As discovered by Ohtomo and Hwang, a large sheet charge density with high mobility exists at the interface between SrTiO 3 and LaAlO 3 . Based on transport, spectroscopic, and oxygen-annealing experiments, we conclude that extrinsic defects in the form of oxygen vacancies introduced by the pulsed laser deposition process used by all researchers to date to make these samples is the source of the large carrier densities. Annealing experiments show a limiting carrier density. We also present a model that explains the high mobility based on carrier redistribution due to an increased dielectric constant.

The molecular design based on o-carborane dyads is described for preparing multifunctional luminescent molecules such as dual emissions, aggregation and crystallization-induced emission enhancements, and luminescent color changes. The pyrene-substituted o-carborane dyads were synthesized via the insertion reaction between decaborane and 1-ethynylpyrene in the presence of Lewis base in a good yield. Finally, extremely bright luminescent compounds with solid-state emission properties (ΦPL > 0.99) were obtained.

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We have grown and studied high quality SrRuO 3 films grown by MBE as well as PLD. By changing the oxygen activity during deposition we were able to make SrRuO 3 samples that were stoichiometric (low oxygen activity) or with ruthenium vacancies (high oxygen activity). Samples with strontium vacancies were found impossible to produce since the ruthenium would precipitate out as RuO 2 . The volume of the unit cell of SrRuO 3 becomes larger as more ruthenium vacancies are 2 introduced. The residual resistivity ratio (RRR) and room temperature resistivity were found to systematically depend on the volume of the unit cell and therefore on the amount of ruthenium vacancies. The RRR varied from ~30 for stoichiometric samples to less than two for samples that were very ruthenium poor. The room temperature resistivity varied from 190 μΩ cm for stoichoimetric samples to over 300 μΩ cm for very ruthenium poor samples. UPS spectra show a shift of weight from the coherent peak to the incoherent peak around the Fermi level when samples have more ruthenium vacancies. Core level XPS spectra of the ruthenium 3d lines show a strong screened part in the case of stoichiometric samples. This screened part disappears when ruthenium vacancies are introduced. Both the UPS and the XPS results are consistent with the view that correlation increases as the amount of ruthenium vacancies increase.

Magnetic Weyl semimetals have novel transport phenomena related to pairs of Weyl nodes in the band structure. Although the existence of Weyl fermions is expected in various oxides, the evidence of Weyl fermions in oxide materials remains elusive. Here we show direct quantum transport evidence of Weyl fermions in an epitaxial 4d ferromagnetic oxide SrRuO3. We employ machine-learning-assisted molecular beam epitaxy to synthesize SrRuO3 films whose quality is sufficiently high to probe their intrinsic transport properties. Experimental observation of the five transport signatures of Weyl fermions—the linear positive magnetoresistance, chiral-anomaly-induced negative magnetoresistance, π phase shift in a quantum oscillation, light cyclotron mass, and high quantum mobility of about 10,000 cm2V−1s−1—combined with first-principles electronic structure calculations establishes SrRuO3 as a magnetic Weyl semimetal. We also clarify the disorder dependence of the transport of the Weyl fermions, which gives a clear guideline for accessing the topologically nontrivial transport phenomena.

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Materials informatics exploiting machine learning techniques, e.g., Bayesian optimization (BO), has the potential to offer high-throughput optimization of thin-film growth conditions through incremental updates of machine learning models in accordance with newly measured data. Here, we demonstrated BO-based molecular beam epitaxy (MBE) of SrRuO3, one of the most-intensively studied materials in the research field of oxide electronics, mainly owing to its unique nature as a ferromagnetic metal. To simplify the intricate search space of entangled growth conditions, we ran the BO for a single condition while keeping the other conditions fixed. As a result, high-crystallinequality SrRuO3 film exhibiting a high residual resistivity ratio (RRR) of over 50 as well as strong perpendicular magnetic anisotropy was developed in only 24 MBE growth runs in which the Ru flux rate, growth temperature, and O3-nozzle-to-substrate distance were optimized. Our BO-based search method provides an efficient experimental design that is not as dependent on the experience and skills of individual researchers, and it reduces experimental time and cost, which will accelerate materials research. The itinerant ferromagnetic perovskite SrRuO3 is one of the most promising materials for oxide electronics. 1 -10 Owing to its compatibility with other perovskitestructured oxides, as well as its high conductivity and chemical stability, 3 SrRuO3 is widely used as an epitaxial conducting layer in oxide heterostructures. However, a thorough understanding of its transport properties, electronic structure, and origin of its ferromagnetism remains elusive despite tremendous efforts for over five decades. While high-quality specimens are indispensable for exploring electronic states, it is difficult to make high-quality bulk single crystals of SrRuO3, and hence, thin film specimens have been making a significant contribution to such research. The residual resistivity ratio (RRR), which is defined as the ratio of resistivity at 300 K [ρ(300 K)] to that at 4 K [ρ(4 K)], is a good measure of the purity of a metallic system, and accordingly, the quality of single-crystalline SrRuO3 thin films: RRR is very sensitive to defects and offstoichiometry. 8,11,12 More specifically, only SrRuO3 films with high RRR values above 40 and 60 have enabled observation of sharp dispersive quasiparticle peaks near the Fermi level by angle-resolved photoemission spectroscopy (ARPES) and of quantum oscillations in the electrical resistivity, respectively. 13,14 Such high-quality films maintain their metallic and ferromagnetic character even when the thickness is reduced to a monolayer, 15 providing a rare example of two-dimensional ferromagnetism. This means that extremely thin SrRuO3 films can serve as a two-dimensional spin-polarized electron system, as the existence of a spin-polarized electron current has been established in thicker SrRuO3-based magnetic tunnel junctions. 16,17 Accordingly, high-quality SrRuO3 thin films are also promising for future spintronics application...