Spectroscopic and theoretical investigation of the electronic states of layered perovskite oxyfluoride $\mathrm{S}{\mathrm{r}}_2\mathrm{Ru}{\mathrm{O}}_3{\mathrm{F}}_2$ thin films

We investigated the electronic structure of a layered perovskite oxyfluoride $\mathrm{S}{\mathrm{r}}_2\mathrm{Ru}{\mathrm{O}}_3{\mathrm{F}}_2$ thin film by hard x-ray photoemission spectroscopy (HAXPES) and soft x-ray absorption spectroscopy (XAS) as well as density functional theory (DFT)-based calculations. The core-level HAXPES spectra suggested that $\mathrm{S}{\mathrm{r}}_2\mathrm{Ru}{\mathrm{O}}_3{\mathrm{F}}_2$ is a Mott insulator. The DFT calculations described the total and site-projected density of states and the band dispersion for the optimized crystal structure of $\mathrm{S}{\mathrm{r}}_2\mathrm{Ru}{\mathrm{O}}_3{\mathrm{F}}_2$, predicting that $\mathrm{R}{\mathrm{u}}^{4+}$ takes a high-spin configuration of ${\left(xy\right)}^{\uparrow }{\left(yz,zx\right)}^{\uparrow \uparrow }{\left(3z^2-r^2\right)}^{\uparrow }$ and that $\mathrm{S}{\mathrm{r}}_2\mathrm{Ru}{\mathrm{O}}_3{\mathrm{F}}_2$ has an indirect band gap of 0.7 eV with minima at the $M,A$ and $X,R$ points. HAXPES spectra near the Fermi level and the angular-dependent O $1s$ XAS spectra of the $\mathrm{S}{\mathrm{r}}_2\mathrm{Ru}{\mathrm{O}}_3{\mathrm{F}}_2$ thin film, corresponding to the valence band and conduction band density of states, respectively, were drastically different compared to those of the $\mathrm{S}{\mathrm{r}}_2\mathrm{Ru}{\mathrm{O}}_4$ film, suggesting that the changes in the electronic states were mainly driven by the substitution of an oxygen atom coordinated to Ru by fluorine and subsequent modification of the crystal field.

Figure 1

Core-level HAXPES spectra of (a) Ru $2s$, (b) Ru $2p$, (c) Ru 3p, (d) Sr $2s$, (e) Sr $2p$, and (f) Sr $3d$ of the $\mathrm{S}{\mathrm{r}}_2\mathrm{Ru}{\mathrm{O}}_4$ and $\mathrm{S}{\mathrm{r}}_2\mathrm{Ru}{\mathrm{O}}_3{\mathrm{F}}_2$ thin films. These spectra have been normalized to the maximum peak heights. The insets in (b), (d), and (e) show magnified views of Ru $2p_{3/2}$, Sr $2s$, and Sr $2p_{3/2}$, respectively.

Figure 2

Core-level spectra of Sr $3p_{1/2}$ and Ru $3d$ for the $\mathrm{S}{\mathrm{r}}_2\mathrm{Ru}{\mathrm{O}}_4$ and $\mathrm{S}{\mathrm{r}}_2\mathrm{Ru}{\mathrm{O}}_3{\mathrm{F}}_2$ thin films. These spectra have been normalized to the maximum peak heights.

Figure 3

(a) $\mathrm{S}{\mathrm{r}}_2\mathrm{Ru}{\mathrm{O}}_3{\mathrm{F}}_2$ with $\mathrm{S}{\mathrm{r}}_2\mathrm{Ti}{\mathrm{O}}_3{\mathrm{F}}_2$-type structure. (b) Two types of in-plane magnetic orderings of Ru spins. Upward and downward arrows denote up- and down-spin moment, respectively.

Figure 4

(a) $c$-axis lengths of $\mathrm{S}{\mathrm{r}}_2\mathrm{Ru}{\mathrm{O}}_3{\mathrm{F}}_2$ calculated for various $U$ values. The figures indicate five different phases: check-A, -B, -C and plain-A, -B. The experimental $c$-axis value is shown by a broken line. (b) Relative energy of the five phases as a function of $U$.

Figure 5

(a) The reported crystal structure of $\mathrm{S}{\mathrm{r}}_2\mathrm{Ru}{\mathrm{O}}_4$ in Ref. [33] and (b) the structure of $\mathrm{S}{\mathrm{r}}_2\mathrm{Ru}{\mathrm{O}}_3{\mathrm{F}}_2$ calculated by the GGA + $U$ method. The lattice constants and Ru-O/F bond lengths (Å) are shown in the figures. Total and site-projected DOS for (c) $\mathrm{S}{\mathrm{r}}_2\mathrm{Ru}{\mathrm{O}}_4$ and (d) $\mathrm{S}{\mathrm{r}}_2\mathrm{Ru}{\mathrm{O}}_3{\mathrm{F}}_2$. The vertical dashed lines in (c) and (d) denote $E_{\mathrm{F}}$.

Figure 6

PDOS for the Ru-$4d$ electrons in (a) $\mathrm{S}{\mathrm{r}}_2\mathrm{Ru}{\mathrm{O}}_4$ and (b) $\mathrm{S}{\mathrm{r}}_2\mathrm{Ru}{\mathrm{O}}_3{\mathrm{F}}_2$. The lines in positive and negative regions denote the PDOS of the majority and minority of spin electrons, respectively. (c) The local structures around Ru in $\mathrm{S}{\mathrm{r}}_2\mathrm{Ru}{\mathrm{O}}_4$ and $\mathrm{S}{\mathrm{r}}_2\mathrm{Ru}{\mathrm{O}}_3{\mathrm{F}}_2$, and the schematic diagrams for $4d^4$ configurations under the two types of crystal field.

Figure 7

Band dispersion image of $\mathrm{S}{\mathrm{r}}_2\mathrm{Ru}{\mathrm{O}}_3{\mathrm{F}}_2$ and the Brillouin zone for a simple tetragonal lattice; the critical points are denoted. The horizontal dashed line in the lower illustration denotes $E_{\mathrm{F}}$.

Figure 8

Valence band spectra of the $\mathrm{S}{\mathrm{r}}_2\mathrm{Ru}{\mathrm{O}}_4$ and $\mathrm{S}{\mathrm{r}}_2\mathrm{Ru}{\mathrm{O}}_3{\mathrm{F}}_2$ thin films. The spectra have been normalized to the area from −1 to 12 eV.

Figure 9

O $1s$ XAS spectra of (a) $\mathrm{S}{\mathrm{r}}_2\mathrm{Ru}{\mathrm{O}}_4$ and (b) $\mathrm{S}{\mathrm{r}}_2\mathrm{Ru}{\mathrm{O}}_3{\mathrm{F}}_2$ films. The spectra are shown at ${\theta }_{\mathrm{inc}}=0^{\circ },{30}^{\circ }$, and 60°; ${\theta }_{\mathrm{inc}}$ is the angle between the incident beam and the surface normal. The structures labeled $A$ to $H$ are discussed in the text.