Seebeck effect in Fe1+xTe1-ySey single crystals

We present measurements of resistivity and thermopower S of Fe1+xTe1-ySey 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 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 >= 0.2, superconductivity occurs and the optimum Se-doping level for a maximum T-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.