Yuki K. Wakabayashi

Ultrathin complex-oxide membranes provide a powerful platform for strain engineering, interfacial control, and heterogeneous integration; however, their formation remains constrained by the availability and performance of suitable water-soluble sacrificial layers. This study demonstrates that barium oxide (BaO) serves as a highly efficient and rapidly dissolving water-soluble sacrificial layer, enabling the epitaxial lift-off and transfer of ultrathin La2/3Sr1/3MnO3 (LSMO) membranes onto SiOx/Si substrates. LSMO membranes with a thickness of ∼8 nm are released using a BaO sacrificial layer grown by molecular beam epitaxy, while high crystallinity is preserved and Ba interdiffusion is limited to a narrow interfacial region of ∼0.5 nm. Post-transfer oxygen annealing at 600 °C increases the Curie temperature (TC) from 342 to 346 K by eliminating Mn2+ states associated with oxygen vacancies generated through oxygen extraction into the BaO layer. These results show that BaO provides a fast, scalable, and compositionally simple route for complex-oxide membrane release, while brief oxygen annealing is essential to restore the optimal Mn valence state and achieve the intrinsic high TC. These findings are directly relevant to oxide-based electronic and spintronic applications.

Self-driving laboratories are powerful tools for navigating high-dimensional process spaces, yet Bayesian-optimization decision layers optimize black-box objective functions without distilling transferable process understanding. Here, we demonstrate a mathematically interpretable self-driving laboratory framework that transforms autonomous optimization data into human-executable growth rules. As a benchmark, we apply this framework to sputtering, addressing a challenge in sputtering epitaxy: realizing high-quality β -Ga ₂ O ₃ heteroepitaxy and single-crystalline β -Ga ₂ O ₃ homoepitaxy. By combining Bayesian optimization with automated optical evaluation of the Urbach energy as a sub-bandgap disorder metric, the self-driving system identifies heteroepitaxial conditions yielding an Urbach energy of 182 meV, below previously reported values for sputtered β -Ga ₂ O ₃ films. Importantly, the optimized growth window is transferable across substrates, realizing single-crystalline β -Ga ₂ O ₃ homoepitaxy, corroborated by scanning transmission electron microscopy. To convert closed-loop data into interpretable growth rules, we train a random forest surrogate and reveal that the growth landscape is described by additive contributions from four growth parameters, with a temperature–oxygen interaction. This structure suggests a human-executable strategy with sequential one-dimensional tuning followed by focused two-dimensional refinement, validated by human-executed re-optimization, yielding a reduced Urbach energy of 163 meV. This establishes an interpretable self-driving workflow that converts autonomous optimization data into validated, human-usable growth rules.

The (111)-oriented 4d ferromagnetic perovskite SrRuO3 (SRO) offers a unique triangular-lattice geometry, making it a promising platform for exploring Berry-curvature-driven and spin–orbit-coupled transport. Here, we present a systematic study of the structure, magnetism, and magnetotransport of high-quality SRO (111) thin films with thicknesses t = 1.2–60 nm grown on SrTiO3 (111) substrates by machine-learning-assisted molecular beam epitaxy. We achieved a residual resistivity ratio of 45.5 in a 60 nm-thick film, the highest reported for this orientation, enabling access to intrinsic electronic and magnetic behavior. Temperature-dependent resistivity confirms Fermi-liquid transport below ∼15 K in both coherently strained (t = 10, 20 nm) and strain-relaxed (t = 60 nm) films, thereby enabling detailed magnetotransport and magnetic measurements. The linear, non-saturating positive magnetoresistance persists up to 14 T, while Hall-effect measurements and temperature scaling separate intrinsic (Karplus–Luttinger) and extrinsic (side-jump) contributions to the anomalous Hall effect, with the relative weight tuned by (111) epitaxial strain. X-ray magnetic circular dichroism at the Ru M2,3 and O K edges, together with SQUID magnetometry, demonstrates an intrinsically low-spin Ru ground state for both coherently strained and relaxed films, resolving ambiguities among prior reports. These detailed crystalline, electrical, and magnetic characterizations provide a rigorous foundation for understanding and engineering quantum transport in SRO (111).

The (111)-oriented 4d ferromagnetic perovskite SrRuO3 (SRO) offers a unique triangular-lattice geometry, making it a promising platform for exploring Berry-curvature-driven and spin-orbit-coupled transport. Here, we present a systematic study of the structure, magnetism, and magnetotransport of high-quality SRO (111) thin films with thicknesses t = 1.2-60 nm grown on SrTiO3 (111) substrates by machine-learning-assisted molecular beam epitaxy. We achieved a residual resistivity ratio of 45.5 in a 60 nm-thick film, the highest reported for this orientation, enabling access to intrinsic electronic and magnetic behavior. Temperature-dependent resistivity confirms Fermi-liquid transport below 15 K in both coherently strained (t = 10, 20 nm) and strain-relaxed (t = 60 nm) films, thereby enabling detailed magnetotransport and magnetic measurements. The linear, non-saturating positive magnetoresistance persists up to 14 T, while Hall-effect measurements and temperature scaling separate intrinsic (Karplus-Luttinger) and extrinsic (side-jump) contributions to the anomalous Hall effect, with the relative weight tuned by (111) epitaxial strain. X-ray magnetic circular dichroism at the Ru M2,3 and O K edges, together with SQUID magnetometry, demonstrates an intrinsically low-spin Ru ground state for both coherently strained and relaxed films, resolving ambiguities among prior reports. These detailed crystalline, electrical, and magnetic characterizations provide a rigorous foundation for understanding and engineering quantum transport in SRO (111).

We present a physics-informed Bayesian optimization (PIBO) with a concise modification to its acquisition function to incorporate the physical prior knowledge. Specifically, this method multiplies the expected improvement (EI) by a weight encoding prior crystal growth physics. When applied to LaAlO3 molecular-beam epitaxy, the weighting function defines a flat stoichiometric window and penalizes off-window proposals, thereby steering the optimization toward physically plausible regions while maintaining controlled exploration. In a closed-loop optimization, relative to the bare EI, which often proposes off-stoichiometric conditions, the weighted EI constrains the search toward stoichiometric regions while retaining sufficient flexibility to explore neighboring conditions, eventually identifying an optimum slightly beyond the stoichiometric window. Within only 15 growth runs, the lattice constant of the grown LaAlO3 film converged to the bulk value, evidencing efficient and rapid optimization for the ideal stoichiometric growth. Because physics knowledge is incorporated solely through the weighting function, the approach requires only minimal modification to standard BO workflows and is readily applicable to other material systems, offering a general and practical route to AI-driven materials synthesis.

SrTiO3 (STO) has attracted considerable interest as a wide-bandgap semiconductor for advanced high-k capacitors and photocatalytic applications. Although previous angle-resolved photoemission spectroscopy (ARPES) studies have characterized the valence band structure originating from O 2p orbitals, the conduction band arising from Ti 3d orbitals upon electron doping, which is called electron pockets, remains poorly understood. In this study, polarization-dependent ARPES measurements were performed on Nb 1%-doped SrTiO3 (001), enabling the direct, orbital-selective visualization of the electron pockets. From the measured band dispersion, we quantitatively determined their effective masses, anisotropy, and electron density. Our results revealed the formation of an electron pocket at the Γ point induced by Nb doping, yielding a direct bandgap energy of 3.79 eV, consistent with previous optical measurements. Furthermore, the effective masses of m1 = 0.63m0 (short-axis direction) and m2 = 8.0m0 (long-axis direction) were identified, where m0 is the free electron mass, and the Fermi surface has been shown to be ellipsoidal. The electron density derived from these dispersions was found to be 3.58 × 1020 cm−3. These findings provide a comprehensive picture of the conduction-band electronic structure that will be crucial in the design of STO-based functional devices.

Achieving robust ferromagnetism and high conductivity in atomically thin oxide materials is critical for advancing spintronic technologies. Here, we report the growth of a highly conductive and ferromagnetic single monolayer SrRuO3 (SRO) having a high Curie temperature of ∼ 154 K on DyScO3 (110) substrates. The SrTiO3 capping layer effectively suppresses surface reactions, which typically hinder ferromagnetism in atomically thin films. X-ray absorption spectroscopy and x-ray magnetic circular dichroism measurements revealed strong orbital hybridization between Ru 4d and O 2p orbitals in the SRO monolayer, which contributes to the enhancement of the conductivity and ferromagnetic ordering of both the Ru 4d and O 2p orbitals. The resistivity of the single monolayer SRO is comparable to that of the bulk SRO. This study highlights the potential of monolayer SRO as a platform for two-dimensional magnetic oxide systems, offering opportunities for the exploration of spintronic devices and quantum transport phenomena.

In transition-metal compounds, the transition-metal d electrons play an important role in their physical properties; however, the effects of the electron correlation between the ligand p electrons have not been clear yet. In this Letter, the Ru 4d and O 2p partial density of states (PDOS) in transition-metal oxide SrRuO_{3} involving Weyl fermions are investigated by resonant photoemission spectroscopy. The observations demonstrate that the O 2p PDOS is distorted from that predicted by first-principles calculations than the Ru 4d PDOS. The results indicate that the electron correlation in the ligand orbitals will be important to understand the electronic structure of the p-d hybridized state in strongly correlated electron systems, even with topological states.