Growth and characterization of the La3Ni2O7-δ thin films: Dominant contribution of the d x2-y2 orbital at ambient pressure

Using the pulsed-laser-ablation technique, we successfully grew La3Ni2O7-delta thin films with c-axis orientation perpendicular to the film surface. X-ray diffraction showed that the (00l) peaks can be well-indexed to the La3Ni2O7-delta phase. Resistive measurements showed that the samples could be tuned from weak insulating to metallic behavior by adjusting the growth conditions. Surprisingly, no curves of rho - T in the temperature region of 2 similar to 300 K showed anomalies corresponding to either the spin-density or charge-density wave orders, as seen in bulk samples. Hall effect measurements showed a linear field dependence on the dominant hole charge carriers, but the Hall coefficient RH = rho xy/H exhibited strong temperature dependence. The magnetoresistance above about 50 K was positive but very weak, indicating a weakened or absent multiband effect. However, negative magnetoresistance was observed at low temperatures, which shows the delocalization effect of the magnetic field. Detailed analysis of the magnetoresistance suggested the delocalization effect at low temperatures is due to the Kondo-like effect rather than Anderson weak localization. Our transport results suggest that the electronic conduction is fulfilled by the dx2-y2 orbital with holes as the dominant charge carriers, while the interaction through Hund's coupling with the localized dz2 orbital plays an important role in the charge dynamics.