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A4: Theory - bulk materials

Ambigous role of spin-orbit interactions in thermoelectric properties of Mg2X (X= Si, Ge, Sn) compounds

K. Kutorasinski, B. Wiendlocha, S. Kaprzyk, J. Tobola
AGH University of Science and Technology, Faculty of Physics and Applied Computer Science, Mickiewicza 30, 30-059 Krakow, Poland

A theoretical study of the influence of relativistic effects on the electronic band structure and thermopower in Mg2X (X= Si, Ge, Sn)  semiconductors using the full potential Korringa-Kohn-Rostoker (KKR) method is presented. A detailed comparison between the fully  relativistic and semirelativistic electronic structure features allows to elucidate that the spin-orbit (SO) interaction splits the  valence bands at specific points of the Brilloiun zone, which remains in excellent agreement with the experimental data. As expected this effect strongly depends on X atom and the SO modifies the topology of the hole-like Fermi surface  pockets, which leads to a change in electron transport properties. The thermoelectric properties are investigated using the Boltzmann approach, which has been recently implemented to the KKR method. It is found that the SO coupling of the valence bands reduces the effective mass and therefore signifcantly lowers the thermopower, primarily in Mg2Sn. A detrimental influence of the SO interaction on the thermoelectric performance of p-type Mg2Si is analyzed as a function of temperature and carrier concentration. Interestingly, similar calculations in n-type Mg2X show a negligible effect of the SO interaction on the lowest conduction bands, and consequently, also on the Seebeck of Mg2X semiconductors[1]. So, it seems really important to evidence that the relativistic effects do not destroy the band convergence feature of electronic structure and should not decrease the thermoelectric properties of n-type materials. Paradoxally, the n-type thermopower at lower carrier concentrations benefits from the degradation of the p-type Seebeck coefficient due to the reduction of the bipolar effect, which is well seen in Mg2Ge and Mg2Sn compounds [2].

This work was supported by the Polish National Science Center (NCN) under Grants DEC-2011/02/A/ST3/00124 and UMO-2011/03/N/ST3/02644.

[1] Kutorasinski et al., Phys. Rev. B 87, 195205 (2013) and references therein.

[2] Kutorasinski et al., Phys. Rev. B 89, 115205 (2014).