Estudo de impureza de európio em PbSe e PbTe. Uma investigação de primeiros princípios

Abstract

In this work properties were studied the structural, electronic and magnetic semiconductors PbSe and PbTe doped with europium (Eu) in the crystalline phase. The investigation is performed within the density functional theory (DFT). We observe that due a strong spin-orbit coupling, relativistic corrections are necessary to describe the PbSe and PbTe. To obtain a good description of crystals where Eu is present (EuSe and EuTe) the standard DFT fails. This fail is due to the fact that the Eu atom presents f electrons in the valence band, which are strongly localized. To describe the localized f electrons an additional procedure from the many body theory is necessary. We observe that using an additional term (the U term) from the Hubbard model, the DFT is able to describe the localized f electrons, such the theory now is called DFT+U. The formation energy results show that the Eu atom is more stable in Pb sites for both PbSe and PbTe. Eu in a Pb site in PbSe introduces electronic levels inside the band gap while the new electronic levels from Eu in a Pb site in PbTe are resonant with the top of the valence band. The analysis from the character of these electronic levels reveled that they come from the 4f electrons from the Eu atom. These results allow us to conclude that Eu is a good dopant to improve the thermoelectric properties of PbTe while the same is not observed for PbSe. Finally, we investigate the magnetic properties for Eu doping PbSe and PbTe, we observe that in both semiconductors there is a magnetic moment of the 1.0 μB localized in the Eu atom.