Electronic excitations around the substituted atom in La${}_2$Cu${}_{1-y}$Ni${}_y$O${}_4$ as seen via resonant inelastic x-ray scattering

We perform a resonant inelastic x-ray scattering (RIXS) study of Ni-substituted La${}_2$CuO${}_4$ and Cu-substituted La${}_2$NiO${}_4$. Resonantly enhanced charge-transfer excitations are successfully observed by tuning the incident photon energy to the $K$ edge of the substituted element. These excitations display a very weak momentum dependence, evidencing their local character. We find a shift in the energy of the onset edge of these features compared with the nonsubstituted compounds La${}_2$CuO${}_4$ and La${}_2$NiO${}_4$. This shift is quantitatively reproduced by exact diagonalization calculations of the RIXS spectra. RIXS is shown to be a powerful tool to probe the electronic states of substituted elements and to understand their interaction with surrounding atoms.

Figure 1

Incident photon energy dependence of RIXS spectra of (a) La${}_2$Cu${}_{0.95}$Ni${}_{0.05}$O${}_4$ near the Ni $K$ edge and (b) La${}_2$Cu${}_{0.05}$Ni${}_{0.95}$O${}_4$ near the Cu $K$ edge. The spectra at representative incident energies are plotted in (c). (d) Polarization-dependent Ni-$K$ x-ray absorption spectra of La${}_2$Cu${}_{0.95}$Ni${}_{0.05}$O${}_4$. The arrow indicates the incident photon energy used for the measurement of momentum dependence.

Figure 2

Momentum dependence of RIXS spectra of (a) La${}_2$Cu${}_{0.05}$Ni${}_{0.95}$O${}_4$ and (b) La${}_2$Cu${}_{0.95}$Ni${}_{0.05}$O${}_4$.

Figure 3

(a) RIXS spectra of La${}_2$Cu${}_{1-y}$Ni${}_y$O${}_4$ measured at the Brillouin zone center. The spectra of $y=0$ and 1 are taken from the literature.[20, 21] The upper and lower two spectra were measured at the Cu and Ni $K$ edge, respectively. The dashed and dotted lines in $y=0$ are the spectra at the momentum of $(\pi ,0)$ and $(\pi ,\pi )$, respectively. The onset edges of the CT excitations are indicated by the arrows. Schematics of the charge transfer excitation in La${}_2$Cu${}_{1-y}$Ni${}_y$O${}_4$ with (b) $y=0$, (c) 0.95, (d) 0.05, and (e) 1. Solid squares, open circles, and arrows denote CuO${}_4$ (NiO${}_4$) plaquettes, Cu (Ni) atoms, and holes, respectively. A core hole is created at the Cu or Ni atom by an incident photon (denoted by a wavy arrow) and a hole transfers to neighboring oxygen. The RIXS spectra calculated on Cu${}_{4-y}$Ni${}_y$O${}_{16}$ clusters with $y=0$ (f), 3 (g), 1 (h), and 4 (i). The spectra in (f, g) are at the Cu $K$ edge and those in (h, i) are at the Ni $K$ edge. Arrows indicate the leading edges of CT excitations.

Figure 4

Schematic local density of states (LDOS) for La${}_2$Cu${}_{1-y}$Ni${}_y$O${}_4$ with $y=0$ (a), 0.95 (b), 0.05 (c), and 1 (d). Vertical ticks between the white and shaded structures indicate the Fermi level. White areas denote the LDOS above the Fermi level at the core-hole-created site, i.e. the Cu site in (a,b) and the Ni site in (c,d). Shaded areas denote the LDOS below the Fermi level at the neighboring oxygen sites. Broken lines represent the energy positions of the Zhang-Rice singlet (ZRS), the Zhang-Rice doublet (ZRD), the Cu 3$d^{10}$ state, and the Ni 3$d^9$ state, respectively, from left to right.