Ce doping in T-La2CuO4 films: Broken electron-hole symmetry in high-Tc superconductivity

We attempted Ce doping in La$_{2}$CuO$_{4}$ with the K$_{2} $NiF$_{4}$ ($T)$ structure by molecular beam epitaxy. With low growth temperature and appropriate substrate choice, we found that Ce can be incorporated into the K$_{2}$NiF$_{4}$ lattice up to $x \quad \sim $ 0.06, which has not yet been realized in bulk synthesis. The doping of Ce made $T$-La$_{2-x}$Ce$_{x}$CuO$_{4}$ more insulating, which is in sharp contrast to Ce doping in La$_{2}$CuO$_{4}$ with the Nd$_{2}$CuO$_{4}$ structure, which made the compounds superconducting. The observed smooth increase in resistivity from hole-doped side ($T$-La$_{2-x}$Sr$_{x}$CuO$_ {4})$ to electron-doped side ($T$-La$_{2-x}$Ce$_{x}$CuO$_{4})$ indicates that electron-hole symmetry is broken in the $T$-phase materials. We propose that the nature of the insulating state in $T$-La$_{2-x}$Ce$_{x} $CuO$_{4}$ is of a Kondo insulator instead of a Mott insulator. The insulating mechanism based on Kondo interaction between Cu3d spins and O2p holes explains the global evolution of the resistivity and also the pseudo gap phenomenon from hole-doping to electron doping.