Competition between screening channels in core-level x-ray photoemission as a probe of changes in the ground-state properties of transition-metal compounds

Core-level x-ray photoemission spectra (XPS) for copper, manganese, and ruthenium compounds are calculated. A strong dependence of the spectral line shape on electron and/or hole doping, magnetic and orbital ordering is observed. The changes can be explained in terms of the competition between local and nonlocal screening effects. In contrast to earlier claims, we find that the changes do not result from additional quasiparticle states at the Fermi level but from a strong coupling of the different screening channels to changes in the ground state. The strong sensitivity of core-level XPS on the surrounding transition-metal atoms enables the study of temperature- and doping-induced changes in orbital occupation and ordering.

Figure 1

(Color online) (a) Schematic representation of the ground state in the undoped cuprates with on average one hole (indicated by the arrows) per $\mathrm{Cu}{\mathrm{O}}_4$ plaquette. Shown are the $x^2\text{−}{y}^2$ orbitals for the copper and the $\sigma$-bonding $p$ orbitals of the oxygen. (b) Removal of a core electron leads to an XPS final state where the core hole (indicated by the blue circle) is unscreened. (c) Electrons can flow in from the oxygens surrounding the site with the core hole, thereby screening the core hole. This is known as local screening. (d) Screening electrons can come from neighboring $\mathrm{Cu}{\mathrm{O}}_4$ plaquettes to screen the core hole. The state with two electrons is known as a Zhang-Rice singlet. (e) Schematic picture of the electron-doped system. (f) The lowest XPS final state where the doped electrons screen the strong core-hole potential.

Figure 2

(Color online) Cu $2p$ XPS spectrum calculated for an undoped [(blue) solid line], a 20% electron-doped [(red) dotted line], and a 20% hole-doped [(green) dashed line] ${\mathrm{Cu}}_5{\mathrm{O}}_{16}$ cluster.

Figure 3

(Color online) (a) Calculations for the manganites are done for a reduced basis set, where effective $e_g$ orbitals correspond to a $\mathrm{Mn}{\mathrm{O}}_6$ unit. (b) Schematic picture of the ground state of the undoped system, where there is on average one $e_g$ electron per site. (c) A locally screened final state, where the core hole (indicated by the thick green circle) is screened by electrons on the $\mathrm{Mn}{\mathrm{O}}_6$ unit. (d) A nonlocally screened final state, where screening electrons come from neighboring $\mathrm{Mn}{\mathrm{O}}_6$ units. (e) For doped systems, final states can be reached where there are no $e_g$ electrons that screen the core hole. (f) Schematic picture of the reduction of the nonlocal screening channel for hole-doped manganites. Doping reduces the number of $e_g$ electrons that could screen the core hole. (g) Reduction of the nonlocal screening channel due to magnetic disorder. When the core spin fluctuate, the double exchange mechanism hampers the hopping between different sites.

Figure 4

(Color online) Mn $2p$ XPS line shapes calculated for a ${\mathrm{Mn}}_8$ cluster. (a) Doping dependence for $R_{1-x}{A}_x\mathrm{Mn}{\mathrm{O}}_3$ with $x=0$, 0.125, 0.25, 0.375, and 0.5 (coresponding to eight, seven, six, five, and four $e_g$ electrons in the ${\mathrm{Mn}}_8$ cluster). (b) Comparison of the Mn $2p$ XPS for $x=0$ (eight $e_g$ electrons) for different magnetic and orbital structures. The dotted and solid lines show the ferromagnetic and $A$ type magnetic structures, respectively. $3x^2-r^2∕3y^2-r^2$ orbital ordering is induced by a Jahn-Teller splitting between the ${(y∕x)}^2-z^2$ and $3{(x∕y)}^2-r^2$ orbitals of $E_{\mathrm{JT}}=0$ (no orbital ordering), 0.1, 0.3, and $0.5\mathrm{eV}$. (c) Temperature dependence of the Mn $2p$ XPS for $x=0.25$ (six $e_g$ electrons) for $T∕T_c=0$, 0.6, 0.8, 0.9, 0.95, and 1.

Figure 5

(Color online) Ru $3d$ XPS line shapes for a ${\mathrm{Ru}}_4$ cluster for different orbital occupations. The ratio between the ${\underset{̱}{n}}_{xy}$ and ${\underset{̱}{n}}_{yz∕zx}$ holes is varied by introducing an energy difference $\Delta E$ between the $xy$ and $yz∕zx$ orbitals. Spectra are given for ${\underset{̱}{n}}_{xy}:{\underset{̱}{n}}_{yz∕zx}=1:1,\frac{1}{2}:\frac{3}{2}$, and 0:2 for $\Delta E=0.01,0$, and $-0.32\mathrm{eV}$, respectively.