Coupling Charge and Topological Reconstructions at Polar Oxide Interfaces

In oxide heterostructures, different materials are integrated into a single artificial crystal, resulting in a breaking of inversion symmetry across the heterointerfaces. A notable example is the interface between polar and nonpolar materials, where valence discontinuities lead to otherwise inaccessible charge and spin states. This approach paved the way for the discovery of numerous unconventional properties absent in the bulk constituents. However, control of the geometric structure of the electronic wave functions in correlated oxides remains an open challenge. Here, we create heterostructures consisting of ultrathin ${\mathrm{SrRuO}}_3$, an itinerant ferromagnet hosting momentum-space sources of Berry curvature, and ${\mathrm{LaAlO}}_3$, a polar wide-band-gap insulator. Transmission electron microscopy reveals an atomically sharp $\mathrm{LaO}/{\mathrm{RuO}}_2/\mathrm{SrO}$ interface configuration, leading to excess charge being pinned near the ${\mathrm{LaAlO}}_3/{\mathrm{SrRuO}}_3$ interface. We demonstrate through magneto-optical characterization, theoretical calculations and transport measurements that the real-space charge reconstruction drives a reorganization of the topological charges in the band structure, thereby modifying the momentum-space Berry curvature in ${\mathrm{SrRuO}}_3$. Our results illustrate how the topological and magnetic features of oxides can be manipulated by engineering charge discontinuities at oxide interfaces.

Figure 1

Atomic characterization. High-angle annular dark-field images of (a) $\mathrm{STO}/\mathrm{SRO}/\mathrm{STO}$ and (b) $\mathrm{STO}/\mathrm{SRO}/\mathrm{LAO}$ heterostructures with the insets showing bright-field images of the interface regions. (c) Intensity profile along the growth axis for the atomic A (gray) and B sites (blue) of (b). (d) Illustration of the charge frustration and the resulting profile of the chemical potential $E_C$ close to the Fermi energy $E_F$ at the $\mathrm{LaO}/{\mathrm{RuO}}_2$ interface. (e) $c/a$ ratio along the growth axis for (a) and (b). (f) $\mathrm{O}─\mathrm{M}─\mathrm{O}$ bond angles for the A and B sites of the $\mathrm{STO}/\mathrm{SRO}/\mathrm{LAO}$ heterostructure, defined with respect to the STO substrate.

Figure 2

Magneto-optical Faraday effect. (a) Illustration of the experimental geometry for probing the Faraday rotation. (b) Faraday rotation as a function of applied magnetic field for an $\mathrm{SRO}(5)/\mathrm{LAO}(10)$ heterostructure for various temperatures. (c) The amplitude of the hysteresis loop ${\theta }_{\max }$ as a function of applied field for $\mathrm{SRO}(5)/\mathrm{STO}(10)$ (gray), $\mathrm{SRO}(5)/\mathrm{STO}(2)/\mathrm{LAO}(8)$ (orange), and $\mathrm{SRO}(5)/\mathrm{LAO}(10)$ (blue) heterostructures. The solid lines represent Landau fits.

Figure 3

Excess charge profile and topological reconstruction. (a) Layer-resolved charge and magnetization profile for $\mathrm{STO}/\mathrm{SRO}(4)/\mathrm{STO}$ (gray) and $\mathrm{STO}/\mathrm{SRO}/\mathrm{LAO}$ (blue) calculated using density-functional theory (DFT). (b) Effective tight-binding model of two coupled SRO monolayers, with excess charge $q=-0.5e$ in the top layer, as well as an on site electrostatic potential $V$ and interlayer coupling $\gamma t$. (c),(d) The 12 Ru $t_{2g}$ bands for the effective model with (c) representing a weak pinning ($\gamma =1$) and (d) a strong pinning ($\gamma =0.5$) scenario, including an orbital Rashba correction ${\lambda }_R=0.04t$ and an on site potential $V=-0.2t$ in the top layer. (e) The calculated anomalous Hall conductivity ${\sigma }_{xy}^{\mathrm{AH}}$, in which the gray dashed line represents the reference case with two ${\mathrm{Ru}}^{4+}$ layers ($q=0$) and the colored solid lines represent the scenarios with excess charge $q=-0.5e$ with either weak (green) or strong pinning (blue).

Figure 4

Anomalous Hall effect. (a) Illustration representing the evolution of the momentum-space topological charges. Upon increasing the charge pinning, the system moves through a Weyl point in the synthetic space spanned by $k_x$, $k_y$ and the charge pinning parameter $\gamma$. (b) The measured anomalous Hall resistivity ${\rho }_{xy}^{\mathrm{AH}}$ for SRO films of varying thickness capped by both STO and LAO as a function of temperature. The inset in (b) shows an example of the magnetic-field dependence of the AHE, from which the amplitude ${\rho }_{xy}^{\mathrm{AH}}$ is extracted.