Since facile routes to fabricate freestanding oxide membranes were previously established, tremendous efforts have been made to further improve their crystallinity, and fascinating physical properties have been also reported in heterointegrated freestanding membranes. Here, we demonstrate our synthetic recipe to manufacture highly crystalline perovskite SrRuO 3 freestanding membranes using new infinite-layer perovskite SrCuO 2 sacrificial layers. To accomplish this, SrRuO 3 /SrCuO 2 bilayer thin films are epitaxially grown on SrTiO 3 (001) substrates, and the topmost SrRuO 3 layer is chemically exfoliated by etching the SrCuO 2 template layer. The as-exfoliated SrRuO 3 membranes are mechanically transferred to various nonoxide substrates for the subsequent BaTiO 3 film growth. Finally, freestanding heteroepitaxial junctions of ferroelectric BaTiO 3 and metallic SrRuO 3 are realized, exhibiting robust ferroelectricity. Intriguingly, the enhancement of piezoelectric responses is identified in freestanding BaTiO 3 /SrRuO 3 heterojunctions with mixed ferroelectric domain states. Our approaches will offer more opportunities to develop heteroepitaxial freestanding oxide membranes with high crystallinity and enhanced functionality.

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NiTe2, a type-II Dirac semimetal with a strongly tilted Dirac band, has been explored extensively to understand its intriguing topological properties. Here, using density functional theory calculations, we report that the strength of the spin–orbit coupling (SOC) in NiTe2 can be tuned by Se substitution. This results in negative shifts of the bulk Dirac point (BDP) while preserving the type-II Dirac band. Indeed, combined studies using scanning tunneling spectroscopy and angle-resolved photoemission spectroscopy confirm that the BDP in the NiTe2–x Se x alloy moves from +0.1 eV (NiTe2) to −0.3 eV (NiTeSe) depending on the Se concentrations, indicating the effective tunability of type-II Dirac Fermions. Our results demonstrate an approach to tailor the type-II Dirac band in NiTe2 by controlling the SOC strength via chalcogen substitution. This approach can be applicable to different types of topological materials.

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Electric polarization can only be well defined in insulators, not in metals, and there is no general scheme to induce and control bulk polarity in metals. Here, we circumvent this limitation by utilizing a pseudo-electric field generated by inhomogeneous lattice strain, namely a flexoelectric field, as a universal means of polarizing and controlling a metal. Using heteroepitaxy and atomic-scale imaging, we show that flexoelectric fields polarize the bulk of an otherwise centrosymmetric metal SrRuO3, with remarkable off-center displacements of Ru ions. This further impacts the electronic bands and lattice anisotropy of the flexo-polar SrRuO3, potentially leading to an enhancement of electron correlation, ferromagnetism, and its anisotropy. Beyond conventional electric fields, flexoelectric fields may universally engender novel electronic states and their control via pure atomic displacements.