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

TL;DR

This study demonstrates that hole doping in La$_3$Ni$_2$O$_7$ thin films leads to decoupling of $d_{x^2-y^2}$ and $d_{z^2}$ orbitals, challenging the rigid band model and revealing new electronic behavior.

Contribution

It provides experimental evidence of orbital decoupling in doped La$_3$Ni$_2$O$_7$, contrasting with previous theoretical predictions.

Findings

Hole doping increases carrier density significantly.

Hall coefficient remains temperature independent, indicating orbital decoupling.

High-pressure oxygen annealing suppresses low-temperature upturns and high-temperature instabilities.

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.