Multiband Optical Absorption Controlled by Lattice Strain in Thin-Film ${\mathrm{LaCrO}}_3$

Experimental measurements and ab initio modeling of the optical transitions in strained $G$-type antiferromagnetic ${\mathrm{LaCrO}}_3$ resolve two decades of debate regarding the magnitude of the band gap and the character of the optical absorption spectrum in the visible-to-ultraviolet (up to $\sim 5\mathrm{eV}$) range in this material. Using time-dependent density functional theory and accounting for thermal disorder effects, we demonstrate that the four most prominent low-energy absorption features are due to intra-Cr $t_{2g}\mathrm{\text{−}}{e}_g$ (2.7, 3.6 eV), inter-Cr $t_{2g}\mathrm{\text{−}}{t}_{2g}$ (4.4 eV), and interion O $2p$-Cr $3d$ (from $\sim 5\mathrm{eV}$) transitions and show that the excitation energies of the latter type can be strongly affected by the lattice strain.

Figure 1

Geometrical structure of bulk ${\mathrm{LaCrO}}_3$ and spin densities for sublattices of the spin-up $\mathrm{Cr}\uparrow$ (a) and spin-down $\mathrm{Cr}\downarrow$ (b) sites. (c) Total density of states and its projection on the $d$ states of $\mathrm{Cr}\uparrow$ (light color, yellow) and $\mathrm{Cr}\downarrow$ (dark color, blue) sublattices. (d) Schematics of the Cr $3d$ bands and spin-allowed optical transitions—four from the occupied $t_{2g}$ states (open red arrows) and four from the O $2p$ band (blue shaded arrows).

Figure 2

(a) Experimental OA spectrum measured for LCO on $a\mathrm{\text{−}}{\mathrm{SiO}}_2$ and fitted to seven Gaussians. The shaded Gaussians represent intra- and inter-Cr $d\mathrm{\text{−}}d$ transitions, as assigned on the basis of ab initio simulations. (b) Spectroscopic ellipsometry measurement of the extinction coefficient for $\mathrm{LCO}/{\mathrm{SrTiO}}_3$(001) after background subtraction. Positions of all Gaussians in (a), except for the index mismatch contribution at 2.0 eV, are shown with arrows.

Figure 3

(a) Observed and calculated OA spectra for unstrained LCO. (b) Calculated OA spectra for pseudocubic unstrained ($a\mathrm{\text{−}}{\mathrm{SiO}}_2$) and strained (LSAT, LAO) LCO. (c) Onset of the O $2p\to e_g(l)$ transitions. Vertical arrows indicate $E_g$ values. Experimental lattice cell parameters and $\mathrm{Cr}\times 4$ clusters were used in all cases. Inset in (a) shows the values of $E_g$ extrapolated to infinite cluster size.

Figure 4

Effect of lattice strain on the one-electron band gaps $G$ (a),(b) and the width of the bands $W$ (c),(d). Vertical lines show the $a$ lattice parameter (a),(c) and the tetragonality ratio (b),(d) in ${\mathrm{LaCrO}}_3$ supported on ${\mathrm{LaAlO}}_3$ (001).