Observation of orbital selective charge transfer in a $\mathrm{Fe}\text{/}\mathrm{BaTi}{\mathrm{O}}_3$ interfacial two-dimensional electron gas

Two-dimensional electron gas (2DEG) states at oxide interfaces between two ferroic materials have been fertile ground to realize controllable multiferroicity. Here, we investigate the 2DEG states at the interface of ferroelectric $\mathrm{BaTi}{\mathrm{O}}_3$ and a magnetic layer of iron using angle-resolved photoemission spectroscopy. Orbital-selective charge transfer occurs on the surprisingly robust 2DEG. Based on first-principles calculations, we show how the interfacial hybridization can give rise to the unexpected charge transfer in the magnetic 2DEG. Our study reveals a close interplay on a 2DEG between magnetic and ferroelectric interfaces, which sheds light on future design principles of multiferroic 2DEG states.

Figure 1

Structure of interface-induced magnetic two-dimensional electron gas (2DEG). (a) A schematic of the interface between a magnetic layer of Fe and a ferroelectric layer of 10-unit-cell (uc) $\mathrm{BaTi}{\mathrm{O}}_3$ (BTO). (b) Reflection high-energy electron diffraction (RHEED) intensity plot during the growth of a 10-uc BTO film. (c) RHEED patterns along the ${\left[11\right]}_{\mathrm{pc}}$ direction before and after depositing Fe atoms on a 10-uc BTO film. (d) Reciprocal space mapping of a 10-uc BTO film.

Figure 2

Robust 2DEG prior to and after Fe deposition. (a), (b) Fermi surfaces of 2DEG in BTO and Fe/BTO films. Red squares represent the Brillouin zone boundary. (c) Schematic of the Fermi surface of the 2DEG. A black (green) solid line indicates a $d_{xy}$ ($d_{xz/yz}$) band. (d) Energy distribution curves of 2DEG in BTO (black) and Fe/BTO (red) films at ($k_{(-1,1)}, k_{\left(1,1\right)}$) = (0, 1.9). Filled inverted triangles indicate coherent peaks near $E_{\mathrm{F}}$. After Fe deposition, incoherent spectral weight appears around $E=E_{\mathrm{F}}-0.7\mathrm{eV}$ (open-inverted triangle).

Figure 3

Fe-induced, orbital-selective charge transfer at the interface. (a), (b) $E-k$ dispersion of BTO and Fe/BTO films along the $k_y$ direction. Here, we define $k_y$ at ${\mathrm{\Gamma }}_{11}$ as zero. 2D curvature band dispersions [60] and fit results with a Lorentzian function are plotted on right. (c) Momentum distribution curves (MDCs) at the Fermi level ($E_{\mathrm{F}}$) and $E_{\mathrm{F}}-20\mathrm{meV}$. Open (closed) inverted triangles represent peaks for the $d_{xy}$ ($d_{xz}$) bands. (d) Peak positions of $d_{yz}$ and $d_{xy}$ bands of BTO and Fe/BTO films. Black (red) open circles represent data from BTO (Fe/BTO) film.

Figure 4

Ab initio results for the $\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_2$/BTO interface. (a) Geometry of the monolayer $\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_2$/BTO heterostructure. (b) Ab initio calculated band structures and density of states (DOS) of (left) BTO and (right) $\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_2$/BTO films. Atom-projected DOS of the BTO ($\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_2$/BTO) film is plotted at the center with black (red) depending on the type of atom. A red open triangle is placed at $E=E_{\mathrm{F}}-0.7\mathrm{eV}$. (c) (left) Enlarged 2D curvature band dispersions with fit results [adapted from Figs. 3 and 3] and (right) calculated $E-k$ dispersions for bands with mainly Ti $3d$ orbital character. The enlarged regions are represented with blue rectangles in (b). (d)–(f) Schematic band structures and atomic orbital interaction in (d) BTO and (e),(f) $\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_2$/BTO films. The double-ended arrows represent electron hopping between orbitals.