Seebeck effect in Fe1+xTe1-ySey single crystals

We present measurements of resistivity and thermopower 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=8T. 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 Tc 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. The support of a theoretical background which could provide band resolved parameters such as carrier density, mobility and electron-phonon coupling of each band would allow to extract from our data valuable quantitative information on the transport and superconducting mechanisms of these iron chalcogenides.