Strain-induced structural transitions in (111)-oriented (LaMnO$_3$)$_{2n}|$(SrMnO$_3$)$_n$ superlattices

Abstract

By means of first-principles electronic structure calculations, we hereby investigate the structural transitions induced by epitaxial strain in (111)-oriented (LaMnO$_3$)$_{2n}|$(SrMnO$_3$)$_n$ superlattices, with $n=2,4,6$. All superlattices in the explored range of strain are shown to prefer a half-metallic ferromagnetic order where the local magnetic moments are coupled to volume-breathing distortions. More in detail, our results reveal that thickness plays a crucial role in the response to epitaxial strain, which is particularly evident in the resulting tilt pattern of the oxygen octahedra. The thinnest superlattice, for $n=2$, always adopts the $a^-a^-a^-$ tilt pattern and the competing $a^-a^-c^+$ tilt pattern can be stabilized as a metastable state only in presence of compressive strain. Instead, the superlattice with $n=4$ favours the $a^-a^-c^+$ tilt pattern at equilibrium conditions, but the in-phase rotations around the third pseudocubic axis are so fragile that the $a^-a^-a^-$ pattern is recovered under a tiny amount of either compressive or tensile strain. The superlattice with $n=6$ exhibits a more nuanced behaviour: compressive strain drives a transition from $a^-a^-c^+$ to $a^-a^-a^-$, whereas tensile strain preserves the $a^-a^-c^+$ tilt pattern and significantly accentuates the structural differences between the two inequivalent sublattices within this symmetry. In fact, the Jahn-Teller distortions are quenched in one of the sublattices, leading to enhanced volume-breathing distortions and corresponding enhanced charge and spin oscillations. This suggests that Hund's physics may be more relevant in this regime of tensile strain, maximizing the interplay between strong electronic correlations and structural effects.

Figures (4)

• Figure 1: Illustration of the structural characteristics of the (111)-oriented (LMO)$_{2n}|$(SMO)$_n$ superlattices. La, Sr, Mn and O ions are depicted as green, yellow, purple and red spheres, respectively. Top left panels: $a^-a^-a^-$ and $a^-a^-c^+$ tilting patterns as they relax in the SMO and LMO regions. Top right panel: local environment around a Mn atom, showing six connections (dashed yellow lines) with three Mn atoms towards an adjacent layer and three Mn atoms towards the opposite adjacent layer. Mid panel: side view of the $n=6$ superlattice, with illustration of the two Mn sublattices $S_e$ and $S_o$, shown in red and blue planes respectively; the interfacial (IF) layers are indicated by the blue arrows, while the layer count of the LMO and SMO regions is also emphasized. The $S_e$ and $S_o$ planes repeat in alternating patterns across the Mn-layers for the A-type AFM solution. Bottom panel: sketch of the layer count in the component regions for all the superlattices investigated in this study.
• Figure 2: Layer-resolved van Vleck distortions (left panels) as well as charge and spin distributions (right panels) for (111)-oriented (LMO)$_{2n}|$(SMO)$_n$ superlattices with $n=2$ in the FM ground state. Results obtained for compressive strain ($-3\%$), without strain ($0\%$) and for tensile strain ($+3\%$) are shown, respectively from top to bottom. The tilt pattern obtained for each strain is shown in the leftmost panels and is also indicated by a different color of the quadrants: aquamarine for $a^-a^-c^+$ and light brown for $a^-a^-a^-$. Finally, the $S_o$ and $S_e$ sublattices are identical by symmetry for the $a^-a^-a^-$ tilt pattern ($R\bar{3}c$ space group).
• Figure 3: Layer-resolved van Vleck distortions (left panels) as well as charge and spin distributions (right panels) for (111)-oriented (LMO)$_{2n}|$(SMO)$_n$ superlattices with $n=4$ in the FM ground state. Results obtained for compressive strain ($-3\%$), without strain ($0\%$) and for tensile strain ($+3\%$) are shown, respectively from top to bottom. The tilt pattern obtained for each strain in shown in the rightmost panels and is also indicated by a different color of the quadrants: aquamarine for $a^-a^-c^+$ and light brown for $a^-a^-a^-$. Finally, the $S_o$ and $S_e$ sublattices are identical by symmetry for the $a^-a^-a^-$ tilt pattern ($R\bar{3}c$ space group). For the $a^-a^-c^+$ tilt pattern, the small in-phase rotation around the third pseudocubic axis makes $S_o$ and $S_e$ quasidegenerate, so only one of them is reported in the plots.
• Figure 4: Layer-resolved van Vleck distortions (central columns) and charge/spin distributions (left and right columns) for (111)-oriented (LMO)$_{2n}|$(SMO)$_n$ superlattices with $n=6$ in the FM ground state. Results are reported separately for the sublattice $S_o$ (left side) and for the sublattice $S_e$ (right side). Results obtained for compressive strain ($-3\%$), without strain ($0\%$) and for tensile strain ($+3\%$) are shown, respectively from top to bottom. The tilt pattern obtained for each strain in shown in the leftmost panels and is also indicated by a different color of the quadrants: aquamarine for $a^-a^-c^+$ and light brown for $a^-a^-a^-$.