Hall effect in superconducting Fe ( Se0.5 Te0.5 ) thin films

The Hall effect is investigated for eight superconducting Fe (Se0.5 Te0.5) thin films grown on MgO and LaSrAlO4 substrates with different transition temperatures (Tc). The normal Hall coefficients (RH) have positive values with magnitude of 1□1.5 10^-3 cm3 /C at room temperature for the all samples. With decreasing temperature, we find two characteristic types of behavior in RH (T) depending on Tc. For thin films with lower Tc (typically Tc <5K), RH start decreasing approximately below T=250K toward a negative side, some of which shows sign reversal at T=50□60K, but turns positive toward T=0K. On the other hand, for the films with higher Tc (typically Tc >9K), RH leaves almost unchanged down to T≈100K, and then starts decreasing toward a negative side. Around the temperatures when RH changes its sign from positive to negative, obvious nonlinearity is observed in the field-dependence of Hall resistance as to keep the low-field RH positive while the high-field RH negative. Thus, the electronic state just above Tc is characterized by ne (electron density) > nh (hole density) with keeping μe < μh. These results suggest the dominance of electron density to the hole density is an essential factor for the occurrence of superconductivity in Fe-chalcogenide superconductors.