Decoupling between $d_{x^2-y^2}$ and $d_{z^2}$ orbitals in hole doped La$_3$Ni$_2$O$_7$

Abstract

Through Sr and Ca doping to the La sites, we successfully obtained the hole doped La$_{3-x}$A$_x$Ni$_2$O$_7$ (A = Sr and Ca) thin films by using the pulsed-laser deposition technique. Temperature dependent resistivity shows an upturn at low temperatures, but some clear instabilities, either due to structure or the releasing of strain between the film and substrate, occur at high temperatures. After annealing the films under high pressure of oxygen atmosphere, the upturn at low temperatures is strongly suppressed; the high temperature instability is completely removed. Hall effect measurements show a clear hole-charge carrier behavior with the carrier density of an order of magnitude higher compared with the undoped films. Surprisingly, it is found that the Hall coefficient is almost temperature independent in the whole temperature region, indicating the absence of multiband effect and suggesting the decoupling of the $d_{x^2-y^2}$ and $d_{z^2}$ orbitals. This is contradicting to the rigid band picture of a bonding $d_{z^2}$ band just below the Fermi energy in the pristine sample.

Figures (4)

• Figure 1: Structural characterizations of the La$_{3-x}$A$_{x}$Ni$_{2}$O$_{7}$ films. (a) XRD 2$\theta$-$\omega$ scans of the La$_{2.55}$Sr$_{0.45}$Ni$_{2}$O$_{7}$ thin film before and after annealing. (b) XRD 2$\theta$-$\omega$ scans of the La$_{2.55}$Ca$_{0.45}$Ni$_{2}$O$_{7}$ thin film before and after annealing. The annealing temperature for both samples is $500\rm{^\circ C}$. The dashed lines in red and blue represent the diffraction peak positions of La$_{3-x}$A$_{x}$Ni$_{2}$O$_{7}$ and La$_{4-x}$A$_{x}$Ni$_{3}$O$_{10}$ phases, respectively.
• Figure 2: $\rho$-T curves of the LSNO (a) and LCNO (b) thin films before and after anneal. The "warm up" and "cool down" in the legend represent the results from the measurement by rising or cooling temperature, respectively. The solid lines are the $\alpha$+$\beta$$T^{n}$ fits of the $\rho$-T at low temperatures.
• Figure 3: Hall coefficients of the LSNO and LCNO thin films before and after annealing. The anneal temperature is $500\rm{^\circ C}$. The inset shows the magnetic field dependent $\rho_{xy}$ of the as-grown and annealed LSNO thin film at different temperatures.
• Figure 4: Magnetoresistance (MR) of the as-grown (a-c) and annealed (d-f) LSNO thin films. (g) Temperature dependent magnetoresistance of the as-grown LSNO thin film at 9 T. The inset shows a partial enlargement of (g). (h) The MR of as-grown LSNO thin film at temperatures above 30 K plotted by following Kohler's law.