Aplikace Berryho fáze na výpočty termoelektrických vlastností CuCr2X4 (X = S, Se, Te).

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Recent studies of phenomena arising from the coupling between spin and heat currents as well as types of anomalous Hall effects (AHE) in various magnetic materials have triggered renewed interest in the anomalous Nernst effect (ANE) as one of the topologically nontrivial phenomena and for its potential application to thermoelectric devices. The ferrimagnetic chalcogenide spinel series CuCr2X4 (X = S, Se, Te) is one of the good candidate for this kind of phenomena, exhibiting long-range magnetic ordering below T~ 380, 430 and 325 K for X = S, Se, and Te, respectively, with the saturated magnetic moment at low temperature around 5 μB. It was found that the behavior of AHE and ANE in CuCr2Se4 corresponds to the intrinsic mechanism explained by Karplus-Luttinger theory and its modern generalization based on the Berry-phase method.

In this presentation, we focus on the comparison of the experimentally determined AHE and ANE of CuCr2X4 with the calculated properties obtained using the Berry's phase approach. First, the basic concept of Berry's phase and its link to electronic structure calculations are presented. By analysing the topological equation of transverse conductivity, we discuss that broken time-reversal symmetry and spin-orbit coupling play an important role in AHE and ANE. Finally, we discuss the overall trends of ANE that are successfully reproduced by Berry-phase calculations for CuCr2X4. The ANE is negative at room temperature for all phases and the highest absolute value ~ 1.5 μV/K is observed for X = Te and Se around room temperature, whereas ANE for X = S is much smaller. A sign change of ANE to positive is observed at 285 K for X = S and at 65 K for X = Se.

Anomalous Nernst conductivity of CuCr<sub>2</sub>X<sub>4</sub>
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 Comparison of the experimental and theoretical anomalous Nernst conductivity (ANC) of CuCr2X4. Experimental ANC was calculated as α Axy = σ xxAxy ( σxx – electrical conductivity, SAxy – anomalous Nernst effect). Theoretical ANC was calculated using Berry phase method.