Thermoelectric power and Hall coefficient measurements on $\text{Ba}{\left({\text{Fe}}_{1-x}{T}_x\right)}_2{\text{As}}_2$ ($T=\text{Co}$ and Cu)

Temperature-dependent thermoelectric power (TEP) data on $\text{Ba}{\left({\text{Fe}}_{1-x}T{M}_x\right)}_2{\text{As}}_2$ ($TM=\text{Co}$ and Cu), complemented by the Hall coefficient data on the samples from the same batches, have been measured. For Co doping we clearly see a change in the temperature-dependent TEP and Hall coefficient data when the sample is doped to sufficient $e$ (the number of extra electrons associated with the $TM$ doping) so as to stabilize low-temperature superconductivity. Remarkably, a similar change is found in the Cu-doped samples at comparable $e$ value, even though these compounds do not superconduct. These changes possibly point to a significant modification of the Fermi surface/band structure of $\text{Ba}{\left({\text{Fe}}_{1-x}T{M}_x\right)}_2{\text{As}}_2$ at small electron doping, that in the case of Co doping is just before, and probably allows for, the onset of superconductivity. These data further suggest that suppression of the structural/magnetic phase transition and the establishment of a proper $e$ value are each necessary but, individually, not sufficient conditions for superconductivity.

Figure 1

(Color online) Transition temperature as a function of (a) extra electrons per $\text{Fe}/TM$ site and (b) measured, $x_{WDS}$, $TM$ concentration, phase diagrams for $\text{Ba}{\left({\text{Fe}}_{1-x}{\text{Co}}_x\right)}_2{\text{As}}_2$ and $\text{Ba}{\left({\text{Fe}}_{1-x}{\text{Cu}}_x\right)}_2{\text{As}}_2$ from Refs. 6, 10. Data points (filled—structural transition, crossed—magnetic transition, and half-filled—superconducting transition) inferred from TEP and Hall coefficient data (shown below) are also shown and often overlap literature data.

Figure 2

(Color online) ${\rho }_H/H$ (Hall coefficient) as a function of temperature for $\text{Ba}{\left({\text{Fe}}_{1-x}{\text{Co}}_x\right)}_2{\text{As}}_2$. Inset: enlarged scale to show data for higher $x$ values.

Figure 3

(Color online) Thermoelectric power as a function of temperature for $\text{Ba}{\left({\text{Fe}}_{1-x}{\text{Co}}_x\right)}_2{\text{As}}_2$.

Figure 4

(Color online) Derivative criteria used to infer upper (structural) and lower (magnetic) phase transitions from transport data (see Refs. 6, 12 for further discussion). The dotted lines are the values of the transition temperatures inferred from the resistivity data. It should be noted that the transition inferred from the TEP data for this Co concentration are slightly lower than those inferred from the resistivity and Hall coefficient data.

Figure 5

(Color online) The low-temperature $\left(T=25\text{K}\right)$ Hall coefficient data as a function of extra electron count, $e$, for $\text{Ba}{\left({\text{Fe}}_{1-x}{\text{Co}}_x\right)}_2{\text{As}}_2$ and $\text{Ba}{\left({\text{Fe}}_{1-x}{\text{Cu}}_x\right)}_2{\text{As}}_2$.

Figure 6

(Color online) ${\rho }_H/H$ (Hall coefficient) as a function of temperature for $\text{Ba}{\left({\text{Fe}}_{1-x}{\text{Cu}}_x\right)}_2{\text{As}}_2$. Inset: field-dependent Hall resistivity, ${\rho }_H$, of $\text{Ba}{\left({\text{Fe}}_{0.985}{\text{Cu}}_{0.015}\right)}_2{\text{As}}_2$ measured at 1.85, 25, and 305 K.

Figure 7

(Color online) Thermoelectric power as a function of temperature for $\text{Ba}{\left({\text{Fe}}_{1-x}{\text{Cu}}_x\right)}_2{\text{As}}_2$.