Unusual electron-doping effects in ${\mathrm{Sr}}_{2-x}{\mathrm{La}}_x\mathrm{Fe}\mathrm{Mo}{\mathrm{O}}_6$ observed by photoemission spectroscopy

We have investigated the electronic structure of electron-doped ${\mathrm{Sr}}_{2-x}{\mathrm{La}}_x\mathrm{Fe}\mathrm{Mo}{\mathrm{O}}_6$ ($x=0.0$ and 0.2) by photoemission spectroscopy and band-structure calculations within the local density $\text{approximation}+U$ scheme. A characteristic double-peak feature near the Fermi level $\left(E_{\mathrm{F}}\right)$ has been observed in the valence-band photoemission spectra of both $x=0.0$ and 0.2 samples. A photon-energy dependence of the spectra in the $\mathrm{Mo}4d$ Cooper minimum region compared with the band-structure calculations has shown that the first peak crossing $E_{\mathrm{F}}$ consists of the $(\mathrm{Fe}+\mathrm{Mo})$ $t_{2g\downarrow }$ states (feature A) and the second peak well below $E_{\mathrm{F}}$ is dominated by the $\mathrm{Fe}{e}_{g\uparrow }$ states (feature B). Upon La substitution, the feature A moves away from $E_{\mathrm{F}}$ by $\sim 50\mathrm{meV}$, which is smaller than the prediction of our band theory, $112\mathrm{meV}$. In addition, an intensity enhancement of both A and B has been observed, although B does not cross $E_{\mathrm{F}}$. Those two facts are apparently incompatible with the simple rigid-band shift due to electron doping. We point out that such phenomena can be understood in terms of the strong Hund’s rule energy stabilization in the $3d^5$ configuration at the Fe sites in this compound. From an observed band-narrowing, we have also deduced a mass enhancement of $\sim 2.5$ with respect to the band theory, in good agreement with a specific heat measurement.

Figure 1

(Color online) Photoemission spectra of the valence band with shallow core levels of ${\mathrm{Sr}}_{2-x}{\mathrm{La}}_x\mathrm{Fe}\mathrm{Mo}{\mathrm{O}}_6$ at $20\mathrm{K}$.

Figure 2

(Color online) Valence-band photoemission spectra of ${\mathrm{Sr}}_{2-x}{\mathrm{La}}_x\mathrm{Fe}\mathrm{Mo}{\mathrm{O}}_6$ at $20\mathrm{K}$.

Figure 3

(Color) (a) Near-$E_{\mathrm{F}}$ photoemission spectra of ${\mathrm{Sr}}_{2-x}{\mathrm{La}}_x\mathrm{Fe}\mathrm{Mo}{\mathrm{O}}_6$ at $T=20\mathrm{K}$. Red $(x=0.0)$ and blue $(x=0.2)$ bars indicate the locations of features $A$ and $B$. A gold spectrum at $120\mathrm{eV}$ is also shown. (b) Photon-energy dependence of the $x=0.0$ and the $x=0.2$ near-$E_{\mathrm{F}}$ spectra. The data are same as in (a). (c) Normalized spectral weights of $A$ and $B$ calculated using windows of $-0.8\text{to}0.1\mathrm{eV}$ $\left(A\right)$ and $-1.95\text{to}-0.95\mathrm{eV}$ $\left(B\right)$. The windows are indicated in Panel (b). The spectral weight at $64.5\mathrm{eV}$ is set to unity. Note that the weight for $B$ includes an offset of 1.0.

Figure 4

(Color online) Total and partial DOS of ${\mathrm{Sr}}_2\mathrm{Fe}\mathrm{Mo}{\mathrm{O}}_6$ in the near-$E_{\mathrm{F}}$ region calculated by the $\mathrm{LDA}+U$ method. Vertical lines denote expected locations of $E_{\mathrm{F}}$ deduced from the calculation, assuming the rigid band shift due to electron doping.

Figure 5

(Color online) Experimental and theoretical $E_{\mathrm{F}}$ spectral weights plotted as functions of La concentration $x$. The $E_{\mathrm{F}}$ spectral weight was evaluated from integration within a $\pm 0.1\mathrm{eV}$ window at $E_{\mathrm{F}}$ and normalized with respect to the $x=0.0$ one. The rigid-band shift is assumed in the theoretical curves and the photoionization cross sections of $\mathrm{Fe}3d$, $\mathrm{Mo}4d$, and $\mathrm{O}2p$ states are taken into account. Results by Navarro et al. (Ref. 14) are also shown for comparison.