Stable and Tunable Current-Induced Phase Transition in Epitaxial Thin Films of Ca2RuO4

Real-space imaging of periodic nanotextures in thin films via phasing of diffraction data

New properties and exotic quantum phenomena can form due to periodic nanotextures, including Moire patterns, ferroic domains, and topologically protected magnetization and polarization textures. Despite the availability of powerful tools to characterize the atomic crystal structure, the visualization of nanoscale strain-modulated structural motifs remains challenging. Here, we develop nondestructive real-space imaging of periodic lattice distortions in thin epitaxial films and report an emergent periodic nanotexture in a Mott insulator. Specifically, we combine iterative phase retrieval with unsupervised machine learning to invert the diffuse scattering pattern from conventional X-ray reciprocal-space maps into real-space images of crystalline displacements. Our imaging in PbTiO3/SrTiO3 superlattices exhibiting checkerboard strain modulation substantiates published phase-field model calculations. Furthermore, the imaging of biaxially strained Mott insulator Ca2RuO4 reveals a strain-induced nanotexture comprised of nanometer-thin metallic-structure wires separated by nanometer-thin Mott-insulating-structure walls, as confirmed by cryogenic scanning transmission electron microscopy (cryo-STEM). The nanotexture in Ca2RuO4 film is induced by the metal-to-insulator transition and has not been reported in bulk crystals. We expect the phasing of diffuse X-ray scattering from thin crystalline films in combination with cryo-STEM to open a powerful avenue for discovering, visualizing, and quantifying the periodic strain-modulated structures in quantum materials.

Defect Engineering in A2 BO4 Thin Films via Surface-Reconstructed LaSrAlO4 Substrates

Ruddlesden-Popper oxides (A₂ BO₄ ) have attracted significant attention regarding their potential application in novel electronic and energy devices. However, practical uses of A₂ BO₄ thin films have been limited by extended defects such as out-of-phase boundaries (OPBs). OPBs disrupt the layered structure of A₂ BO₄ , which restricts functionality. OPBs are ubiquitous in A₂ BO₄ thin films but inhomogeneous interfaces make them difficult to suppress. Here, OPBs in A₂ BO₄ thin films are suppressed using a novel method to control the substrate surface termination. To demonstrate the technique, epitaxial thin films of cuprate superconductor La_{2- x} Sr_x CuO₄ (x = 0.15) are grown on surface-reconstructed LaSrAlO₄ substrates, which are terminated with self-limited perovskite double layers. To date, La_{2- x} Sr_x CuO₄ thin films are grown on LaSrAlO₄ substrates with mixed-termination and exhibit multiple interfacial structures resulting in many OPBs. In contrast, La_{2- x} Sr_x CuO₄ thin films grown on surface-reconstructed LaSrAlO₄ substrates energetically favor only one interfacial structure, thus inhibiting OPB formation. OPB-suppressed La_{2- x} Sr_x CuO₄ thin films exhibit significantly enhanced superconducting properties compared with OPB-containing La_{2- x} Sr_x CuO₄ thin films. Defect engineering in A₂ BO₄ thin films will allow for the elimination of various types of defects in other complex oxides and facilitate next-generation quantum device applications.

Superconducting Sr2RuO4 Thin Films without Out-of-Phase Boundaries by Higher-Order Ruddlesden-Popper Intergrowth

Ruddlesden-Popper (RP) phases (A_n+1B_nO_3n+1, n = 1, 2,···) have attracted intensive research with diverse functionalities for device applications. However, the realization of a high-quality RP-phase film is hindered by the formation of out-of-phase boundaries (OPBs) that occur at terrace edges, originating from lattice mismatch in the c-axis direction with the A'B'O₃ (n = ∞) substrate. Here, using strontium ruthenate RP-phase Sr₂RuO₄ (n = 1) as a model system, an experimental approach for suppressing OPBs was developed. By tuning the growth parameters, the Sr₃Ru₂O₇ (n = 2) phase was formed in a controlled manner near the film-substrate interface. This higher-order RP-phase then blocked the subsequent formation of OPBs, resulting in nearly defect-free Sr₂RuO₄ layer at the upper region of the film. Consequently, the Sr₂RuO₄ thin films exhibited superconductivity up to 1.15 K, which is the highest among Sr₂RuO₄ films grown by pulsed laser deposition. This work paves the way for synthesizing pristine RP-phase heterostructures and exploring their unique physical properties.