed in the temperature range 4.2–300 K. For FeTe, a substantial increase of χ under pressure is found. The calculated paramagnetic susceptibility exhibits a strong dependence on the unit cell volume V and especially the height of chalcogen species from the Fe plane. The calculations have explained the experimental data on χ(T) and χ(P) for FeSe and FeTe, respectively.