Strain-induced metal-insulator transition in $t_{2g}$ electron system of perovskite titanate $\mathrm{S}{\mathrm{m}}_{0.5}\mathrm{C}{\mathrm{a}}_{0.5}\mathrm{Ti}{\mathrm{O}}_3$ films

We have demonstrated a strain-induced metal-insulator transition for weak Jahn-Teller effects in a $t_{2g}$ electron system of $\mathrm{S}{\mathrm{m}}_{0.5}\mathrm{C}{\mathrm{a}}_{0.5}\mathrm{Ti}{\mathrm{O}}_3$, which has metallic ground states in the bulk. A clear variation of electronic properties in $\mathrm{S}{\mathrm{m}}_{0.5}\mathrm{C}{\mathrm{a}}_{0.5}\mathrm{Ti}{\mathrm{O}}_3$ thin films was investigated as a function of epitaxial stain by changing a substrate and the film thickness. Under strong biaxial tensile strain, metallic behaviors were completely eliminated, as evidenced by the temperature dependence of resistivity. The observed metal-insulator transition was consistent with the behavior of the Ti $3d$ density of states at the Fermi level seen in valence band hard x-ray photoemission spectra.

Figure 1

Characterization of the SCTO films on the DSO substrates. (a) RHEED intensity oscillations for the specular beam spot during the initial growth. The insets show the RHEED and AFM images of the SCTO film. (b) Out-of-plane XRD pattern around 220 DSO reflections (indicated by the asterisk) The 220 SCTO reflection is indicated by the solid triangle. (c) X-ray rocking curves of 220 SCTO reflections.

Figure 2

Temperature dependence of resistivity for the SCTO films. The curves for the films on DSO substrates’ thickness dependence, while those with thickness of 50–55 nm on DSO, LSAT, and NGO strain dependence. The dashed curve indicates the bulk single crystal ($\delta =0.4$) [14].

Figure 3

Epitaxial structures of SCTO films on DSO substrates. Reciprocal space maps around (a)–(c) 444 and (d)–(f) 260 DSO reflections, of which in-plane $d$ vectors are nearly orthogonal to each other. F and S denote reflections from films and substrates, respectively. Thicknesses of the films are (a), (d) 50 nm, (b), (e) 100 nm, and (c), (f) 170 nm.

Figure 4

Electronic structures of the SCTO films on the DSO (red) and LSAT (blue) substrates. (a) Sm $3d$ core-level spectra. The inset shows the same photoemission spectra around the Sm $3d_{5/2}$ core level expanded to the lower binding energy side. (b) Valence band spectra normalized by the highest intensity. (c) Magnified spectra near the Fermi level.

Figure 5

(a) Schematic illustration of $\mathrm{GdFe}{\mathrm{O}}_3$-type perovskite structure. xyz and abc axes are defined in text. (b) Energy diagram of $d_{xy},d_{yz}$, and $d_{zx}$ orbitals in SCTO bulk and films on DSO under strong tensile strain. (c) Schematic illustration of electronic phase diagrams for $R_{1-x}\mathrm{C}{\mathrm{a}}_x\mathrm{Ti}{\mathrm{O}}_{3+y/2}$. The phase boundary with the bold solid line is taken from Ref. [14]. The solid circle indicates SCTO. MIT in SCTO occurs when the orbital degeneracy ($N_{\mathrm{d}}$) is reduced from 3 to 1.