Evolution of a band insulating phase from a correlated metallic phase

We investigate the evolution of the electronic structure in $\mathrm{Sr}{\mathrm{Ru}}_{1-x}{\mathrm{Ti}}_x{\mathrm{O}}_3$ as a function of $x$ using high resolution photoemission spectroscopy, where $\mathrm{Sr}\mathrm{Ru}{\mathrm{O}}_3$ is a weakly correlated metal and $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ is a band insulator. Increase in $x$ leads to an introduction of a local potential different from Ru-O sublattice. The features in the bulk spectra could be captured remarkably well using first principles approaches, and the electron correlation strengths are found to be similar in all the compositions. The surface spectra exhibit a metal-insulator transition at $x=0.5$ by opening up a soft gap at the Fermi level ${ϵ}_F$ and a hard gap appears at higher $x$ values. These results presumably provide an experimental demonstration of the predictions of metal-insulator transition due to local potential and electron correlation in a two-dimensional bipartite lattice using the ionic Hubbard model.

Figure 1

(Color online) (a) Integral background subtracted XP valence band spectra of $\mathrm{Sr}{\mathrm{Ru}}_{1-x}{\mathrm{Ti}}_x{\mathrm{O}}_3$ for various values of $x$. Solid line represents the $\mathrm{O}2p$ part for $x=0.6$. The inset demonstrates the background subtraction procedure. Open circles, solid line, and closed circles represent raw data and background and subtracted spectra of $\mathrm{Sr}{\mathrm{Ru}}_{0.4}{\mathrm{Ti}}_{0.6}{\mathrm{O}}_3$. (b) $\mathrm{Ru}4d$ spectra after the subtraction of the $\mathrm{O}2p$ contributions as shown in (a). (c) $\mathrm{Ru}4d$ band obtained from $\mathrm{He}\mathrm{II}$ spectra.

Figure 2

(Color online) $S\left(ϵ\right)$ obtained from (a) $\mathrm{He}\mathrm{II}$ and (b) XP spectra of $\mathrm{Sr}{\mathrm{Ru}}_{1-x}{\mathrm{Ti}}_x{\mathrm{O}}_3$. (b) $S\left(ϵ\right)$ in (a) are plotted as a function of (c) ${\mid ϵ-{ϵ}_F\mid }^{0.5}$ and (d) ${\mid ϵ-{ϵ}_F\mid }^{1.25}$. $S\left(ϵ\right)$ obtained from (e) XP and (f) $\mathrm{He}\mathrm{II}$ spectra of ${\mathrm{Ca}}_{1-x}{\mathrm{Sr}}_x\mathrm{Ru}{\mathrm{O}}_3$.

Figure 3

(Color online) Extracted (a) bulk and (b) surface spectra of $\mathrm{Sr}{\mathrm{Ru}}_{1-x}{\mathrm{Ti}}_x{\mathrm{O}}_3$ for various values of $x$. The SDOs obtained from bulk and surface spectra are shown in (c) and (d), respectively.

Figure 4

(Color online) Real and imaginary parts of the self-energy of (a) $\mathrm{Sr}\mathrm{Ru}{\mathrm{O}}_3$ and (b) $\mathrm{Sr}{\mathrm{Ru}}_{0.5}{\mathrm{Ti}}_{0.5}{\mathrm{O}}_3$ obtained by second order perturbation method following the method of Treglia et al. (Ref. 22). Spectral functions for various values of $U$ of (c) $\mathrm{Sr}\mathrm{Ru}{\mathrm{O}}_3$ and (d) $\mathrm{Sr}{\mathrm{Ru}}_{0.5}{\mathrm{Ti}}_{0.5}{\mathrm{O}}_3$. Calculated experimental spectra for different values of $U$ of (e) $\mathrm{Sr}\mathrm{Ru}{\mathrm{O}}_3$ and (f) $\mathrm{Sr}{\mathrm{Ru}}_{0.5}{\mathrm{Ti}}_{0.5}{\mathrm{O}}_3$.