Spin-polarized two-dimensional $t_{2g}$ electron gas: Ab initio study of EuO interface with oxygen-deficient $\mathrm{SrTi}{\mathrm{O}}_3$

Using first-principles calculations we predict the existence of a spin-polarized two-dimensional electron gas (2DEG) at the interface of a ferromagnetic insulator EuO and oxygen-deficient $\mathrm{SrTi}{\mathrm{O}}_3$. The carriers are generated by oxygen vacancies in $\mathrm{SrTi}{\mathrm{O}}_3$ near the interface and have predominantly Ti-$t_{2g}$ orbital character. At the interface, the split-off $d_{xy}$-derived conduction band of $\mathrm{SrTi}{\mathrm{O}}_3$ is fully spin-polarized and the in-gap vacancy-induced state, found below the conduction-band edge, is aligned ferromagnetically with EuO. The calculations suggest a possible mechanism for generating spin-polarized 2DEG for spintronic applications.

Figure 1

Simulation cell of ${\left(\mathrm{EuO}\right)}_3/\mathrm{Ti}{\mathrm{O}}_2-{\left(\mathrm{SrO}-\mathrm{Ti}{\mathrm{O}}_2\right)}_6/{\left(\mathrm{EuO}\right)}_3$ heterostructure. (a) Top view of two different types of interfaces: Eu on top of a hollow site in a $\mathrm{Ti}{\mathrm{O}}_2$ plane (top-H) and on top of an oxygen in a $\mathrm{Ti}{\mathrm{O}}_2$ plane (top-O); (b) $2\times 2$ supercell with a single vacancy at the subinterface SrO plane. Only half of the cell is presented due to mirror symmetry. The oxygen vacancy site is marked in black.

Figure 2

DOS projected on a specific atom for each layer in top-H heterostructure without and with a vacancy. Only the results for the upper half cell are shown due to symmetry. Eu, O, Ti, and Sr states are marked by magenta, red, dark blue, and green, respectively. EuO surface states are indicated with the square. Panels (a) and (c) correspond to the spin-up while (b) and (d) correspond to spin-down components, respectively.

Figure 3

DOS projected on the Ti $d$ states in each layer. Layer 1 represents the central “bulk” part of $\mathrm{SrTi}{\mathrm{O}}_3$, while layer 4 represents the interfacial layer. (a) DOS of top-H heterostructure with a vacancy; (b) DOS of top-H heterostructure without vacancies but artificially doped with two extra electrons.

Figure 4

Similar to Fig. 2, the atom-projected DOS for each layer in the heterostructure containing a vacancy with $U_{\mathrm{Ti}-\mathrm{d}}=8\mathrm{eV}$.

Figure 5

(a) The $e_g-t_{2g}$ splitting in bulk $\mathrm{SrTi}{\mathrm{O}}_3$ for $U_{\mathrm{Ti}-\mathrm{d}}=5\mathrm{eV}$ and (b) $U_{\mathrm{Ti}-\mathrm{d}}=8\mathrm{eV}$.