Origin of Interfacial Orbital Reconstruction in Perovskite Superlattices

The competition between on-site electronic correlation and local crystal field stands out as a captivating topic in research. However, its physical ramifications often get overshadowed by influences of strong periodic potential and orbital hybridization. The present study reveals this competition may become more pronounced or even dominant in two-dimensional systems, driven by the combined effects of dimensional confinement and orbital anisotropy. This leads to electronic orbital reconstruction in certain perovskite superlattices or thin films. To explore the emerging physics, we investigate the interfacial orbital disorder-order transition with an effective Hamiltonian and how to modulate this transition through strains.

Figure 1

(a) Sketch of competition between electronic correlation and global periodic field. (b) Sketch of competition between electronic correlation and local crystal field. (c) Layer number of 2DEG vs Hubbard $U$ in LAO/STO from first-principle calculations. The blue insets are electron cloudy of 2DEGs with $U\le 3$ (left) and $U>3\mathrm{eV}$ (right).

Figure 2

(a) Distortion of interfacial ${\mathrm{TiO}}_6$ octahedron from ${\mathrm{O}}_{\mathrm{h}}$ to ${\mathrm{C}}_{4\mathrm{v}}$ symmetry, and normal (left) and abnormal (right) $d$-orbital splitting of Ti. (b) First-principle calculated the projected density of states (spin up) of interfacial Ti with Hubbard $U=0$ and 4 eV, with the filled areas representing the occupied states. (c) Splitting energy ($\mathrm{\Delta }E$) between $d_{xy}$ and $d_{xz}/d_{yz}$ vs Hubbard $U$ from first-principle calculations. The red curve in the inset is the splitting energy vs Hubbard $U$ for stretched STO bulk, which is simulated using a model comprised of four cubic unit cells.

Figure 3

(a)–(c) represent the electronic $d_{xy}\text{−}{d}_{xy}$, $d_{xz}\text{−}{d}_{xz}$, and $d_{xz}\text{−}{d}_{yz}$ hopping on 2D ${\mathrm{TiO}}_2$ lattice, respectively. $\mathit{t}$ and ${\mathit{t}}^{\prime }$ represent the hopping amplitude along $\sigma$ and $\pi$ bonds, respectively. ${\mathit{t}}^{\prime \prime }$ represents one between two different orbitals. (d) Electron occupation numbers on in-plane (red line) and out-plane (black line) orbitals from DMFT calculations. (e),(f) Projected density of states for the Hubbard model with $U=3\mathrm{eV}$ and 4 eV from DMFT calculations.

Figure 4

Phase diagram of orbital order (OO) of interfacial 2DEGs vs Hubbard $U$ and uniaxial straining from first-principle calculations. The insets are the spatial structures of 2DEGs under the corresponding sections of staining and $U$ values.