Seebeck effect in ${\text{Fe}}_{1+x}{\text{Te}}_{1-y}{\text{Se}}_y$ single crystals

We present measurements of resistivity and thermopower $S$ of ${\text{Fe}}_{1+x}{\text{Te}}_{1-y}{\text{Se}}_y$ single crystalline samples with $y=0$, 0.1, 0.2, 0.3, and 0.45 in zero field and in a magnetic field $B=8\text{T}$. 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 $y\ge 0.2$, 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.

Figure 1

(Color online) Resistivity as a function of temperature for the six samples; inset: zoom of the low-temperature region.

Figure 2

(Color online) (a) Seebeck coefficient as a function of temperature for the six samples; dashed black lines show the fit $S=A+B/T$, with $A$ and $B$ as fitting constants. (b) From top to bottom, the following quantities are plotted as a function of the Se content $y$ (atoms per unit formula): value of the Seebeck coefficient at $T=300\text{K}$, crossover temperature at which $S$ changes in sign, value of the Seebeck coefficient at the minimum of the $S$ curve and superconducting transition temperature, extracted from resistivity (90% of normal-state resistivity criterion) and Seebeck measurements. In the case of $S_{\text{min}}$, different symbols have been used for superconducting and nonsuperconducting samples, in order to emphasize visually the correspondence of $S_{\text{min}}$ and $T_{\text{c}}$ in superconducting samples, described in the text.

Figure 3

(Color online) Seebeck coefficient as a function of temperature for the six samples in the low-temperature region, in zero field and in a magnetic field $B=8\text{T}$.