Hall effect in superconducting Fe(Se_0.5Te_0.5) thin films

The Hall effect is investigated for eight superconducting Fe(Se_0.5Te_0.5) thin films grown on MgO and LaSrAlO₄ substrates with different transition temperatures (T_c). The normal Hall coefficients (R_H) 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 R_H(T) depending on T_c. For thin films with lower T_c (typically T_c<5  K), R_H start decreasing approximately below T=250  K toward a negative side, some of which shows sign reversal at T=50~60  K, but turns positive toward T=0  K. On the other hand, for the films with higher T_c (typically T_c>9  K), R_H leaves almost unchanged down to T100  K, and then starts decreasing toward a negative side. Around the temperatures when R_H changes its sign from positive to negative, obvious nonlinearity is observed in the field-dependence of Hall resistance as to keep the low-field R_H positive while the high-field R_H negative. Thus, the electronic state just above T_c is characterized by n_e (electron density) >n_h (hole density) with keeping µ_e<µ_h. 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. ©2010 The American Physical Society