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.

This letter reports synchronization phenomena and mathematical modeling on a frustrated system of living beings, or Japanese tree frogs (Hyla japonica). While an isolated male Japanese tree frog calls nearly periodically, he can hear sounds including calls of other males. Therefore, the spontaneous calling behavior of interacting males can be understood as a system of coupled oscillators. We construct a simple but biologically reasonable model based on the experimental results of two frogs, extend the model to a system of three frogs, and theoretically predict the occurrence of rich synchronization phenomena, such as triphase synchronization and 1:2 antiphase synchronization. In addition, we experimentally verify the theoretical prediction by ethological experiments on the calling behavior of three frogs and time series analysis on recorded sound data. Note that the calling behavior of three male Japanese tree frogs is frustrated because almost perfect antiphase synchronization is robustly observed in a system of two male frogs. Thus, nonlinear dynamics of the three-frogs system should be far from trivial.

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...

This paper reports theoretical and experimental studies on spatio-temporal dynamics in the choruses of male Japanese tree frogs. First, we theoretically model their calling times and positions as a system of coupled mobile oscillators. Numerical simulation of the model as well as calculation of the order parameters show that the spatio-temporal dynamics exhibits bistability between two-cluster antisynchronization and wavy antisynchronization, by assuming that the frogs are attracted to the edge of a simple circular breeding site. Second, we change the shape of the breeding site from the circle to rectangles including a straight line, and evaluate the stability of two-cluster and wavy antisynchronization. Numerical simulation shows that two-cluster antisynchronization is more frequently observed than wavy antisynchronization. Finally, we recorded frog choruses at an actual paddy field using our sound-imaging method. Analysis of the video demonstrated a consistent result with the aforementioned simulation: namely, two-cluster antisynchronization was more frequently realized.