Seebeck effect inFe1+xTe1−ySeysingle crystals

Abstract

We present measurements of resistivity and thermopower $S$ of ${\text{Fe}}_{1+x}{\text{Te}}_{1\ensuremath{-}y}{\text{Se}}_{y}$ single crystalline samples with $y=0$, 0.1, 0.2, 0.3, and 0.45 in zero field and in a magnetic field $B=8\text{ }\text{T}$. We find that the shape of thermopower curves appears quite peculiar in respect to that measured in other Fe-based superconducting families. We propose a qualitative analysis of the temperature behavior of $S$, where the samples are described as almost compensated semimetals: different electron and hole bands with similar carrier concentrations compete and their relative contribution to the thermoelectric transport depends on the respective filling, mobility, and coupling with phonons. For $y\ensuremath{\ge}0.2$, superconductivity occurs and the optimum Se-doping level for a maximum ${T}_{\text{c}}$ of 13 K turns out to be $y=0.3$. At low temperatures, evidence of a contribution to $S$ by an excitation-drag mechanism is found while at high temperatures a strikingly flat behavior of $S$ is explained within a narrow-band Hubbard model.