Growth and characterization of the La_3Ni_2O_7-δ thin films: dominant contribution of the d_x^2-y^2 orbital at ambient pressure

By using the pulsed-laser-ablation technique, we have successfully grown the La_3Ni_2O_7-δ thin films with c-axis orientation perpendicular to the film surface. X-ray diffraction shows that the (00l) peaks can be well indexed to the La_3Ni_2O_7-δ phase. Resistive measurements show that the samples can be tuned from weak insulating to metallic behavior through adjusting the growth conditions. Surprisingly, all curves of ρ-T in the temperature region of 2∼300 K do not show the anomalies corresponding to either the spin density wave or the charge density wave orders as seen in bulk samples. Hall effect measurements show a linear field dependence with the dominant hole charge carriers, but the Hall coefficient R_H=ρ_xy/H exhibits strong temperature dependence. The magnetoresistance above about 50 K is positive but very weak, indicating the absence of multiband effect. However, a negative magnetoresistance is observed at low temperatures, which shows the delocalization effect. Detailed analysis on the magnetoresistance suggests that the delocalization effect at low temperatures is due to the Kondo-like effect, rather than the Anderson weak localization. Our transport results suggest that, the electronic conduction is fulfilled by the d_x^2-y^2 orbital with holes as the dominant charge carriers, while the interaction through Hund's coupling with the localized d_z^2 orbital plays an important role in the charge dynamics.