Electronic structure and correlation in $\beta \text{−}T{\mathrm{i}}_3{\mathrm{O}}_5$ and $\lambda \text{−}T{\mathrm{i}}_3{\mathrm{O}}_5$ studied by hard x-ray photoelectron spectroscopy

We have conducted hard x-ray photoelectron spectroscopy investigations of the electronic structure changes and electron correlation phenomena which take place upon the photoinduced reversible phase transition between β- and $\lambda \text{−}T{\mathrm{i}}_3\mathrm{O}$. From valence band spectra of β- and $\lambda \text{−}T{\mathrm{i}}_3{\mathrm{O}}_5$, we have identified the bipolaron caused by the σ-type bonding of $d_{xy}$ orbitals in $\beta \text{−}T{\mathrm{i}}_3{\mathrm{O}}_5$ and the π stacking between the $d_{xy}$ orbitals between different Ti sites in $\lambda \text{−}T{\mathrm{i}}_3{\mathrm{O}}_5$, previously predicted by ab initio calculations. This indicates that the single electron band picture is valid for the description of photoinduced phase transitions. On the other hand, the Ti $2p$ and Ti $1s$ core level spectra exhibit nonlocal screening satellite features, which are typical spectroscopic signs of strong electron correlation in the coherent $\mathrm{Ti}{t}_{2g}$ states. The most striking result we obtain is that correlation in the valence band also manifests to reduce the plasmon energy, which results in an enhancement of the valence electron mass by a factor of 2.7.

Figure 1

(a) Comparison of experimental valence band spectra for the β (blue) and λ (red) phases, respectively. These spectra have been normalized to integrated intensity after eliminations of Shirley backgrounds. (b) Blue and red dots show the data points of experimental spectra in the band gap region beneath the Fermi level for the β and λ phases, respectively, on an enlarged scale. The blue and red curves are curve-fit results for the two phases, and the dashed curves show the individual Voigt function components and backgrounds. It is clear that the $t_{2g}$ state in the λ phase (red) is well fitted by a single Voigt function with an intense background in the band gap region between the $t_{2g}$ states and O $2p$-like main band. For the β phase (blue), two Voigt components with a weaker background are necessary to fit the experimental spectra. The onset of the λ-phase spectrum just coincides with the Fermi level, whereas onset of the β-phase spectrum is 0.2 eV below the Fermi level.

Figure 2

(a) and (b) DOSs and PDOSs. (c) and (d) Comparisons of experimental and simulated spectra for the λ phase [(a) and (c)] and β phase [(b) and (d)], respectively. The experimental and simulated $t_{2g}$ spectra are shown on enlarged scales in the inset to (c) and (d).

Figure 3

Unit cells of β phase [(a) and (c)] and λ phase $\mathrm{T}{\mathrm{i}}_3{\mathrm{O}}_5$. The three different sites of Ti atoms are indicated as Ti(1), Ti(2), and Ti(3).

Figure 4

Experimental (a) and simulated (b) Ti $2p$ spectra, shown at different vertical magnifications. In the highest magnification (30×) experimental spectra, the data points are shown by dots, and 30 point smoothed spectra are shown by solid curves. The gray and black (thin blue and red) represent the β and λ phases, respectively. (c) The main and lower energy satellite peaks (A) of $\mathrm{Ti}2p_{3/2}$ are separated by fitting using two Voigt functions after subtracting Shirley backgrounds for the β (gray, thin blue on line) and λ (black, red on line) phases.

Figure 5

Experimental (a) and simulated (b) Ti $1s$ spectra, shown at different vertical magnifications. In the highest magnification $(100\times )$ experimental spectra, the data points are shown by dots, and 30 point smoothed spectra are shown by solid curves. The gray and black (thin blue and red) represent the β and λ phases, respectively.

Figure 6

(a) O $2s$, Ti $3p$, and Ti $3s$ spectra for the β (blue) and λ (red) phases. (b) O $1s$ and Ti $2s$ spectra for the β (gray, thin blue) and λ (black, red) phases. (c) O $1s$ spectra on an enlarged vertical scale.