Strain-Stabilized Interfacial Polarization in Altermagnetic RuO₂/TiO₂ Heterostructures

Interfacial polarization – charge accumulation at the heterointerface – is a well-established tool in semiconductors, but its influence in metals remains unexplored. In this talk, we will demonstrate that interfacial polarization can robustly modulate surface work function in metallic rutile RuO₂ layers in epitaxial RuO₂/TiO₂ heterostructures grown by hybrid molecular beam epitaxy (MBE). Using multislice electron ptychography, we directly visualize polar displacements of transition metal ions relative to oxygen octahedra near the interface, despite the conductive nature of RuO₂. This interfacial polarization enables over 1 eV modulation of the RuO₂ work function, controlled by small thickness variations (2–3 nm), as measured by Kelvin probe force microscopy, with a critical thickness of ~ 4 nm – corresponding to the transition from fully-strained to relaxed films [1]. We also demonstrate that ultrathin, fully strained RuO₂ films retain metallicity and exhibit a sizable anomalous Hall effect (AHE) at a significantly lower magnetic field (<9 T). Together with density functional theory calculations, these results reveal that epitaxial strain stabilizes a non-compensated altermagnetic ground state in epitaxial RuO₂ (110) thin films [2,3].

1. S. G. Jeong, B. Y. X. Lin, M. Jin, I. H. Choi, S. Lee, Z. Yang, S. Nair, R. Choudhary, J. Parikh, A. Santhosh, M. Neurock, K. A. Stoerzinger, J. S. Lee, T. Low, Q. Tu, J. M. LeBeau, and B. Jalan, Strain-Stabilized Interfacial Polarization Tunes Work Function Over 1 eV in RuO₂/TiO₂ Heterostructures, under review (2025), https://arxiv.org/abs/2507.07575.

2. S. G. Jeong, S. Lee, B. Lin, Z. Yang, I. H. Choi, J. Y Oh, S. Song, S. W. Lee, S. Nair, R. Choudhary, J. Parikh, S. Park, W. S. Choi, J. S. Lee, J. M. LeBeau, T. Low, and B. Jalan, Metallicity and Anomalous Hall Effect in Epitaxially-Strained, Atomically-thin RuO₂ Films, PNAS 122(24), e2500831122 (2025).

3. S. G. Jeong, I. H. Choi, S. Nair, L. Buiarelli, B. Pourbahari, J. Y. Oh, N. Bassim, D. Hirai, A. Seo, W. S. Choi, R. M. Fernandes, T. Birol, L. Zhao, J. S. Lee, and B. Jalan, Altermagnetic polar metallic phase in ultra-thin epitaxially-strained RuO₂ films, under review (2025), https://arxiv.org/abs/2405.05838.