Growth and characterization of the La$_{3}$Ni$_{2}$O$_{7-\delta}$ thin films: dominant contribution of the $d_{x^{2}-y^{2}}$ orbital at ambient pressure

Abstract

By using the pulsed-laser-ablation technique, we have successfully grown the La$_{3}$Ni$_{2}$O$_{7-\delta}$ 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$_{3}$Ni$_{2}$O$_{7-\delta}$ 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 $\rho-T$ in the temperature region of 2$\sim$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 $R_{H}=\rho_{xy}/H$ exhibits strong temperature dependence. The magnetoresistance above about 50~K is positive but very weak, indicating a weakened or absence of multiband effect. However, a negative magnetoresistance is observed at low temperatures, which shows the delocalization effect by magnetic field. 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.