Manifestation of screening effects and $A\text{−}\mathrm{O}$ covalency in the core level spectra of $A$ site elements in the $AB{\mathrm{O}}_3$ structure of ${\mathrm{Ca}}_{1-x}{\mathrm{Sr}}_x\mathrm{Ru}{\mathrm{O}}_3$

We investigate the evolution of $\mathrm{Ca}2p$ and $\mathrm{Sr}3d$ core level spectra in ${\mathrm{Ca}}_{1-x}{\mathrm{Sr}}_x\mathrm{Ru}{\mathrm{O}}_3$ using photoemission spectroscopy. Core level spectra in this system exhibit multiple features and unusual evolution with the composition and temperatures. Analysis of the core level spectra in conjunction with the band structure results indicates final state effects due to different core hole screening channels. Such screening in the photoemission final states can be attributed to the large $A\text{−}\mathrm{O}$ covalency in these systems. Changes in the core level spectra with temperature and composition suggest significant modification in $A\text{−}\mathrm{O}$ $(A=\mathrm{Ca}∕\mathrm{Sr})$ covalency in Ca-dominated samples, which gradually reduces with the increase in Sr content and becomes insignificant in $\mathrm{Sr}\mathrm{Ru}{\mathrm{O}}_3$. This study thus provides a direct evidence of cation-oxygen covalency and its evolution with temperature, which may be useful in understanding the unusual ground state properties of these materials.

Figure 1

X-ray diffraction patterns of ${\mathrm{Ca}}_{1-x}{\mathrm{Sr}}_x\mathrm{Ru}{\mathrm{O}}_3$ for various values of $x$.

Figure 2

(Color online) $\mathrm{O}1s$ core level spectra of $\mathrm{Sr}\mathrm{Ru}{\mathrm{O}}_3$ at $20\mathrm{K}$ and room temperature. The solid line represents the smooth asymmetric (Doniach-Šunjić line shape) curve passing through the spectrum at $20\mathrm{K}$.

Figure 3

(Color online) $\mathrm{Ca}2p$ (left column) and $\mathrm{Sr}3d$ (right column) core level spectra in ${\mathrm{Ca}}_{1-x}{\mathrm{Sr}}_x\mathrm{Ru}{\mathrm{O}}_3$ for different values of $x$. Triangles, open circles, and solid circles represent the spectra collected using $\mathrm{Mg}K\alpha$ radiations at $300\mathrm{K}$, $\mathrm{Al}K\alpha$ radiations at $300\mathrm{K}$, and $\mathrm{Al}K\alpha$ radiations at $20\mathrm{K}$, respectively.

Figure 4

(Color online) Experimental (circles) and simulated (solid lines) $\mathrm{Ca}2p$ spectra for different $x$ in ${\mathrm{Ca}}_{1-x}{\mathrm{Sr}}_x\mathrm{Ru}{\mathrm{O}}_3$. The dashed lines represent the $2p_{3∕2}$ and $2p_{1∕2}$ features delineated from the experimental spectra.

Figure 5

(Color online) Experimental (circles) and simulated (solid lines) $\mathrm{Sr}3d$ spectra for different $x$ in ${\mathrm{Ca}}_{1-x}{\mathrm{Sr}}_x\mathrm{Ru}{\mathrm{O}}_3$. The dashed lines represent the $3d_{5∕2}$ and $3d_{3∕2}$ features delineated from the experimental spectra.

Figure 6

(Color online) Experimental and simulated (a) $\mathrm{Ca}2p$ and (b) $\mathrm{Sr}3d$ spectra of ${\mathrm{Ca}}_{0.5}{\mathrm{Sr}}_{0.5}\mathrm{Ru}{\mathrm{O}}_3$ exhibiting all the component features, separately. The open symbols represent spin-orbit split components in every case. Notably, the intensity of feature F in $\mathrm{Sr}3d$ spectra is the sum of the intensities of features M and N as shown in the figures. Here, $I_B∕I_A=0.9$ and $I_M∕I_E=1.7$. (c) $\mathrm{Sr}3d$ spectra collected using $\mathrm{Al}K\alpha$ $\left(\lambda \right)$ and $\mathrm{Mg}K\alpha$ $\left(0.91\lambda \right)$ photons at normal emission and $\mathrm{Al}K\alpha$ photons at an emission angle of 45° $\left(0.71\lambda \right)$.

Figure 7

(Color online) (a) Crystal structure of $\mathrm{Sr}\mathrm{Ru}{\mathrm{O}}_3$ and $\mathrm{Ca}\mathrm{Ru}{\mathrm{O}}_3$. $\mathrm{Gd}\mathrm{Fe}{\mathrm{O}}_3$ distortion in the structure leads to large $\mathrm{Sr}\text{−}\mathrm{O}∕\mathrm{Ca}\text{−}\mathrm{O}$ covalency due to the closeness of O(1) sites toward $\mathrm{Sr}∕\mathrm{Ca}$ sites. Band structure results for (b) $\mathrm{Sr}\mathrm{Ru}{\mathrm{O}}_3$ and (c) $\mathrm{Ca}\mathrm{Ru}{\mathrm{O}}_3$ in the real and equivalent cubic structures are shown in lower left and right panels, respectively. Solid lines represent the $\mathrm{O}2p$ PDOS and dashed lines represent $\mathrm{Sr}4d∕\mathrm{Ca}3d$ PDOS.