Thermoelectric power and Hall coefficient measurements on Ba(Fe_1−xT_x)₂As₂ (T=Co and Cu)

Temperature-dependent thermoelectric power (TEP) data on Ba(Fe_1−xTM_x)₂As₂ (TM=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 Ba(Fe_1−xTM_x)₂As₂ 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.