Strain-induced insulator-to-metal transition in LaTiO${}_3$ within DFT + DMFT

We present results of combined density functional theory plus dynamical mean-field theory (DFT + DMFT) calculations, which show that the Mott insulator LaTiO${}_3$ undergoes an insulator-to-metal transition under compressive epitaxial strain of about $-2$%. This transition is driven by strain-induced changes in the crystal-field splitting between the Ti $t_{2g}$ orbitals, which in turn are intimately related to the collective tilts and rotations of the oxygen octahedra in the orthorhombically distorted $Pbnm$ perovskite structure. An accurate treatment of the underlying crystal structure is therefore crucial for a correct description of the observed metal-insulator transition. Our theoretical results are consistent with recent experimental observations and demonstrate that metallic behavior in heterostructures of otherwise insulating materials can emerge also from mechanisms other than genuine interface effects.

Figure 1

(a) Calculated $c/a$ ratio (top), octahedral tilt angles (middle), and Ti-O bond distances (bottom) in LaTiO${}_3$ as a function of in-plane strain. The $c/a$ ratio is calculated from the orthorhombic lattice parameters ($c/a\approx \sqrt{2}$ for zero strain) and is compared to experimental data from Ref. [7] (filled diamonds). (b) and (c) show projections of the orthorhombically distorted $Pbnm$ perovskite structure. The three different Ti-O bond distances are denoted as L1–L3. The angles $\theta$ and $\varphi$ measure in-plane “rotations” and out-of-plane “tilts,” respectively, and are related to specific bond angles, which are indicated in (b) and (c) by capital letters $\Theta$ and $\Phi$.

Figure 2

Trace of the local Green's function at $\tau =\beta /2$ as function of the interaction parameter $U$, calculated for different values of epitaxial strain.

Figure 3

(a) Strain dependence of the three “crystal-field levels” corresponding to the effective Ti $t_{2g}$ Wannier orbitals. (b) and (c) Occupations and “effective” chemical potential $\mu -\mathrm{Re}\Sigma \left({\omega }_0\right)$, respectively, as a function of the interaction parameter $U$, calculated for different strain values. The three different orbitals for each strain are indicated by solid, dotted, and dashed lines, in order of decreasing occupation. The dashed and dot-dashed horizontal lines in (c) indicate the band-edges of the noninteracting system for 0% and 3.76% strain, respectively. In (a) and (c), the Fermi level was used as zero energy reference for each strain value.

Figure 4

Orbitally resolved spectral functions for $U=4.78$ eV and $\epsilon =0$ (bottom), $\epsilon =-1.75$% (middle), and $\epsilon =-3.76$% (top). Orbitals 1, 2, and 3 correspond to the eigenfunctions of the occupation matrix and are ordered with decreasing occupation.