Observation of mirror-odd and mirror-even spin texture in ultra-thin epitaxially-strained RuO2 films

TL;DR

The study investigates ultrathin, epitaxially strained RuO$_2$ films to resolve their magnetic and spin textures. Using spin-resolved ARPES and complementary ab-initio calculations, it uncovers a dual spin-texture: mirror-odd components from inversion-symmetry breaking and an intrinsic mirror-even component suggesting time-reversal symmetry breaking. Symmetry analysis points to a magnetic point group m'm2' with in-plane moments, compatible with ferromagnetic or d-wave altermagnetic order stabilized by strain. The work demonstrates a strain-driven, non-relativistic spin texture in oxide heterostructures, offering new avenues for symmetry-breaking phenomena and spintronic functionalities in ultrathin oxide films.

Abstract

Recently, rutile RuO$_2$ has attracted renewed interest due to expectations of prominent altermagnetic spin-splitting. However, accumulating experimental evidence suggests that in its bulk and thick-film forms, RuO$_2$ does not display any form of magnetic ordering. Despite this, the spin structure of RuO$_2$ remains largely unexplored in the ultra-thin limit, where substrate-imposed epitaxial strain can be substantial. Here, we employ spin-resolved angle-resolved photoemission spectroscopy, supported by ab-initio calculations, to reveal the electronic structure of 2.7~nm-thick epitaxial RuO$_2$ heterostructures. We observe an unconventional spin texture characterized by the coexistence of mirror-even and mirror-odd momentum-dependent components. A comprehensive symmetry analysis rules out nonmagnetic origins of this spin texture. These findings suggest an emergent non-relativistic spin structure enabled by epitaxial strain in the ultra-thin limit, marking a distinct departure from the behavior of relaxed or bulk RuO$_2$. Our work opens new perspectives for exploring symmetry-breaking mechanisms and spin textures in oxide heterostructures.

Figures (5)

• Figure 1: Proposed spin texture relevant in epitaxially-strained RuO$_2$. (A) Schematic illustration of the charge dipole produced at the (110) RuO$_2$/TiO$_2$ interface and the associated Rashba-type spin splitting within the (110)-plane. (B) Decorated local chemical environment of the two Ru sublattices in RuO$_2$ related by a C$_4$ rotational symmetry and the schematic illustration of nonrelativistic altermagnetic spin splitting in $k$-space. (C) Bulk Brillouin zone (BZ), its projection along [110] (light blue and gray planes) and the (110) surface Brillouin zone (red rectangle) of RuO$_2$. (D) A schematic summary of the observations from spin-resolved ARPES in the projected (110) BZ. The blue arrow outside the BZ indicates a net moment along [001].
• Figure 2: Design and structural characterization of fully strained metallic RuO$_2$ (110) heterostructures grown by hMBE. (A) XRR and (B) XRD 2$\theta$-$\theta$ scans of RuO$_2$ heterostructures. Scattered symbols and solid lines in (A) represent the experimental data and corresponding fitting results, respectively. The inset in (A) shows a schematic illustration of the heterostructure architecture comprising of 2.7 nm RuO$_2$/ 2 nm TiO$_2$/ Nb:TiO$_2$ (110). (C) RHEED patterns acquired after growth along the [1$\bar{1}$0] direction reveal streaky features with Kikuchi lines, indicative of high crystalline quality. (D) AFM image demonstrating atomically smooth surface morphology. (E) Representative rotational anisotropy SHG results with both fundamental and SHG polarizations parallel to the incidence plane. Fitting curves (solid lines) based on non-centrosymmetric mm2 symmetry agree well with the experimental data (scattered symbols). Full polarization analysis is included in the Supplementary Text.
• Figure 3: Narrow bands (NBs) in the ultra-thin epitaxially-strained RuO$_2$. (A) Constant energy contour (CEC) at $E$ - $E_{\rm F}$ = -0.1 eV measured by 62 eV $p$-polarized (indicated on the left top) photons, with an energy integration window of 20 meV, emphasizing the $\alpha$ narrow bands ($\alpha$-NB) and $\beta$ narrow bands ($\beta$-NB) denoted by the red and blue arrows. (B) CEC at $E$ - $E_{\rm F}$ = -0.3 eV showing the $\beta$-NB. (C) Measured electronic band dispersions along the high symmetry $\Bar{\Gamma}-\Bar{M}$ direction spanned by the red dotted arrow in (B), overlaid with spin-polarized bulk density functional theory (DFT) calculations carried out with full TiO$_2$ substrate strain. The energy distribution curve (EDC) integrated across the presented momentum range is shown on the right in red, while the black EDC is the single EDC at $\Bar{\Gamma}$. (D) Non-spin-polarized DFT calculated $\Bar{\Gamma}-\Bar{M}$ band structure
• Figure 4: Measured photoelectron spin polarization along the in-plane [001] direction with respect to the (1$\mathbf{\bar{1}}$0)-mirror plane. (A) Fermi surface probed by 55 eV photons. (B) Band dispersions along the $\bar{\Gamma}-\bar{M}$ direction indicated by the horizontal dashed double-arrow in (A). (C and D) Same as (A and B), but taken using the 62 eV photon energy. The magenta symbols in (A to D) indicate where the spin-resolved energy distribution curves (EDCs) are measured. (E and F) Raw spin-resolved EDCs integrated across the magenta bars on either sides of the (1$\bar{1}$0)-mirror in
• Figure 5: Out-of-plane and in-plane photoelectron spin polarization with respect to the (001)-mirror plane. (A) Fermi surface reproduced from Fig. \ref{['fig:fig4']}A but highlighting the (001)-mirror and the momentum positions of the measured spin-resolved energy distribution curves (EDCs) using circular and pentagonal symbols. (B) Electronic band dispersions measured along $\Bar{\Gamma}-\Bar{Z}$. Similarly, the width of the vertical magenta bars provides a visualization of the momentum resolution in the spin-resolved measurement mode. (C and D) Raw spin-resolved EDCs selectively probing only the out-of-plane [110] spin polarization on the upper and lower sides of the (001)-mirror. (E and F) Converted out-of-plane spin polarization from (C and D). (G, H, I, J) Same as (C, D, E, F), but probing the in-plane spin polarization along [001]. All measurements here utilized a photon energy of 55 eV. See Materials and Methods for details about the background normalization and error bar calculation.